\section{Nuclear Astrophysics in Canada} \label{sec:nucastro}
\subsection{Overview}
The ultimate goal of all world-wide efforts in Nuclear Astrophysics is the complete understanding of the astrophysical origin and the production processes that make up all the visible matter around us. This quest is strongly connected to the theoretical understanding of the quantum many-body problem of the atomic nucleus that would enable a reliably prediction of the properties of all nuclei from the proton- to the neutron-dripline.

The interpretation of the observed solar abundances (see Fig.~\ref{Fig:NA-Abundances}), from hydrogen up to uranium, is a long-standing problem. The basic foundation was already laid in 1957 by Cameron \footnote{\href{https://iopscience.iop.org/article/10.1086/127051/pdf}{A.G.W. Cameron, Publ. Astron. Soc. of the Pac. 69, 201 (1957)}} and Burbidge, Burbidge, Fowler, and Hoyle \footnote{\href{https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.29.547}{E.M. Burbidge et al., Revs. Mod. Phys. 29, 547 (1957)}}. Since then, generations of stellar modellers have worked hand in hand with experimental nuclear astrophysicists and theoretical nuclear physicists to better understand the underlying astrophysical processes and to finetune and constrain the astrophysical and nuclear physics inputs.

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\begin{figure}[htb!]
\centering
\includegraphics[width=0.8\columnwidth]{Figs/NA_abundances.jpg}
\caption{\label{Fig:NA-Abundances} Measured (observed) solar abundances of the stable nuclides. The different regions for astrophysical productions processes are marked in color.
}
\end{figure}
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How well do we understand the different nucleosynthesis processes nowadays? Despite the tremendous efforts in the past six decades, experimental nuclear astrophysics has just scratched at the surface of its possibilities. So far only half of the total number of nuclei that are expected to exist between the neutron- and proton-dripline have been discovered, about 3450 nuclei. Another $\approx$4000 nuclei are awaiting discovery, the vast majority of them are on the neutron-rich side and will decay by $\beta$-delayed neutron emission.

However, not all of these nuclei and their nuclear properties need to be investigated to improve our understanding of the creation of elements. So-called ''sensitivity studies" have helped to identify the most important nuclear properties and regions of nuclei that need to be measured to better understand the creation of nuclei in stars and astrophysical events. Such efforts are important to guide the future experimental program where competition for beamtime will be large.  

With the detection techniques established and refined at the present facilities, the community is looking forward to an exciting future with the several of the next generation of radioactive ion beam (RIB) facilities becoming operational in the next decade.


\subsection{The Canadian program}
The Canadian community is focusing their efforts on main aspects around the Big Question \textit{``What is the role of radioactive nuclei in shaping the visible matter in the universe?"}. There is also a very lively experimental program around astrophysical reaction studies of stable nuclei but the focus in the upcoming decades will be to fully exploit the new possibilities with radioactive beams, at domestic facilities like ISAC and ARIEL at TRIUMF, as well as at major RIB facilities and at smaller accelerators abroad.

The detection of the gravitational wave signal GW170817 from a binary neutron star merger (see Sec.~\ref{sec:intro:NA}) has led to a worldwide boost in public awareness for the creation of elements and how radioactive beam facilities and stellar modellers help to understand these complex topics better since we are all made of stardust.

Canadian scientists are already deeply involved in this ''multi-messenger" astrophysics since many years and have taken leadership roles thanks to the diverse nuclear (astro)physics research that is enabled by the ISAC facility at TRIUMF. Still, the next decade will be the most exciting since developments on the experimental frontier (new facilities, like ARIEL, FRIB, FAIR) and on the theoretical frontier (new models and descriptions) will allow unprecedented progress in nuclear astrophysics. 

\subsubsection{Summary of the program}
The majority of the domestic experimental program is carried out at the ISAC facility at TRIUMF (Sec.~\ref{sec:CanFac:ISAC}) and complemented by offshore-activities in Germany, Japan, and the USA (Sec.~\ref{sec:IntFac}). This program is closely connected to the recent
\href{https://arxiv.org/abs/1910.09712}{White Paper for the Canadian Long Range Plan for Astronomy and Astrophysics 2020} where many of the Canadian nuclear astrophysicists were co-authors.

The experimental efforts (see Sec.~\ref{sec:NA:facilities}) aim at the measurement of various nuclear physics quantities that are direct or indirect input parameters in astrophysical calculations of various stellar events: 
\begin{itemize}
    \item The CREX/PREX-II collaboration at Jefferson Lab is aiming at a more precise determination of the neutron skin thickness of $^{48}$Ca and $^{208}$Pb. These measurements will help to constrain the density-dependence of the symmetry energy of neutron-rich nuclear matter, which has for example implications on the equation of state and thus on the structure of neutron stars and supernovae explosions. 
    \item The DRAGON and EMMA recoil separators and their auxiliary detectors, as well as the TUDA detector measure the resonance strength and cross sections of reactions that take place in various nucleosynthesis processes. 
    \item  Ion trap facilities like TITAN and CPT are measuring the masses of the participating radioactive nuclei to high precision. Masses are the most important nuclear physics input parameter for astrophysical abundance calculations since they determine the reaction path.
    \item High-efficiency decay spectrometers like GRIFFIN and TIGRESS in combination with their many auxiliary detectors allow to investigate the evolution of the shell structure or reaction rates. GRIFFIN can measure decay half-lives and lifetimes of excited states as well as extract deformation parameters and neutron emission probabilities which e.g. influence the reaction path in the rapid neutron capture process. TIGRESS coupled to EMMA allows e.g. the tagging of $\gamma$-rays emitted in radiative capture reactions.
    \item Moderated neutron detectors like the BRIKEN array at RIKEN Nishina Center or the BELEN detector at GSI/FAIR measure the half-lives and neutron emission probabilities of the most neutron-rich heavy nuclei that can presently be produced. These nuclear properties are important ingredients for a better description of the final abundance distribution of the r process.
    \item Time-of-flight neutron scintillators like DESCANT and the pseudo-bar neutron array of TexNeut and its planned Canadian successor allow to detect neutrons and their energies from decays and reactions. They can be coupled to a multitude of setups at ISAC, e.g. DESCANT with GRIFFIN and TIGRESS, and the pseudo-bar neutron array will allow operation with EMMA and IRIS.
    \item Indirect studies of neutron capture cross sections via $(d,p)$ reactions are carried out with the EMMA recoil separator and the TIGRESS $\gamma$-ray spectrometer with either TI-STAR or SHARC as particle detectors, or offshore via $\beta$-decay spectroscopy in the USA with the SuN total absorption spectrometer. These studies help to constrain the nuclear level density and $\gamma$-ray strength function for a better theoretical description of yet unmeasured cross sections. 
\end{itemize}

The efforts of the theoretical astrophysics community in Canada are also very diverse and support many aspects of the aforementioned measurements. The two main groups covered under the CINP umbrella are located at the University of Guelph and at TRIUMF:
\begin{itemize}
    \item The nuclear (astro)theory group at the University of Guelph aims to provide answers to long-standing overarching questions related to nuclear forces, novel states in nuclei and matter, as well as the behaviour of matter at thermodynamic extremes found in neutron stars, their mergers, accretion disks around black holes, and supernovae. This by will be achieved by employing novel chiral effective field theory ($\chi$EFT) interactions for both medium-mass nuclei and neutron-star matter.
    \item At TRIUMF, the goal of the theory group is to develop a predictive \textit{ab-initio} theory of nuclear structure and nuclear reactions for light and medium mass nuclei. The method primarily used in this project is the no-core shell model with continuum (NCSMC), capable of simultaneous description of bound and unbound states from first principles. A novel many-body approach is the valence-space formulation of the in-medium similarity renormalization group (VS-IMSRG), which can be thought of as an \textit{ab-initio} shell model approach to atoms and nuclei, using electromagnetic and the latest two- and three-nucleon forces, respectively. 
    %, i.e., from accurate nucleon-nucleon and three-nucleon interactions derived within chiral effective field theory ($\chi$EFT). 
    \item TRIUMF in collaboration with nuclear astrophysicists from Los Alamos National Laboratory and the University of Notre Dame in the USA is investigating new "reverse engineering" tools for constraining yet unmeasured nuclear physics parameters from the deduced solar abundances for the r-process. This method will help to guide future experimental campaigns for the measurement of nuclear properties in the neutron-rich "Terra Incognita".
\end{itemize}

Theoretical and experimental nuclear astrophysics efforts in Canada are strongly connected by two Joint Centers, the ''Joint Institute for Nuclear Astrophysics - Center for the Evolution of Elements" (JINA-CEE) in the USA and the Astronomy Research Center (ARC) at the University of Victoria. 

\paragraph{Joint Institute for Nuclear Astrophysics - Center for the Evolution of Elements (JINA-CEE)} \label{sec:NA:jina} (USA) McGill, TRIUMF, Victoria;
{\intl Australia, Brasil, China, Germany, Hungary, Italy, Israel, Japan, Jordan, Netherlands, Sweden, UK, USA} \\
In the USA the \href{https://www.jinaweb.org}{Joint Institute for Nuclear Astrophysics - Center for the Evolution of Elements (JINA-CEE)} is a multi-institutional Physics Frontiers Center funded by the US National Science Foundation. JINA was founded in 1999 as joint venture between the University of Notre Dame, Michigan State University, and the University of Chicago and has been supported from 2002-2020 as a NSF Physics Frontiers Center. The interdisciplinary and multinational JINA-CEE network involves 26 institutions and about 300 scientific participants from all over the world, including about 240 students and postdocs. Participating Canadian institutions include scientists from the University of Victoria, TRIUMF, and McGill University.

JINA-CEE addresses fundamental questions about the cosmos, like \textit{``Where do the elements come from that make up our world?''} and \textit{``What are basic properties of matter when compressed to high density?''}. It has organized many workshops and conferences where Canadian Nuclear Astrophysicists took part and also received (partial) travel support. JINA-CEE has also co-sponsored the \href{https://tsi.triumf.ca/2017/}{TRIUMF Summer Institute about ''Modern Tools in Nuclear Astrophysics"} in 2017 and provided travel support for participating students.

\paragraph{International Research Network for Nuclear Astrophysics (IReNA)} \label{sec:NA:irena} (USA) Victoria; {\intl Canada, Europe, Japan, USA} \\
\href{https://www.irenaweb.org/}{IReNA} is US National Science Foundation AccelNet Network of Networks. It connects six interdisciplinary research networks across 17 countries to foster collaboration, complement and enhance research capabilities in the US and abroad, and thus greatly accelerate progress in science. An important component of IReNA is the training of students and other young researchers in an unique interdisciplinary, collaborative, and international environment. 

IReNA connects this broad range of observations with the extraordinarily broad range of experimental and theoretical nuclear physics studies and advanced computational models needed to truly create new windows into the physics of the universe. 



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\paragraph{Astronomy Research Centre (ARC)}\label{sec:NA:arc} (University of Victoria) NRC Dominion, NRC Herzberg, TRIUMF \\
The \href{https://www.uvic.ca/research/centres/arc/}{Astronomy Research Center (ARC)} was launched in 2015 as a communication platform to increase awareness and opportunities in astronomical research at the University of Victoria. It brings together world-renowned researchers with the expertise to answer many basic questions about our universe. Scientists from the University of Victoria work closely with local researchers from BC at the NRC Herzberg Astronomy \& Astrophysics Research Centre, the NRC Dominion Radio Astronomy Observatory, TRIUMF accelerator laboratory, and with industrial partners across Canada. 

Currently, ARC hosts an NSERC-CREATE training program on New Technologies for Canadian Observatories, and is involved in the CFI-funded GIRMOS instrument being built at NRC-Herzberg for the Gemini-South Observatory. In addition, several members of ARC are involved in a large CFI proposal for the final design phase of a new 11-meter spectroscopic survey telescope, the Maunakea Spectroscopic Explorer.  

ARC has co-hosted the \href{https://tsi.triumf.ca/2017/}{TRIUMF Summer Institute about ''Modern Tools in Nuclear Astrophysics"} in 2017 and provided travel support for participating students. 


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\subsubsection{Creation of nuclei in stars}\label{sec:NA:light}
\paragraph{Light element nucleosynthesis}
Apart from the lightest nuclei (hydrogen, helium, lithium) which are created in Big Bang nucleosynthesis, the vast majority of heavier nuclei are produced in the interior of stars (in so-called ''burning phases") or through explosive stellar events. The mass of a star determines the duration and sequence of the burning phases.  Stars between 0.08 and 8 solar masses ($M_\odot$) can only ignite hydrogen and helium burning and produce nuclei up to $^{12}$C and $^{16}$O, whereas ''massive" stars with $> 8~M_\odot$ continue through advanced burning phases and produce a core of iron-group elements before they end their lives in Core Collapse Supernovae.

In binary systems also explosive burning phases can be triggered. For example, in ''classical novae" hydrogen-rich material from a Red Giant is transferred to the surface of a dense White Dwarf and explosive hydrogen burning reactions ignited. These reactions can be detected as luminosity increase as part of the material is ejected. Another example are X-ray bursters which consist of a neutron star which is accreting hydrogen-rich material from an accompanying donor star. This material forms a dense layer due to the extremely high gravitational field. After hours of accumulation and gravitational compression, nuclear fusion starts and leads to a thermonuclear runaway. Explosive stellar nucleosynthesis is ignited via the hot CNO cycle and continues into the ``rapid proton-capture ($rp$) process''. The $rp$-process proceeds close to the proton-dripline up to mass $A\approx$110. Within seconds most of the accreted material is burned, powering a bright X-ray burst that is observable with X-ray telescopes and carries information about the produced radioactive isotopes in its light curves.

All these scenarios require the accurate knowledge of proton- and $\alpha$-capture reactions on stable and radioactive isotopes within the astrophysically relevant energy range (''Gamow window''). These charged-particle reaction face the challenge of Coulomb penetrability and have thus extremely small cross sections. The reactions are studied in the laboratory via direct measurements at the appropriate energies (where possible), i.e. reactants and products interacting as they do in the astrophysical environment. This requires radioactive beams like the ones produced by the ISAC facility, and the instruments optimized to study the reactions. In cases where the reaction cannot be studied directly, the approach is an indirect study, either via an appropriate nucleon transfer reaction, or extraction of nuclear parameters via scattering observables or decay properties of resonances. 

Three setups at the ISAC facility at TRIUMF, namely the DRAGON  and EMMA recoil separators and the TUDA detector, are well-suited for these measurements. Their broad goal is to effectively measure the cross sections of nuclear reactions using radioactive and stable beams to provide input to global nuclear reaction rate libraries. In this way they help to reduce or eliminate those reactions as sources of uncertainty between astronomical observations and astrophysical models, or elucidate their roles in the nucleosynthesis or energy generation. In addition, studies more focused on nuclear structure allow refinement of modern {\em ab initio} no-core shell model theory calculations, which in turn drive theoretical extrapolations of experimental data to astrophysical energies with improving accuracy. 

This measurement program is in line with the proposed aims of the $\gamma$-ray astronomy community whose recent White Paper to NASA on a new MeV-scale $\gamma$-ray mission \footnote{\href{https://ui.adsabs.harvard.edu/link_gateway/2019BAAS...51c.122F/PUB_PDF}{C. Fryer et al., Bull. Amer. Astron. Soc. 51, Vol. 3, 122 (2019)}} contained motivations for observing characteristic $\gamma$-ray lines from classical novae, as well as core-collapse supernovae.

Recent focus of the Canadian community were measurements of reactions in the region of the endpoint of nova nucleosynthesis around $A\approx$40 since a significant disagreement between abundance predictions and spectroscopic observations of the ejecta was found. From a nuclear-physics point of view, the $^{38}$K$(p,\gamma)^{39}$Ca reaction is one of the main reactions this region for which more experiments are required. Studies exploring the sensitivity of nova nucleosynthesis to reaction rate uncertainties have found that this reaction has a strong impact on the abundances of several elements near $A$= 40 that are potentially observable in the nova ejecta, for example, $^{38}$Ar, $^{39}$K and $^{40}$Ca. This reaction has now been measured at DRAGON and further indirect measurements in this mass region have been carried out and are under investigation, e.g. at the (now closed) Maier-Leibniz Laboratory (MLL) in Garching/ Germany and at Triangle Universities Nuclear Laboratory (TUNL). Further progress on this question must rely on improved comparisons between new spectroscopic observations of ejecta elemental abundances from this mass region, with reliable yield predictions from the models. 

Another focus has been on reactions of producing and destroying radioactive $^{7}$Be, which plays an important role for the solar neutrino production during the proton-proton chains in hydrogen burning. Recent studies of the $\alpha(^{3}He,^{3}He)\alpha$ reactions at DRAGON with the scattering chamber SONIK have provided insights into the $^{3}$He($\alpha,\gamma$)$^{7}$Be reaction. The $^{7}$Be$(\alpha,\gamma)^{11}$C reaction was also investigated since it was found to play an important role in the late phase of the $\nu$p-process in core-collapse supernovae, affecting the abundances of $A$= 100--110 nuclei by producing intermediate-mass nuclei that remove protons from the environment (proton ''poisons"). Upcoming studies will continue this program with $^{7}$Be + p and $^{7}$Be + $\alpha$ elastic scattering, and in the longer term with e.g. the first precise inverse kinematics measurement of the important $^{7}$Be$(p,\gamma)^{8}$B reaction.


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\paragraph{Heavy element nucleosynthesis}\label{sec:NA:heavy}
Nuclei beyond iron up to uranium are produced by different reactions mechanisms since nuclear fusion cannot generate energy anymore for heavier nuclei. The main processes driving the nucleosynthesis here are neutron capture processes in different astrophysical scenarios. They are distinguished by the timescale of neutron capture relative to $\beta$-decay: in the ''slow" neutron capture (s) process the neutron capture is slow compared to the $\beta$-decay, thus the reaction path runs along the line of stability. The reaction flow of the ''rapid" neutron capture (r) process proceeds far off stability through short-lived, neutron-rich nuclei since the neutron-capture rate is much higher than the $\beta$-decay rate. And recently, a third neutron capture process has gained a lot of attraction since it can nicely explain certain abundance pattern in stars, the so-called ''intermediate" neutron capture (i) process. 

For the remaining 1\%, notably 32--35 neutron-deficient, stable nuclei with low elemental abundances, a superposition of charged-particle and photon-induced reaction mechanisms is summarized under the term ''p processes".

All of three neutron capture processes are located on the neutron-rich side of the chart of nuclides and create the vast majority ($\approx$99\%) of the amount of stable nuclei heavier than iron. One of the most important missing puzzle piece for all of these three neutron capture processes are experimental neutron capture cross sections on short-lived radionuclides. Whereas the cross sections of the vast majority of stable nuclei have been measured in the last decades and agree well with the statistical Hauser-Feshbach theory (within a factor of 2-3 at stability), this theory unfortunately cannot be reliably applied for the lightest nuclei and at neutron shell closures since the level density becomes too low. Unfortunately, the nuclei at these neutron shell closures are the most important ones in astrophysical terms since they are responsible for the formation of the observed peaks in the abundance distributions. If one moves away from stability towards more neutron-rich nuclei, the predicted rate uncertainty increases to a factor of more than 100 which would lead to large deviations in the predicted abundance. To circumvent these problems, neutron capture cross sections have to be measured for neutron-rich key nuclei. 



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\paragraph{The slow neutron-capture process} \label{sec:NA:s}

The Canadian research around the $s$ process is focused around the investigation of charged-particle reactions that influence the neutron economy. One highlight was the indirect determination of the strength of the key E$_{cm}$ = 706~ keV resonance in $^{22}$Ne$(\alpha, n)^{25}$Mg, the main neutron source for the weak $s$-process \footnote{\href{https://doi.org/10.1016/j.physletb.2020.135256}{S. Ota et al., Phys. Lett. B 802, 135256 (2020)}}. In an experiment at the Texas A\&M Cyclotron Institute (see Sec.~\ref{sec:intfac:texas}) the resonance strength was determined indirectly by measuring neutron/$\gamma$-ray decay branching ratios, following selective population of the resonance in the inverse-kinematics $^{22}$Ne$(^{6}Li,d)^{26}$Mg reaction. The extracted strength was around a factor 3 lower than previous measurements which leads to a factor $\approx$10--100 decrease in the production of A$>$100 nuclides.

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\paragraph{The rapid neutron-capture process} \label{sec:NA:r}
It is not surprising that the focus of nuclear structure and astrophysics research has shifted in recent years towards the investigation of neutron-rich nuclei since the discovery potential in this ``Terra Incognita'' is very high. The $r$-process occurs in explosive astrophysical environments such as core collapse supernovae and neutron star mergers. Since its reaction path lies far from stability, many experimental campaigns world-wide are ongoing to push the known limits of neutron-rich nuclei, especially since up to $\approx$4000 more nuclei are predicted to exist and a large fraction of them will become accessible for the first time with the next generation of RIB facilities \footnote{\href{https://iopscience.iop.org/article/10.1088/1361-6471/ab0849}{C.J. Horowitz et al., J. Phys. G: Nucl. Part. Physics 46, 083001 (2019)}}.

However, not all of these nuclei need to be studied for a better understanding of $r$-process nucleosynthesis. Sensitivity studies for different nuclear physics parameters such as masses, $\beta$-decay half-lives, neutron-capture rates, and $\beta$-delayed neutron emission probabilities have identified key regions of importance \footnote{\href{http://dx.doi.org/10.1016/j.ppnp.2015.09.001}{M.R. Mumpower et al., Progr. Part. Nucl. Phys. 86, 86 (2016)}}. These regions are located around the $N$=50, 82, and 126 shell closures, and in the Rare Earth Element region around mass $A\approx$160. The neutron shell closures are connected to short-lived $r$-process nuclei responsible for the formation of the observed elemental solar abundance peaks at $A$=80, 130, and 195. In contrast, the Rare Earth Element region is connected to nuclei with half-filled neutron- and proton orbitals that are highly deformed and produce an $r$-process mini-abundance peak that is responsible for the production of the stable nuclei between mass $A$=160--170. 
%$^{160}$Gd, $^{161-164}$Dy, $^{165}$Ho, and $^{166}$Er. 

The lightcurve of the kilonova following the neutron star merger event that triggered GW170817 (see Sec.~\ref{sec:intro:NA}) showed two distinctive patterns: a fast, blue component high in iron-group elements and light $r$-process material up to A$\approx$140, and a longer-lasting red component rich in lanthanides \footnote{\href{https://www.nature.com/articles/nature24453}{D. Kasen et al., Nature 551, 80 (2017)}}.  The red colour of the kilonova light curve has been interpreted to originate from the decay of heavy ($A>$140) $r$-process material, like lanthanides ($Z$=58-71, about 1-10\% of the ejected mass).
%which have densely spaced f-shell electrons that are responsible for the much higher opacity of the ejected material. This leads to longer diffusion times and longer-lasting light curves (weeks instead of days), and the lanthanides obscure the optical bands, so that the observed light curve appears in the infrared. 
This was the first direct observation of decaying lanthanides in a stellar light curve and confirmed the production in an $r$-process event during neutron star mergers. 

%From the nuclear structure perspective, the region around the mid-shell nucleus $^{170}$Dy ($Z$=66, $N$=104) is dominated by deformed nuclei. 
There is no question that the Rare Earth Element mini-peak around $A\approx$160 is formed when the $r$-process flow is hindered by nuclear structure effects in this deformed region around the neutron-midshell closure ($N$=104). However, which nuclear properties (half-lives, low-lying isomers, masses, etc.) play a key role here remains an open question due to missing experimental information.

One intermediate step to better constrain yet unknown nuclear properties like masses is the so-called ''reverse-engineering" method (Sec.~\ref{sec:NA:reverse} and Fig.~\ref{Fig:NA-Comp}). In the case of the $r$-process the distinct feature of the Rare Earth Peak has been identified as perfect tool to try to extract yet-unmeasured masses, like e.g. with the Canadian Penning Trap at Argonne National Laboratory in the USA (see Sec.~\ref{sec:NA:CPT}).


The ISAC facility at present and with the cleaner and more intense beams from ARIEL in future is ideally suited to play a key role in the exploration of hard-to-access regions. Mass measurements with the TITAN facility and decay spectroscopy with GRIFFIN and its auxiliary detectors have allowed to increase our knowledge to better understand and constrain the nuclear physics input of r-process nucleosynthesis. Offshore experiments like the campaign to measure half-lives and $\beta$-delayed neutron emission probabilities of the most neutron-rich nuclei with the BRIKEN array in Japan (see Sec.~\ref{sec:NA:briken}) have complemented this knowledge. 

A new addition to the Canadian program are indirect measurements of neutron capture cross sections via $(d,p)$ reactions, for example with the EMMA recoil separator coupled to TIGRESS and its auxiliary detectors. This will allow for the first time to better constrain another important nuclear physics input parameter which had so far to rely entirely on theoretical models.

The new results from PREX-II and CREX at JLab in the USA on the neutron-skin thickness will allow indirectly to constrain the density-dependence of the symmetry energy of neutron-rich nuclear matter. These results are highly anticipated in the astrophysics community due to their implications on the equation of state (EOS) and thus for the structure of neutron stars and supernovae explosions.

Experiments are always guided by theoretical predictions but in turn also help to benchmark models to make them more reliable for extrapolations into the yet-unknown regions. Going to even more neutron-rich, shorter-lived nuclei will be possible in the next decade with the new generation of radioactive beam facilities like e.g. ARIEL at TRIUMF, FRIB in the USA, and FAIR in Germany. Although not all participating neutron-rich nuclei will be accessible and should be investigated, the focus will be on the aforementioned key regions which will allow a more complete description of r-process nucleosynthesis. 

%Radiative neutron-capture cross sections play an important role in determining the final abundance distribution of elements produced in the $r$-process. As an example, the observed abundance peaks at masses $A=130$ (2nd r-process peak) and $A=195$ (3rd r-process peak) are caused by low neutron captures for nuclei with neutron numbers $N=82$ and $N=126$ due to nuclear shell effects. However, theoretical neutron-capture reaction rates vary by a factor 10-100 and higher, obscuring all but the largest abundance pattern features and thus limiting the ability to determine the exact astrophysical conditions of the site(s) responsible for heavy element nucleosynthesis. Only with neutron capture cross sections determined with uncertainties smaller than a factor of 10 can fine features be observed and used as diagnostic tool in reverse-engineering astrophysical conditions.  

%In addition, if supernovae contribute to the lighter $r$-process elements (masses $(A \sim 130)$ and/or these lighter elements can also be produced in the “hot” wind of a neutron-star merger, there will be a point where the nucleosynthesis undergoes freeze-out and falls out of (n, $\gamma$)–( $\gamma$,n) equilibrium. The remaining neutrons, as well as the $\beta$-delayed neutrons, could be captured on the moderately neutron-rich nuclei present at the time of freeze-out, and these final neutron captures could significantly alter the final elemental distribution. Neutron capture rates are also the missing nuclear physics ingredient required to determine whether or not an additional neutron capture process, the so called intermediate (i) process, is present in the universe. 


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\paragraph{The intermediate neutron-capture process} \label{sec:NA:i}

A substantial fraction of so-called carbon-enhanced metal-poor (CEMP) stars have abundance distributions that cannot be described by the $s$- or $r$-processes, or by a combination of the two. The CEMP stars are ``fossil” low-metallicity stars that do not yet carry the signatures of many cycles of nucleosynthesis and thus provide a unique insight into the fundamental building blocks of stellar nucleosynthesis. Recent astrophysical models and observations implied that a third neutron-capture process with ''intermediate" neutron densities (10$^{14}$-10$^{16}$ $n$/cm$^3$) is responsible for these abundances. Its reaction flow proceeds through neutron-rich nuclei 2--6 neutrons away from stability where the majority of nuclear properties (masses, half-lives, $\beta$-delayed neutron emissions) are known experimentally, except for neutron capture cross sections.

The $i$ process was first described in 1977 to occur in He-shell flashes in thermally-pulsing asymptotic giant branch (TP-AGB) and post-AGB stars \footnote{\href{http://articles.adsabs.harvard.edu/pdf/1977ApJ...212..149C}{J.J.Cowan et al., Astrophys. J. 212, 149 (1977)}}. Thermal convection carries hydrogen from the surrounding hydrogen-rich envelope into the helium burning shell and triggers the production of neutrons with the reaction sequence $^{12}$C$(p,\gamma)^{13}$N$(e^+ \nu)^{13}$C$(\alpha,n)^{16}$O. Another possible astrophysical scenario are rapidly accreting white dwarfs in close binary systems \footnote{\href{https://doi.org/10.1093/mnras/stz1921}{P.A. Denissenkov et al., Monthly Not. Royal Astron. Soc. 488, 4258 (2019)}}.

%Recent astrophysical models and observations imply that this third neutron capture process has "intermediate" neutron densities (10$^{14}$-10$^{16}$ $n$/cm$^3$, compared to 10$^{7}$-10$^{10}$ $n$/cm$^3$ in the $s$-process and $>$10$^{20}$ $n$/cm$^3$ in the $r$-process). 

The main nuclear physics uncertainties in the $i$ process comes from unmeasured neutron-capture cross sections of the participating radioactive nuclei close to stability. Direct neutron capture measurements on short-lived nuclei are not yet possible as the radioactive targets cannot be produced in the required $\mu$g quantities (corresponding to $\approx$ 10$^{16}$ atoms) and are hard to handle due to their high activity.

A method to overcome this lack of direct neutron capture data is through $(d,p)$ neutron-transfer reactions with accelerated radioactive beams using for example EMMA at TRIUMF-ISAC (see Sec.~\ref{sec:NA:emma}). In collaboration with colleagues from Norway, South Africa, and the USA a new complementary program has been initiated to study nuclei relevant to the $r$- and $i$-process at TRIUMF and other laboratories, to extract important nuclear physics parameters like nuclear level densities and $\gamma$-ray strength functions with the so-called \emph{''Oslo-method"}. These two quantities are the main sources of uncertainty in the calculation of neutron-capture cross sections within the statistical Hauser-Feshbach model. Without any experimental constraints on these parameters, theoretical predictions for cross sections of neutron-rich nuclei may vary by orders of magnitude. The proposed measurements with EMMA expect to reduce that uncertainty to a factor of 2--3.

As a first step, the $^{142}$Cs$(d,p)^{143}$Cs reaction will be studied at TRIUMF to infer the $(n,\gamma)$ cross section indirectly. These results will be compared with a decay experiment at Argonne National Laboratory that will constrain the neutron capture into $^{143}$Cs via the ''$\beta$-Oslo method" using the SuN total absorption spectrometer. The combined data will offer the first direct comparison of reaction-based and $\beta$-decay techniques for constraining neutron capture rates experimentally.

Recent $i$-process studies have identified four bottleneck reactions which act at different neutron densities and affect the abundance of a different element. The $^{135}$I(n,$\gamma$)$^{136}$I reaction is important at neutron densities of order 10$^{14}$ n/cm$^3$ and affects the abundance distribution of Ba. At lower densities (10$^{13}$ n/cm$^3$) the Ba abundance is affected by the $^{137}$Cs(n,$\gamma$)$^{138}$Cs reaction. On the other hand, the La abundance at low neutron densities is correlated with the $^{139}$Ba(n,$\gamma$)$^{140}$Ba reaction, while at high neutron densities the main bottleneck is the $^{139}$Cs(n,$\gamma$)$^{140}$Cs. Accurate experimental knowledge of all four reaction rates will provide strong constraints on the predicted abundances, and in turn compare to astronomical observations and deduce the neutron densities involved in the $i$ process.

The $^{135}$I(n,$\gamma$)$^{136}$I reaction will be studied at Argonne National Laboratory using the $\beta$-Oslo technique and the SuN detector. The remaining three reactions will be studied at ISAC-II in 2021/22 in transfer reactions with EMMA and TIGRESS and open the door to a larger follow-up campaign towards neutron-capture cross section measurements of shorter-lived nuclei for the $r$-process.





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\subsubsection{Neutron star physics and the equation of state}\label{sec:NA:EOS}
With a radius of roughly 12~km and densities of up to twice the nuclear matter density, neutron stars are the smallest and densest objects in the universe. The behavior of matter under such extreme conditions is governed by quantum chromodynamics (QCD). Neutron stars probe the low temperature and high density region of the nuclear phase diagram and offer a unique opportunity to test and explore the richness of QCD in a regime that is beyond the reach of terrestrial experiments. While laboratory experiments at colliders like RHIC and the LHC explore the high-temperature/ low-density regime, astrophysical studies probe the opposite, low-temperature/high-density regime. 

One of the central questions in the astrophysics of neutron stars is the relation between the pressure and the density, the so-called equation of state (EOS, $P\propto\rho^\alpha$). In contrast to the EOS of white dwarfs which can be described with special relativity, the neutron star EOS needs to consider increased effects from general relativity. When translated into a mass-radius relation, neutron star mass measurements can help constraining these predictions and make better predictions of the behaviour of neutron star matter. With a ''stiffer" EOS ($\alpha\geq$5/3) a larger radius is derived for the same mass and thus a higher maximum neutron star mass. 

%The conservative Tolman-Oppenheimer-Volkoff (TOV) limit (analogous to the Chandrasekhar limit for White Dwarf stars) lies in the range $1.5 - 3.0 M_{\odot}$. The uncertainty in the value reflects the fact that the EOS for extremely dense matter is not well constrained. In a neutron star below the TOV limit, the weight of the star is balanced and the collapse prevented by the strong force and the quantum degeneracy pressure of neutrons. If the mass of the neutron star is above the limit, the star will collapse to some denser form like a Black Hole or via a quark-hadron phase transition into a Quark Star. Since the latter is still hypothetical, it is assumed that neutron stars above the TOV limit collapse directly into a Black Hole.

The ``neutron skin'' is defined as the difference in the root-mean-square neutron and proton distributions of a nucleus, $R_n- R_p$. Precise measurements of the neutron skin thickness in neutron-rich nuclei will help to constrain the density-dependence of the symmetry energy of neutron-rich nuclear matter, which has for example implications on the structure of neutron stars and supernovae explosions.

A model-independent measurement of the neutron radius is carried out at Jefferson Lab via extraction of the parity-violating asymmetry in the elastic scattering of polarized electrons from $^{48}$Ca (CREX) and $^{208}$Pb (PREX-II), see Sec.~\ref{sec:NA:prex-crex}. The PREX-II measurement has confirmed the large central value of the PREX I measurement (0.33$^{+0.16}_{-0.18}$ fm), with a preliminary neutron skin of 0.278 $\pm$0.078 fm. These results are in modest tension (just over 1$\sigma$) with the LIGO results for neutron star tidal polarizability. 

A precise determination of the neutron skin in $^{208}$Pb provides a constraint on the density-dependence of the symmetry energy, in a regime where extrapolation to other nuclear species and up to the scale of bulk nuclear matter (i.e. neutron stars) are possible. However, this extrapolation requires model input to describe the variation of the symmetry energy with the size and atomic mass number of the nucleus (see Fig.~\ref{Fig:NA:pb208}). 

A larger neutron skin in $^{208}$Pb, $R_n$($^{208}$Pb), implies a stiffer EOS with a larger pressure that correlates to a larger radius of a neutron star $r_{NS}$. Recently there has been great progress in deducing $r_{NS}$ from the spectrum and intensity of the X-rays emitted from neutron stars, with model-dependent corrections for the properties of the atmosphere. 

%%%%%%%%%%%%%%%%%%
\begin{figure}[!htb]
\centering
\includegraphics[width=0.5\columnwidth]{Figs/Pb208Densities.pdf}
\caption{ \label{Fig:NA:pb208} 
Predictions of neutron and proton densities in $^{208}$Pb with differing values for the neutron-skin thickness. The inset shows the running sum of neutrons minus protons, indicating how models with larger values for the symmetry pressure $L$ are more effective in pushing the 44 excess neutrons to the surface [J.Piekarewicz et al., Physics Today 72, 7, 30 (2019)]. The preliminary PREX II measurement of 0.278$\pm$0.078~fm is consistent with a larger symmetry pressure, L. Figure courtesy of J. Piekarewicz (Florida State University).}
\end{figure}
%%%%%%%%%%%%%%%%%%


However, from observations of X-ray bursts from three ``ideal'' neutron stars, some theories extract a very small radius of $\approx$ 10~km, implying that the EOS softens at high density and suggesting a transition to an exotic phase in the QCD phase diagram. Other theories using the same three neutron stars plus six more conclude that the radius is nearly 12~km, leading to a prediction of the neutron skin thickness $R_n$($^{208}$Pb) -- $R_p$($^{208}$Pb) = 0.15(2)~fm. This implies a stiffer EOS which leaves little room for softening due to a phase transition at high density.

Models also claim that the neutron skin thickness of $^{208}$Pb is tightly correlated with the dipole polarizability $\alpha_D$ and have used this to infer a new neutron skin thickness for $^{208}$Pb. However, recent works have now shown that the correlations are model-dependent which emphasizes the importance of more model-independent electroweak measurements of $R_n$, like in the CREX and PREX-II experiments. Both experiments will provide crucial information about the neutron matter equation of state (EOS) at nuclear densities, and this data will provide a bridge between Ab-Initio calculations of medium-mass nuclei and Density Functional Theory (DFT) calculations of heavy nuclei. 

The EOS of neutron stars is important for the Laser Interferometer Gravitational-Wave Observatory (LIGO) searching for signals from inspiraling neutron stars. Not only does the expected number of neutron stars observed depend upon how fast the stars cool but also the properties of the gravity waves. 
%\textbf{(Reference to work of Liliana: Theoretical astrophysics calculations at the University of Guelph...}



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsubsection{Decays of nuclides under stellar conditions}\label{sec:NA:HCI}
Hot, dense astrophysical environments cause nuclei to be fully or highly ionized and surrounded by a cloud of free electrons (plasma). These ''stellar" conditions are very different to the terrestrial conditions in the laboratory and can have a large influence on reactions (due to electron screening effects) and even decay modes (due to excited or highly-ionized states). 

Some common modes of electroweak decay, such as orbital electron capture (EC), internal conversion (IC), and bound-state $\beta$-decay ($\beta_b$) proceed through an interaction between the nucleus and bound electrons within the constituent atom. As a result, the probabilities of the respective decays are not only influenced by the structure of the initial and final states in the nucleus, but can also depend strongly on the atomic charge state \footnote{\href{http:\\stacks.iop.org/RoPP/74/016301}{Y. A. Litvinov and F. Bosch, Rep. Progr. Phys. 74, 016301 (2011).}}. 
%These effects become increasingly more significant as the atom is ionized closer to the electron shell with the largest spatial overlap with the nucleus (the atomic K-shell). 

Conditions suitable for the partial or complete ionization of nuclides can be generated artificially to selectively block or accelerate certain decay modes and thus simulate ``stellar" conditions in the laboratory. The Canadian community is involved in two experiments that investigate different aspects of stellar decay modes. 

In stellar environments, nuclear excitation by photoabsorption as well as via coupling to the atomic shells is possible, populating higher excited nuclear states. The two nuclear excitation mechanisms involving atomic electrons are ``nuclear excitation by electron capture" (NEEC) and ``nuclear excitation by electron transition" (NEET). The probability of NEET/NEEC is generally small compared with competing processes but can potentially lead to deviations from the formation paths for heavy element production processes. 

When vacancies in the atomic orbitals are filled by electrons moving from higher-lying to lower-lying orbits, this energy can be emitted via X-rays or transferred to Auger electrons in outer shells. The third possibility is the NEET process that occurs by simultaneous excitation of the nucleus during an atomic transition, provided the energies of the two transitions match. 

In the NEEC process a free electron is captured into a highly-charged ion with the simultaneous excitation of the nucleus, this is the time-reversal process of internal conversion (IC). The NEEC process requires that the kinetic energy of the captured free electron and its binding energy match the transition energy between two nuclear states. If the decay of long-lived nuclear isomeric states could be selectively triggered by NEEC, this method of ''energy storage in isomers" has interesting implications for a next-generation of nuclear batteries. 

Several nuclides with suitable transitions have been identified, and in 2018 claims of the first direct observation of the NEEC process were made in the depletion of the $^{93m}$Mo isomer \footnote{\href{https://www.nature.com/articles/nature25483}{C.J. Chiara et al., Nature 554, 216 (2018)}}. However, this was immediately followed by an intense discussion since theoretical calculations disagreed with the experimental data by $\approx$9 orders of magnitude, in clear conflict with the conclusion that NEEC was the excitation mechanism behind the observed depletion rate of $^{93m}$Mo \footnote{\href{https://doi.org/10.1103/PhysRevLett.122.212501}{Y. Wu et al., Phys. Rev. Lett. 122, 212501 (2020)}}.  

At the TITAN facility at TRIUMF-ISAC the electron beam ion trap (EBIT, see Sec.~\ref{sec:NA:titan}) will be surrounded by up to seven photon detectors for access to in-trap decay spectroscopy of highly-charged ions. With this setup decay processes involving highly-charged ions will be investigated. In the past years the TITAN-EBIT setup has been upgraded, for example with the inclusion of a new Ultra-Low-Energy High Purity Germanium (ULE-HPGe) detector for the detection of low-energy X-ray and $\gamma$-ray transitions, as well as in collaboration with Simon Fraser University with up to five HPGe detectors from the former 8$\pi$ setup. For 2021/22 the controlled measurement of the NEEC process in $^{129m}$Sb is envisioned, and if successful this would be the first low-energy direct measurement.


%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%
A different approach towards the investigation of decay processes of highly-charged ions has been developed at the GSI Helmholtz Center for Heavy Ion Research in Germany, and is now used by the \texttt{ILIMA} (Isomers, Lifetimes, and MAsses) collaboration. This experimental program is based on the combination of the in-flight fragment separator and a system of ion storage rings that allows access to  unique possibilities for the investigation of exotic decay modes, as well as the measurement of masses and half-lives of stored ions. The new Collector Ring (CR) at the Facility for Antiproton and Ion Research (FAIR) will extend this research program in the next decade.

In conjunction with resonant Schottky pickups which have single-ion sensitivity, particle detectors installed in dedicated pocket positions in the storage ring lattice, off-axis from the stored ions and separated from the ultra-high vacuum by a thin stainless steel window, can be used for identification of decay products outside of the ring acceptance as well as additional beam diagnostics. 

A detector prototype, \texttt{CsISiPHOS} (CsI-Silicon Particle detector for Heavy ions Orbiting in Storage rings \footnote{\href{https://doi.org/10.1016/J.NIMA.2016.08.040}{M.A. Najafi, et al., Nucl. Instr. Meth. A836, 1 (2016)}} (see Fig.~\ref{Fig:NA:bound}), has been constructed and commissioned by the German-Canadian collaboration, and used in spring 2020 in the Experimental Storage Ring (ESR) at GSI Darmstadt to measure the half-life of the bound-state $\beta$-decay nucleus $^{205}$Tl$^{81+}$. 

The bound-state $\beta$-decay is the time-mirrored analogue of the orbital electron capture process and occurs in fully ionized nuclei with very low $Q_\beta$ value when the $\beta$ electron is not emitted into the continuum but into bound states in the K and L orbitals. Whereas the half-life of the neutral atom in its ground-state can very long-lived ($^{187}$Re) or even stable ($^{163}$Dy or $^{205}$Tl, see Fig.~\ref{Fig:NA:bound}), removal of all electrons allows for the $\beta$-decay to be accelerated by several orders of magnitude since it can now proceed by ''less hindered" transitions to excited states.

%%%%%%%%%%%%%%%%%%
\begin{figure}[htb!]
\centering
\includegraphics[width=0.8\columnwidth]{Figs/NA_bound.png}
\caption{\label{Fig:NA:bound} (left) Picture of the prototype detector \texttt{CsISiPHOS} that is presently installed at the Experimental Storage Ring (ESR) at GSI Darmstadt. (Right) The bound-state $\beta$-decay transforms stable $^{205}$Tl$^{0+}$ into a radioactive nucleus $^{205}$Tl$^{81+}$ by removing all electrons. The decay can proceed faster to the first excited state in $^{205}$Pb via a first forbidden transition.
%
}
\end{figure}
%%%%%%%%%%%%%%%%%%

Motivation for the measurement of the decay of $^{205}$Tl$^{81+}$ was the LOREX (LORandite EXperiment) solar neutrino experiment \footnote{\href{https://doi.org/10.1063/1.57378}{K.M. Subotic et al., AIP Conf. Proc. 455, 912 (1998)}},\footnote{\href{https://doi.org/10.1016/j.nima.2018.03.039}{M.K. Pavicevic, Nucl. Inst. Meth. A 895 (2018) 62}}, which is one of the biggest geochemical activation experiments on Earth and designed to measure the solar neutrino flux integrated over the last few million years. LOREX uses lorandite, a Tl-bearing mineral (TlAsS$_2$) from the Allchar mine (Macedonia) and measures the $^{205}$Pb content produced from neutrino capture on $^{205}$Tl. The solar proton–proton neutrino flux can be measured over the period of 4.3 million years from the reaction $^{205}$Tl + $\nu_e~\longrightarrow~^{205}$Pb + $e^-$ with a very low threshold of 52~keV, the lowest of any known neutrino-induced nuclear reaction. The measurement of the bound-state $\beta$-decay probability of $^{205}$Tl$^{81+}$ (see Fig.~\ref{Fig:NA:bound}) is one important, yet unknown input parameter for the determination of the ``true" mean solar pp-neutrino flux. Theoretical calculations estimate this half-life to be in the order of $t_{1/2}\approx$120~days. 

The latest ESR experiment has now, for the first time, measured this half-life. This data is presently analyzed by students at GSI and at TRIUMF. In parallel, NSERC funding for a second in-ring particle detector has been 
received, and the construction is ongoing (see Sec.~\ref{sec:nucastro:esr})









%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsubsection{Theoretical nuclear astrophysics efforts}\label{sec:NA:theory}
The theoretical nuclear (astro)physics efforts in Canada are focused in the two groups at TRIUMF and at the University of Guelph. With now two principal investigators the nuclear theory group in Guelph is starting to form a hub, aiming to provide answers to overarching questions related to nuclear forces, novel states in nuclei and matter, as well as the behaviour of matter at thermodynamic extremes found in neutron stars, their mergers, accretion disks around black holes, and supernovae. At TRIUMF, the focus is on {\em ab-initio} methods for light and medium-mass nuclei...

%These groups have developed theoretical descriptions for light- and medium-mass nuclei as systems of nucleons interacting by forces via the fundamental theory of strong interactions (QCD). 
A detailed description of their methods can be found in Secs.~\ref{sec:nucstruc:theory} and \ref{sec:Theory}. In this section applications of these methods for nuclear astrophysics are described.

%A new effort in theoretical nuclear astrophysics is the constraining of yet unknown nuclear physics parameters from astronomical observations, like the solar abundance curve. This so-called ``reverse engineering'' is performed in collaboration with theoretical astrophysicists in the USA.

%%%%%%%%%%%%%%%%%%%%
\paragraph{Astrophysical Signatures} \label{sec:NA:Liliana} Guelph, TRIUMF; {\intl Colombia, USA}\\
A detailed description of the work can be found in Sec.~\ref{sec:theory:guelph}.
The aim is to build a consistent theoretical framework with key physics from nuclear physics, neutrino reactions, and gravity to predict multi-messenger observations, for example elemental abundances and neutrino fluxes. This can be achieved with a coherent understanding of the different emission channels of compact objects like Neutron Stars and Black Holes, their mergers and accretion into them. This is research is timely, given the recent successful debut of the multi-messenger era triggered by GW170817 (see Sec.~\ref{sec:intro:NA}).

These results from these astrophysical calculations will help constrain nuclear interactions and the nuclear matter EOS, offer limits on the detection of neutrinos in the Galaxy and at cosmological distances, and provide input to numerical relativity simulations and experimental searches. 

The research is building on previous work, e.g. for the role of the black hole spin on the observer's inclination for distant black hole accretion disks \footnote{\href{https://doi.org/10.1103/PhysRevD.93.123015}{O.L. Caballero et
al, Phys. Rev. D 93, 123015 (2016)}} and detection of the relic neutrino background from
accretion disks around black holes \footnote{\href{https://doi.org/10.1103/PhysRevD.100.043008}{T.S.H. Schilbach et al., Phys. Rev. D 100, 043008 (2019)}}.

For accreting neutron stars the group has made progress on the understanding of the impact of
degeneracy on neutron capture rates in accreting neutron stars \footnote{\href{https://doi.org/10.3390/universe5010036}{B. Knight and L. Caballero, Universe
5(1), 36 (2019)}}. The research in the next years aims to study neutrino emission properties in curved space-time, to quantify directionality effects of relativistic neutrino absorption on the proton-fraction of outflows and $r$-process nucleosynthesis, as well as to study the effect of gravity on neutrino oscillations for accretion disks and mergers.



%%%%%%%%%%%%%%%%%%%%
\paragraph{Nuclear Many-Body Problem} \label{sec:NA:Alex} Guelph, TRIUMF; {\intl Germany, USA}\\
A detailed description of the work can be found in Sec.~\ref{sec:theory:guelph}.

This research studies the connections between microscopic nuclear interactions and strongly interacting nuclear systems appearing in terrestrial laboratories and in astrophysical settings. Improved formulations of nuclear forces are used to understand the physics of neutron star crusts and cores, neutron-rich nuclei, as well as phenomena at the interface of nuclear physics and ultracold atoms.
The research employs chiral Effective Field Theory ($\chi$EFT) interactions, which are then used in different few- and many-nucleon frameworks. The work on nucleonic matter studies is complementary to the research carried out at the TRIUMF Theory Group.

Recent research results include the use of local chiral EFT in light nuclei and neutron matter \footnote{\href{https://doi.org/10.1103/PhysRevLett.116.062501}{J. Lynn et al, Phys. Rev. Lett. 116, 062501 (2016)}}, novel developments in first-principles or mean-field techniques \footnote{\href{https://doi.org/10.1103/PhysRevC.99.014321}{E. Rrapaj
et al, Phys. Rev. C 99, 014321 (2019)}}, and the use of ab-initio methods to constrain selected aspects of more phenomenological approaches \footnote{\href{https://doi.org/10.1103/PhysRevLett.116.152501}{M. Buraczynski and A. Gezerlis, Phys. Rev. Lett. 116, 152501 (2016)}} ~\footnote{\href{https://doi.org/10.1103/PhysRevLett.122.152701}{M. Buraczynski et al., Phys. Rev. Lett. 122, 152701 (2019)}}.

In the upcoming years, the research program will employ a mixture of pionless and chiral EFT interactions, applied to light or medium-mass nuclei as well as nucleonic matter in the context of quantum Monte Carlo simulations. On the phenomenological
front, a quasi-3D Skyrme-Hartree-Fock self-consistent solver is being finalized, which has been already applied to the problem of the static response of neutron matter. In addition, the future research will employ neural networks to understand the approach to the thermodynamic limit and to produce improved variational wave functions for strongly correlated systems.



%%%%%%%%%%%%%%%%%%%%
\paragraph{Ab-Initio Methods for light and medium-mass nuclei: NCSMC} \label{sec:NA:Petr} TRIUMF; {\intl USA, Germany}\\
A detailed description of the work can be found in Sec.~\ref{sec:theory:abinitio}. The implications of this work for Nuclear Structure can be found in Sec.~\ref{sec:nucstruc:theory}.

%Atomic nuclei are quantum many-body systems made by protons and neutrons interacting with forces that originate from the fundamental theory of Quantum ChromoDynamics (QCD). 
The exact treatment of nuclei starting from the constituent nucleons and the inter-nucleon interactions among them has been a long-standing goal in nuclear physics. The goal of this research is to develop a predictive {\em ab-initio} theory of nuclear structure and nuclear reactions for light and medium mass nuclei. Such a theory is needed for the understanding of exotic nuclei investigated at rare isotope facilities, for nuclear reactions important for astrophysics, for fusion reactions for the future energy generation as well as for the testing of fundamental symmetries in nuclear processes.

In addition, an accurate many-body theory for light and medium mass nuclei provides a feedback about the quality of the inter-nucleon interactions, e.g. those derived from the QCD-based chiral effective field theory ($\chi$EFT), used in the calculations and ultimately helps to improve our knowledge of the nucleon-nucleon (NN) interaction, and in particular of the still-not-completely-understood three-nucleon (3N) interaction.

%The {\em ab-initio} nuclear structure approach is capable of describing bound states of nuclei with a microscopic cluster method developed to describe reactions of nuclear clusters. The resulting method provides a unified description of both bound and unbound nuclear states starting from inter-nucleon interactions among protons and neutrons. The only input for these calculations are nucleon-nucleon and three-nucleon interactions derived within the $\chi$EFT. 
The method primarily used in this project is the no-core shell model with continuum (NCSMC). For example the $^{3}$He$(\alpha,\gamma)^7$Be radiative capture, one of the key reactions for Big-Bang
nucleosynthesis and the solar proton-proton chain has been described successfully with this method \footnote{\href{https://doi.org/10.1140/epja/i2018-12486-5}{J. Dohet-Eralyet al., Phys. Lett. B 757, 430 (2016)}}.

The medium-term goals include investigations of reactions important for astrophysics, including radiative capture reactions such as $(n,\gamma), (p,\gamma), (\alpha,\gamma)$ as well as charge-exchange and transfer reactions involving $\alpha$-particles. Prominent examples for future research are $^8$Be($\alpha,\gamma$)$^{12}$C and $^{12}$C($\alpha,\gamma$)$^{16}$O for helium-burning processes, and reactions like $^{14}$N($n,p$)$^{14}$C and $^{13}$C($\alpha,n$)$^{16}$O that are relevant for the neutron production and destruction in the intermediate ($i$) and slow neutron capture ($s$) processes.



%%%%%%%%%%%%%%%%%%%%%%
\paragraph{Ab-Initio Methods for heavy nuclei: VS-IMSRG} \label{sec:NA:Jason} TRIUMF; {\intl USA, Germany}\\
A detailed description of the work can be found in Sec.~\ref{sec:theory:abinitio}. The implications of this work for Nuclear Structure can be found in Sec.~\ref{sec:nucstruc:theory}.

The central focus of this project is to quickly advance a novel many-body approach, the valence-space formulation of the in-medium similarity renormalization group (VS-IMSRG), which can be thought of as an {\em ab-initio} shell model approach to atoms and nuclei, using the latest two (NN) and three nucleon forces (3N) developed by the community. 

The VS-IMSRG method allows to calculate properties of light to heavy nuclei starting from only
input nuclear forces and electroweak currents. A tremendous level of progress has been made in the past few years in the development of this method into a world-leading and far-reaching {\it ab-initio} tool. With the excellent agreement with data in terms of absolute and separation energies, first global ab-initio calculations of all nuclei from helium to nickel were performed and achieved deviation from absolute experimental ground-state energies on a level that is comparable with nuclear mass models. 
%\textbf{(Is there a paper to cite as example for nuclear astrophysics, e.g. mass measurements with TITAN or so?)}.

A topic of indirect nuclear astrophysics interest are WIMP-nucleus structure functions for dark matter direct detection searches. The group has now implemented these processes into the VS-IMSRG framework and has calculated up to Ge. The goal is to provide first {\em ab-initio} WIMP-nucleus scattering cross sections for all direct detection candidates up to Xe.

The medium-term goals of the VS-IMSRG are to expand the reach towards heavier nuclei and towards the driplines, and to predict the neutron skin thickness of $^{208}$Pb (see PREX-II experiment, Sec.~\ref{sec:NA:prex-crex}). This will also include the investigation of the structure of exotic nuclei and the evolution of magic numbers to the heavy region of nuclei.




%%%%%%%%%%%%%%%%%%%%%%
\paragraph{Reverse engineering of $r$-process abundances} \label{sec:NA:reverse} TRIUMF, UBC; Manitoba, McGill; {\intl USA}\\
 One of the most useful theoretical tools to guide experimental efforts nowadays are so-called ``sensitivity studies" in which one or more nuclear parameter is varied within given boundaries, e.g. nuclear masses within $\pm$500~keV or cross sections by a factor of 10, and the resulting influence on the calculated isotopic abundances investigated with astrophysical reaction networks. This procedure helps to identify ''sensitive" nuclear physics inputs and key areas for further investigations. 

So far, these sensitivity studies were only a one-way street and helped to identify key nuclei for more detailed studies. A new tool that has now become possible thanks to machine learning tools and high performance computing is the so-called ``reverse engineering''  \footnote{\href{https://doi.org/10.1088/1361-6471/44/3/034003}{M. Mumpower et al., J. Phys. G: Nucl. Part. Phys. 44, 034003 (2017)}} where an astrophysical reaction network is repeatedly solved and compared against the observed $r$-process abundances to optimize and constrain the input parameters (e.g. unknown nuclear properties, astrophysical conditions, initial composition, expansion timescales etc.) with Markov-chain Monte Carlo (MCMC) methods (see Fig.~\ref{Fig:NA-Comp}). 

This method has been successfully applied for masses of neutron-rich nuclei in the so-called ''Rare Earth Region" ($A$=160--170). Mass measurements of neutron-rich neodymium ($Z$=60) and samarium ($Z$=62) were carried out at the Canadian Penning Trap (CPT) at Argonne National Laboratory (see Sec.~\ref{sec:NA:CPT}) and were compared with predictions from ''reverse engineering" of the elaborately parameterized mass surface that best reproduces the solar abundances of the stable lanthanide nuclei \footnote{\href{https://doi.org/10.1103/PhysRevLett.120.262702}{R. Orford et al., Phys. Rev. Lett. 120, 262702 (2018)}}. In this way a new semi-empirical mass surface can be extracted that sets constraints on yet unmeasured heavier isotopes.

An extension to this ''reverse engineering" is presently developed by TRIUMF/ UBC scientists in collaboration with astrophysicists at Los Alamos National Laboratory and the University of Notre Dame in the USA. The idea is to improve and generalize the reverse engineering method by replacing the computationally expensive reaction network simulations with much faster neural network ''emulators" that will then allow not only for more robust Bayesian parameter inference thanks to the faster accumulation of data points in MCMC methods, but also allows to apply this method in a variety of mass ranges and nuclear parameters in addition to masses. To achieve this, there are two important steps to add: First, a ''good" emulator has to be developed via training of machine learning algorithms; then the next step is to sample from the posterior distributions of the parameters, which may be reduced or compressed by a ''dimensionality reduction" technique, through an MCMC algorithm coupled to the emulator. Both steps have to be carried out without losing any important information, for example the prominent even-odd staggering in masses or structure changes around (sub-)shell closures. 

These methods cannot replace the respective measurement if this nuclei becomes accessible but help to put tighter constraints on parameter ranges for theoretical models which will stay invaluable for the vast majority of nuclear physics properties which cannot be measured with the next generation of radioactive beam facilities.


%%%%%%%%%%%%%%%%%%
\begin{figure}[!htb]
\centering
\includegraphics[width=0.8\columnwidth]{Figs/NA_MCMC.png}
\caption{\label{Fig:NA-Comp} Example of ''reverse engineering" in nuclear astrophysics: By comparison of the measured (observed) r-process abundances with the abundances from astrophysical reaction network calculations regions of discrepancies can be identified which require further finetuning of the astrophysical or nuclear physics inputs. MCMC techniques can be used to automatize the search for the ''best" input parameters and set constraints yet unmeasured properties.
%
}
\end{figure}
%%%%%%%%%%%%%%%%%%




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\subsection{Experimental Facilities}\label{sec:NA:facilities}
Whereas the domestic experimental Nuclear Astrophysics research program is centered at the ISAC (Isotope Separator and Accelerator) facility at TRIUMF, a diverse offshore program is ongoing at various stable and radioactive beam facilities in Germany, Japan, and the USA. 


\subsubsection{Experiments at the ISAC facilities at TRIUMF}

\paragraph{DRAGON recoil separator}\label{sec:NA:dragon}
(TRIUMF) McMaster, Northern British Columbia, Simon Fraser, St. Mary's; {\intl UK, USA}

DRAGON (Detector of Recoils And Gammas Of Nuclear reactions) is a recoil separator based around a windowless recirculating (hydrogen or helium) gas target which is designed to measure astrophysically important radiative ($\alpha$ and proton) capture rates of stable and radioactive beams. Of particular interest are the reactions which occur in the asymptotic giant branch (AGB) stars, or in explosive environments like classical novae, supernovae, and X-ray bursters. 

The success of DRAGON lies in its ion-optical design, providing it with superior background rejection qualities which allow measurements to be made with a range of beam intensities. Surrounding the gas target is a high-efficiency bismuth germanate (BGO) array to detect $\gamma$'s emitted by radiative capture reactions. Thanks to its world-leading beam suppression (up to 10$^{13}$) and its excellent mass resolving power, detection of recoiling fusion products at the focal plane is possible down to 1 event per week, which allows to measure astrophysically important cross sections on stable nuclei that are otherwise only accessible in deep underground laboratories, or for radioactive nuclei that are accessible nowhere else.

DRAGON has been extremely successful in measuring  challenging radiative capture reactions with radioactive beams, which suffer from high background and low count rates. It has performed the majority of these pioneering measurements where the reaction is measured for the first time directly, e.g. $^{19}$Ne$(p,\gamma)^{20}$Na (Ref.~\footnote{\href{https://doi.org/10.1103/PhysRevLett.119.242701}{R. Wilkinson et al., Phys. Rev. Lett. 119, 242701 (2017)}}) and $^{38}$K$(p,\gamma)^{39}$Ca (Ref.~\footnote{\href{https://doi.org/10.1103/PhysRevC.97.025802}{G. Lotay et al., Phys. Rev. Lett. 116, 132701 (2016), G. Christian et al., Phys. Rev. C 97, 025802 (2018)}}). 

DRAGON can also be set up with the scattering chamber SONIK (Scattering Of Nuclei in Inverse Kinematics, see Fig.~\ref{Fig:NA:SONIK}) which utilizes the windowless gas target to induce scattering, with ejectiles or/and recoils detected at precise angles, simultaneously at different interaction energies. With high intensity radioactive and stable beams, these data are used to measure low-energy elastic scattering differential cross sections, which aid in the extrapolation of capture data to the very lowest astrophysical energies. Recent studies of $\alpha(^3$He$,^3$He$)\alpha$ have provided insights into the $^3$He$(\alpha,\gamma)^7$Be reaction; the system will soon be used to study both $^7$Be$+p$ and $^7$Be$+\alpha$ elastic scattering, and in the longer term other light radioactive beams.

%%%%%%%%%%%%%%%%%%
\begin{figure}[htb!]
\centering
\includegraphics[width=0.8\columnwidth]{Figs/SONIK.jpg}
\caption{\label{Fig:NA:SONIK} The new scattering chamber SONIK which can be set-up around the windowless gas target at DRAGON. (Picture: 2018 TRIUMF Photowalk; Sarah Johnson)
}
\end{figure}
%%%%%%%%%%%%%%%%%%

The plans for the near future include the installation of GRIFFIN clovers around the gas target for a more sensitive $\gamma$-tagging of capture reactions. A CFI application is also prepared for a new, highly efficient LaBr$_3$ array surrounding the gas target. This would allow a sensitivity increase of a factor of $\approx$10.



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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\paragraph{EMMA recoil spectrometer}\label{sec:NA:emma}
(TRIUMF) Guelph, McMaster, Saint Mary's, SFU

The experimental setup and the nuclear structure program of EMMA is discussed in Sec.~\ref{sec:nucstruc:emma}.

The Electromagnetic Mass Analyser (EMMA) is a recently-commissioned vacuum-mode recoil mass spectrometer located in the ISAC-II experimental hall. EMMA's design enables the measurement of fusion evaporation, radiative capture, and transfer reactions for nuclear structure and astrophysics.  Currently, the TIGRESS spectrometer is situated at the EMMA target location, as shown in Fig.~\ref{fig:EMMA-TIGRESS}. 
%Many of the TIGRESS+EMMA experimental campaigns have an astrophysical motivation. 

EMMA is able to perform high-mass radiative capture reactions that are out of reach of the DRAGON facility. The first scientific experiment in 2019 was a measurement of the cross section of the $p(^{83}$Rb,$\gamma)^{84}$Sr reaction, carried out in conjunction with the TIGRESS $\gamma$-ray spectrometer. That study represented the first ever direct measurement of a supernova reaction using a radioactive beam, providing valuable information for models of the astrophysical $p$ process. 

EMMA will extend its experimental program in the upcoming years with TIGRESS and auxiliary detectors like SHARC-II and TI-STAR to indirectly constrain neutron capture cross sections of radioactive nuclei. In particular, $(d,p)$ reactions such as $^{87}$Kr$(d,p)^{88}$Kr and $^{142}$Cs$(d,p)^{143}$Cs will be studied to infer the $(n,\gamma)$ reaction cross section indirectly. In these reactions the ''Oslo method” will be applied to extract the nuclear level density (NLD) and the $\gamma$-ray strength function ($\gamma$SF) which are important ingredients to constrain the neutron capture cross section.  








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\paragraph{GRIFFIN and auxiliary detectors} (TRIUMF) Guelph, SFU, Regina, Queen's; {\intl USA} \label{sec:NA:griffin}

For a detailed description of the array and its auxiliary detectors, see Sec.~\ref{sec:nucstruc:griffin}.
Since its commissioning in 2016 the GRIFFIN spectrometer with its many auxiliary detectors has been proven as powerful device for the spectroscopy of neutron-rich nuclei. In combination with the excellent beam purity achieved with the ion-guide laser ion source (IG-LIS)\footnote{\href{https://doi.org/10.1063/1.4868496}{S. Raeder et al., Rev. Sci. Instrum. 85, 033309 (2014)}} and uranium carbide targets, one region of increased interest for recent decay spectroscopy with GRIFFIN has been the cadmium and indium nuclei around doubly-magic ($Z$=50, $N$=82) $^{132}$Sn.  The first physics result from the new GRIFFIN spectrometer\footnote{\href{https://https://doi.org/10.1103/PhysRevC.93.062801}{R. Dunlop et al., Phys. Rev. C 93, 062801(R) (2016)}} resolved a stunning 23\% discrepancy between two previous half-life measurements for the important $N$=82 $r$-process nucleus $^{130}$Cd, which is mainly responsible for the second abundance peak at $A$=130.  

However, this shift in the adopted $^{130}$Cd half-life to much lower values posed then a problem for shell-model calculations, which used the half-lives of $^{131}$In and $^{130}$Cd as ``anchor points" to extract half-lives for the (at that time) unknown half-lives of  $N$=82 isotones with $Z<47$.
%The previous, longer, $^{130}$Cd half-life resulted in predicted half-lives for $^{129}$In, $^{128}$Pd, and $^{127}$Ru that were much longer than observed. 
A renormalization to the new $^{130}$Cd half-life (corresponding to a reduced quenching of the Gamow-Teller transition strength) resulted in an almost perfect reproduction of recently measured half-lives -- with the exception of $^{131}$In. 
%The first-forbidden $\beta$-decay strength in this region used in the Shell Model calculations is estimated from the $\beta$ decay of the $J^{\pi} = 1/2^{-}$ isomers of $^{129,131}$In and is believed to account for approximately 10–20\% of the decay strength for the $N = 82$ isotones, with the $^{131}$In decay having the largest first-forbidden contribution.  
A follow-up experiment of the $\beta$- and $\beta$-delayed neutron decay of $^{131}$In was performed \footnote{\href{https://doi.org/10.1103/PhysRevC.99.045805}{R. Dunlop et al., Phys. Rev. C 99, 045805 (2019)}} and greatly extended the known decay scheme. With this, a better understanding of the important first-forbidden contributions to the $\beta$-decay strength was achieved that will allow more reliable shell model calculations in this crucial region for $r$-process nucleosynthesis.  

Along the same line, detailed spectroscopic investigations of the decays of the ground and isomeric states of $^{129}$Cd (Ref.~\footnote{\href{https://doi.org/10.1103/PhysRevC.102.024337}{Y. Saito et al., Phys. Rev. C 102, 024337 (2020)}}) also greatly extended the known decay scheme and even surpassed a recent study by the former EURICA setup at RIKEN Nishina Center \footnote{\href{https://doi.org/10.1103/PhysRevC.91.054324}{J. Taprogge et al., Phys. Rev. C 91, 054324 (2015)}}. The log $ft$ values deduced from the $\beta$-feeding intensities suggest that some of the high-lying states in $^{129}$In are populated by the $\nu$0g$_{7/2} \rightarrow \pi$0g$_{9/2}$ allowed Gamow-Teller transition, which indicates that this transition is more dominant in $^{129}$Cd decay than previously thought. In addition, a previous discrepancy between the half-lives of the 11/2$^-$ ground-state and the 3/2$^+$ isomeric state was resolved.

A future focus for spectroscopic investigations with GRIFFIN  will be the neutron-rich rare earth region near the midshell closure at $^{170}$Dy ($Z= 66$, $N= 104$). The development of heavier lanthanide beams is a unique strength of the ISAC facility since yields from fission sources drop quickly towards heavier masses. Also beams from the ARIEL photofission target from Day 1 of its operation will allow to push the decay spectroscopy program to even further neutron-rich nuclei.

%As a first step, decay spectroscopy of neutron-rich Eu isotopes with $A$=160-165 has been carried out. For these investigations, the IGLIS ion source was again a key to success. Rare earth elements have very similar first ionization potentials and are difficult to separate if no further isobaric cleaning is used.  Immediately following the successful development of a laser-ionization scheme for the Eu isotopes by the TRILIS group at TRIUMF, a proposal was submitted for the decay spectroscopy of $^{160-165}$Eu with GRIFFIN.  The decay of $^{160}$Eu was investigated with a high-statistics data set and 41 new transitions and 10 new excited states were indentified [MSc Thesis Daniel Yates, UBC (2019)]. This GRIFFIN dataset also includes lifetime and conversion electron spectroscopy. The analysis of the datasets for $^{161-165}$Eu is ongoing.  A Letter of Intent for the development of additional neutron-rich lanthanide beams at ISAC has been submitted (S1875LoI) to extend this research in the next decade. The development of heavier lanthanide beams is a unique strenth of ISAC since yields from fission sources drop quickly towards heavier masses. 

DESCANT, the DEuterated SCintillator Array for Neutron Tagging, is a 70-element array of deuterated liquid-scintillator detectors that can be used with both the TIGRESS and GRIFFIN. After several commissioning runs it will be used as neutron tagger for the spectroscopy of very neutron-rich nuclei that decay by $\beta$-delayed neutron emission. For more details, see Sec.~\ref{sec:nucstruc:descant}.




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\paragraph{IRIS}
(TRIUMF) Guelph, McMaster, Regina, Saint Mary's, SFU; {\intl Japan, UK, USA, France} \label{sec:NA:IRIS}\\
The ISAC Reaction Induced Spectroscopy station (IRIS) is a facility to study direct reactions by charged-particle spectroscopy using the reaccelerated beams of radioactive isotopes with energies from 5--12 $A$ MeV provided by the ISAC-II facility at TRIUMF. It utilizes nucleon transfer reactions, inelastic, and elastic scattering on a thin windowless solid
H$_2$ or D$_2$ target to investigate light nuclei around the proton- and neutron drip-lines to study nuclear shell structure, new excitation modes, exotic halos and skins,
and nucleosynthesis processes. 

After a very successful past campaign (for more details, see Sec.~\ref{sec:nucstruc:iris}) the focus in the upcoming years will be the coupling to an active target time projection chamber (TPC) in which the detector gas also acts as the reaction target allowing tracking and detection of very low-energy reaction products. An experimental campaign with an active target TPC from collaborators in France (ACTAR, see Fig.~\ref{Fig:NA:ACTAR}) is planned for the near future, and a CFI funding proposal for the construction of a new active target TPC (EXACT-TPC) is presently underway. 
Another future project is the coupling of neutron detectors (Sec.~\ref{sec:NA:n-array}) to IRIS for studying $(d,n)$ reactions. 

%In the next phase of IRIS drip-line nuclei like $^{8}$He, 11Li, 10C and 20Mg will be explored to investigate the halo structure and shell evolution at the limits of nuclear binding. 
Extension of the experimental program from light to medium-mass to neutron-rich Ca, Ga, and Kr isotopes is planned. These projects will use inelastic deuteron and proton scattering, one and two-nucleon transfer reactions mainly with deuteron and proton targets.  First exploratory studies of a $(d,p)$ reaction on neutron-rich $^{93}$Kr were performed with a low beam intensity of $~$200 pps, which is an unprecedented accomplishment.

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\begin{figure}[htb!]
\centering
\includegraphics[width=1.0\columnwidth]{Figs/ACTAR-new.png}
\caption{\label{Fig:NA:ACTAR} Observed particle tracks of a reaction detected in the ACTAR active target time projection chamber. Picture by G. Grinyer (U Regina).}
\end{figure}
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\paragraph{TIGRESS and auxiliary detectors}\label{sec:NA:tigress}
(TRIUMF) Guelph, SFU, Saint Mary's; {\intl France, Spain, UK, USA}

As in the case of GRIFFIN, the sensitivity of TIGRESS $\gamma$-spectrometer is dramatically enhanced through its coupling with a suite of specialized charged-particle and neutron detector systems that can be tailored, for each experiment, to the reaction of interest. For details of the setups and the nuclear structure program, see Sec.~\ref{sec:nucstruc:tigress}.

Two of the main auxiliary detectors for the measurement of astrophysically relevant transfer reactions are SHARC (Silicon  Highly-segmented  Array  for  Reactions  and  Coulex) and TI-STAR (TIGRESS Silicon Tracker Array) which is presently under development. Recently, TIGRESS has been coupled with the EMMA recoil spectrometer (see Sec.~\ref{sec:NA:emma} and \ref{sec:nucstruc:emma} for details about this program).

SHARC is comprised of up to 2000 channels of double-sided silicon strip detectors (DSSDs) with nearly 4$\pi$ solid angle coverage around the reaction target at the centre of the TIGRESS array, developed by collaborators from the University of York in the UK and optimized for light charged particle detection in single- and two-nucleon transfer reactions with accelerated radioactive ion beams in inverse kinematics. It is used to detect charged particles with excellent energy and angular resolution. An upgraded version of SHARC is presently being built and will be ready to use in early 2021. The new SHARC-II vacuum enclosure will permit the simultaneous use of SHARC, TIGRESS, and EMMA for the first time, including DSSD coverage at both large and small laboratory scattering angles without diminishing the angular acceptance of EMMA.

Stable $^{19}$F is thought to be produced in asymptotic giant branch (AGB) stars via the reaction sequence $^{18}$O$(p,\alpha)^{15}$N$(\alpha,\gamma)^{19}$F. At present, however there are inconsistencies between the observed abundances and the modelled ones. Other astrophysical  scenarios might also contribute to the $^{19}$F abundances but are lacking firm observations. It is thus thought that the underproduction is due to uncertainties in reaction rates, including the competing $^{18}$O$(\alpha,\gamma)^{22}$Ne reaction. The resonance properties in $^{22}$Ne were studied following a one-neutron transfer experiment in inverse kinematics using TIGRESS and SHARC to identify the spins of critical resonances which dominates the $\alpha$ capture rate into $^{22}$Ne around 0.1~GK. With this data the production of $^{19}$F can now further be constrained.


TI-STAR is being developed at the University of Guelph and will precisely reconstruct the vertex of nuclear reactions in an extended gas target, providing a new capability to study transfer reactions with TIGRESS using accelerated radioactive beams from ISAC-II. TI-STAR makes use of multiple layers of novel ultra thin silicon detectors of 20$\mu m$ thickness to track light recoils like protons and helium ions. 
%GEANT4 Monte-Carlo simulations of TI-STAR have been finalized and detailed CAD designs for the TI-STAR mechanics and gas system have been completed. 
The current schedule foresees TI-STAR to be operational in fall 2021/spring 2022 for CANREB experiments at TRIUMF.  

A major goal of the TI-STAR project is constraining neutron capture rates for $i$-process and $r$-process nuclei via one-neutron transfer reactions, for which a new program has been initiated at TRIUMF. For this, TI-STAR will include an extended deuterium target that, when combined with its high resolution tracking capability to reconstruct the interaction point, will provide excellent energy resolution, while enhancing the reaction yield by factors of 10--100 for Oslo-type measurements.  Simulations show that TI-STAR will give access to neutron capture rate measurements for a large number of important nuclei determining the final $r$-process abundances. 

This new program will greatly benefit from the new ARIEL facility (Sec.~\ref{sec:CanFac:ariel}) since it will give access to clean and intense radioactive beams for studies in the $i$-process and $r$-process region.  

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\paragraph{TITAN ion trapping facility}\label{sec:NA:titan} (TRIUMF) Calgary, Manitoba, McGill, SFU; {\intl France, Germany, Netherlands, South Korea, UK, USA}

TITAN (TRIUMF's Ion Trap for Atomic and Nuclear science) is a unique ion trap facility, currently consisting of five individual ion traps coupled together. For technical details, see Sec.~\ref{sec:nucstruc:titan}. It is a facility that is ideally suited for high-precision measurements of very short-lived nuclei down to a few ms half-lives since it has the fastest beam preparation and measurement cycle for on-line precision mass measurements and is the only system in the world to provide highly charged radioactive ions, which boost the precision by one to two orders of magnitude. The astrophysics part of the TITAN research program has been focussed on mass measurements for the $r$- and $rp$-process. The commissioning of the Multi-Reflection Time-of-Flight (MR-TOF) spectrometer in 2017 has extended TITAN's capabilities and marked a new era, especially for mass measurements with astrophysical background.

Another new direction at TITAN is the In-Trap Decay Spectroscopy program investigating decay modes of highly-charged ions that cannot be be measured anywhere else (see Sec.~\ref{sec:NA:HCI}). TITAN is currently the only ion trap facility in the world capable of performing decay-spectroscopy on trapped highly charged radioactive ions (HCIs). This work employs the radio-frequency quadrupole (RFQ) linear Paul trap, used to cool and bunch the ISAC beam, and the electron-beam ion trap (EBIT) for increasing the atomic charge-state of the ions and performing the in-trap decay measurements \footnote{\href{https://doi.org/10.1016/j.nima.2014.12.118}{K.G. Leach et al., Nucl. Instr. Meth. A 780, 91 (2015).}}. The TITAN-EBIT features seven ports with optical access to the center of the trap, which are able to house various photon detectors (see Fig.~\ref{Fig:NA:EBIT}). For this future decay spectroscopy program detectors from the former 8$\pi$ setup will be used which are provided in collaboration with Simon Fraser University. The setup will be fully operational in 2021 and will allow to start a new experimental campaign for the controlled measurements of nuclear excitation via electron capture (NEEC) (e.g. in $^{129m}$Sb), and selective decay-mode blocking of first-order atomic and nuclear transitions. 

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\begin{figure}[htb!]
\centering
\includegraphics[width=1.0\columnwidth]{Figs/TITAN-EBIT.png}
\caption{\label{Fig:NA:EBIT} (Left) A schematic view of the TITAN electron beam ion trap (EBIT). (Right) Cross-sectional view of the EBIT configuration for decay spectroscopy demonstrating the access ports where HPGe detectors are located. Figures are adapted from K.G. Leach et al., Nucl. Instr. Meth. A780, 91 (2015). }
\end{figure}
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%ID Nov 2020: No comments from Ania- removing this part. 
%\textbf{Summarize here astrophysically relevant papers, e.g. $n$-rich Ga masses (M.P. Reiter et al., 2019), Indium masses (Izzo et al.). Potentially add a picture of Pascal Reiter and Andrew Jacobs working at MR-TOF from TRIUMF Photowalk 2018. If no feedback, leave out. }

Medium-term upgrades of the TITAN facilities include a cryogenically cooled Penning trap (cryoMPET) using two detection techniques, namely the Time-Of-Flight (TOF) and Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) techniques. The latter can improve the precision, resolving power, and sensitivity of the former, and the precision of both techniques benefits from higher charge states.

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\paragraph{TUDA particle detector} \label{sec:NA:tuda} (TRIUMF) McMaster, Northern British Columbia, Simon Fraser; {\intl UK}

The TUDA (TRIUMF-UK Detector Array) charged-particle scattering and reaction detector array is a collaboration with the University of Edinburgh and the University of York in the UK. It is a versatile  scattering facility based around large area, highly-segmented silicon arrays and a variety of solid or gaseous targets. TUDA is a portable detector which can be installed in both ISAC experimental halls to indirectly constrain reaction rates or to provide important nuclear structure information (e.g. excitation energies, spins, and parities) to guide direct measurements. It is especially well-suited for transfer reactions, elastic scattering, and direct measurements of $(p,\alpha)$ and $(\alpha,p)$ reactions. These reactions address problems in massive star nucleosynthesis, thermonuclear and core-collapse supernovae, and classical novae. 

It has provided a variety of data on astrophysical reactions over the years, most recently for the $^{18}$F$(p,\alpha)^{15}$O, $^{18}$Ne$(\alpha,p)^{21}$Na $^{26}$Al$(d,p)^{27}$Al and $^{23}$Na$(\alpha,p)^{26}$Mg reactions. 




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\subsubsection{Offshore experiments}
Canadian scientists are involved in astrophysics experiments with the main focus on the complementary program at large in-flight fragmentation facilities like the GSI Helmholtz Center and FAIR in Germany, RIKEN Nishina Center in Japan, and NSCL/ FRIB in the USA. In addition, a diverse research program with stable and radioactive beams is also carried out at various smaller accelerator facilities worldwide.

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\paragraph{Nuclear radii measurements}\label{sec:NA:radii}
(GSI/ Germany, RIKEN/ Japan) Saint Mary's, TRIUMF; {\intl China, Germany, Japan, the Netherlands, Slovakia, Spain, UK, USA}

The research program at the Fragment Separator (FRS) at GSI Darmstadt and the RIKEN Nishina Center at RIKEN, Japan uses the reactions of high-energy beams of exotic nuclei on various targets to determine the neutron skin thickness and discover nuclear halos through measurement of their nuclear radii. This research seeks to unveil exotic nuclear forms, neutron halos, neutron skin and new arrangements of nuclear shells in nuclei approaching the edges of the bound nuclear landscape. The thick neutron-dominated surface emerging in these nuclei provides laboratory access to gain knowledge on the equation-of-state of asymmetric nuclear matter that describes the characteristics of neutron-rich cosmic environments such as neutron stars and supernovae. For more details, see Sec.~\ref{sec:nucstruc:relatavistic}.

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\paragraph{Spectroscopy of highly-charged ions in storage rings}\label{sec:nucastro:esr}
(GSI/FAIR) TRIUMF; {\intl China, Germany, Japan, UK}

The Canadian research program at the storage rings at GSI/FAIR in Darmstadt/ Germany is part of the \texttt{ILIMA} (Isomers, Lifetimes, and MAsses) collaboration. In conjunction with conventional resonant Schottky pickups, particle detectors are installed in dedicated pocket positions in the storage rings and can be used for identification of decay products outside of the ring acceptance as well as additional beam diagnostics. This setup allows unique investigations of exotic decay modes of highly-charged ions over several hours (compared to tens of seconds in trap-environments like in the TITAN-EBIT), as well as the parallel (symbiotic) measurement of masses and half-lives of stored ions.

A multi-purpose particle detector prototype, \texttt{CsISiPHOS} (CsI-Silicon Particle detector for Heavy ions Orbiting in Storage rings, see Fig.~\ref{Fig:NA:bound}), has been constructed and commissioned, and used in spring 2020 in the Experimental Storage Ring (ESR) at GSI Darmstadt to measure half-life of the bound-state $\beta$-decay nucleus $^{205}$Tl$^{81+}$ (see~Sec.~\ref{sec:NA:HCI}). 

NSERC funding was awarded in 2020 to construct an advanced version of the \texttt{CsISiPHOS} particle detector, as well as to upgrade the data acquisition system for operation at FAIR. The design of the new detector is the result of an extensive R\&D program performed in collaboration with the TU Munich/ Germany that resulted in a FAIR Technical Design Report (TDR) \footnote{\href{https://edms.cern.ch/document/2088497}{Technical Design Report ILIMA Heavy Ion Detector}}. 
The new particle detector stack is presently under construction at TRIUMF and will be installed and tested at GSI Darmstadt in late 2021. With this second particle detector, e.g. symbiotic measurements of masses, half-lives, and neutron-branching ratios are envisioned in the next decade with FAIR.



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\paragraph{Measurements of $\beta$-delayed neutron emitters with BRIKEN}\label{sec:NA:briken} (RIKEN) TRIUMF, McMaster; {\intl Chile, Hong Kong, Japan, Poland, Spain, UK, USA, Vietnam}

The BRIKEN (Beta-delayed neutron emitters at RIKEN) collaboration (Fig.~\ref{Fig:NA-BRIKEN}) has the goal of measuring half-lives and neutron-branching ratios of $\approx$600 $\beta$-delayed neutron-emitters for direct input into astrophysical models for a better understanding of the nucleosynthesis of heavy elements in the $r$-process. The setup consists of an implantation detector with double-sided silicon strip detectors (DSSDs), a moderated neutron detector with 140 $^{3}$He-filled tubes, and two optional HPGe clover detectors \footnote{\href{https://iopscience.iop.org/article/10.1088/1748-0221/12/04/P04006}{A. Tarife\~no et al., J. Instrum. 12, P04006 (2017)}}. In three years of running between 2016-18, more than 270 nuclei have been investigated between $^{33}$Na and $^{170}$Gd, and $\approx$180 new neutron-branching ratios and $\approx$60 new half-lives have been extracted. First results have now been published (see Refs.~\footnote{\href{https://doi.org/10.1016/j.nima.2019.02.004}{A. Tolosa-Delgado et al., Nucl. instr. Meth. A925, 133 (2019)}}, \footnote{\href{https://doi.org/10.1103/PhysRevC.100.011302}{V. Phong et al., Phys. Rev. C 100, 011302 (2019)}}, and \footnote{\href{https://doi.org/10.1103/PhysRevC.100.031302}{R. Yokoyama et al., Phys. Rev. C. 100, 031302(R) (2019)}}) and many more papers will follow. Two of the six experimental campaigns were lead by Canadian researchers.

%One of the highlights is the first determination of the neutron-branching ratio ($P_{1n}$ value) of doubly-magic $^{78}$Ni. Whereas theoretical predictions range anywhere between 10--80\%, the experimental value is closer to 25\%. 
The BRIKEN campaign will conclude in 2021, and the Canadian participants will transition to future experiments with the BELEN neutron detector at GSI/FAIR Darmstadt as part of the \texttt{DESPEC} collaboration. 

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\begin{figure}[htb!]
\centering
\includegraphics[width=0.8\columnwidth]{Figs/NA_Group_BRIKEN2017.pdf}
\caption{\label{Fig:NA-BRIKEN} (Part of the) BRIKEN group at RIKEN Nishina Center in Japan in 2017.
%
}
\end{figure}
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\paragraph{PREX and CREX: Measuring the neutron skin thickness} \label{sec:NA:prex-crex}
(JLab) Manitoba; {\intl Croatia, Italy, Slovenia, Ukraine, USA}

PREX, the Pb (lead) Radius EXperiment, uses the parity violating weak neutral interaction to probe the neutron distribution in $^{208}$Pb, with the aim of measuring the root-mean-square neutron radius ($R_n$($^{208}$Pb)) to an accuracy of 1\%. Data taking was completed in 2019, and the collaboration has presented preliminary unblinded results at the October 2020 APS DNP meeting. 
%\textbf{(Include a picture and a nice plot of this result)}

CREX, the Calcium Radius Experiment, uses elastic scattering of polarized electrons from $^{48}$Ca to measure $R_n$($^{48}$Ca) to 0.9\% precision. The collaboration started data taking in fall 2019 and is planning to resume in summer 2020 after a COVID19-related break. Data analysis should be completed by the end of 2022. 

In combination, the results will challenge the assumptions of state-of-the-art nuclear structure models, with measurements for atomic mass number $A$ in a regime where microscopic models can be applied to conditions in which the nucleon closely approximates infinite neutron-rich nuclear matter. By itself, the CREX result can help to illuminate specific details such as the role of three-neutron (3N) forces in microscopic calculations.

The Canadian collaborators at the University of Manitoba have made significant contributions to PREX-II and CREX in several crucial areas, for example as co-spokesperson for CREX (J. Mammei) and with simulation and design work. The group has also been responsible for the upgrade of the electron detector for the Hall A Compton polarimeter, and has provided the simulations of magnetic fields used to design the experiment and shielding of neutron backgrounds. 

For more details, see Sec.~\ref{sec:nucstruc:prex}.

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\paragraph{Mass measurements with CPT}\label{sec:NA:CPT} (Argonne) Manitoba, McGill; {\intl USA}

The Canadian Penning Trap (CPT) mass spectrometer was originally constructed for use at the AECL Chalk River Laboratories, and now has been operational at Argonne National Laboratory since 2001 (see Sec.~\ref{sec:IntFac:others:ANL}). In combination with the CARIBU (Californium Rare Isotope Breeder Upgrade) fission fragment source it is ideally suited for high-precision measurements of very neutron-rich isotopes for $r$-process investigations . 

Recent highlights include the measurement of neutron-rich nuclei in the so-called ''Rare Earth Region" ($A$=160--170) which were compared with predictions from ``reverse engineering'' of the mass surface that best reproduces the solar abundances of the stable Rare Earth Peak nuclei (see e.g. Sec.~\ref{sec:NA:reverse}).  In this way a new semi-empirical mass surface can be extracted that sets constraints on yet unmeasured heavier isotopes.
However, Canadian faculty involved in the CPT project has retired or is close to retirement.

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\paragraph{Indirect neutron capture cross sections measurements with SuN}\label{sec:NA:SuN} (NSCL and Argonne) Guelph, TRIUMF; {\intl Norway, South Africa, USA}

The new experimental program that has been started in collaboration with scientists at the NSCL in the USA and Norway aims at constraining neutron-capture cross sections of radioactive nuclei with various indirect methods. This is important for the reaction flow of the $i$ and $r$ process.

While the $^{142}$Cs$(d,p)^{143}$Cs reaction will be studied at TRIUMF to infer the $(n,\gamma)$ cross section indirectly, at Argonne National Laboratory a decay experiment will constrain the neutron capture into $^{143}$Cs via the ''$\beta$-Oslo method" using the SuN total absorption spectrometer. The combined data will offer the first direct comparison of reaction-based and $\beta$-decay techniques for constraining neutron capture rates experimentally.

In a follow-up step, one of the identified key reactions in the $i$ process, the $^{135}$I(n,$\gamma$)$^{136}$I reaction, will also be studied at Argonne National Laboratory.

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\paragraph{Indirect reaction studies}\label{sec:NA:alan} (MLL and TUNL) McMaster; {\intl Germany, USA}

Studies exploring the sensitivity of nova nucleosynthesis to reaction rate uncertainties have found that the $^{38}$K$(p,\gamma)^{39}$Ca reaction has a strong impact on the abundances of several elements near $A$= 40 that are potentially observable in the nova ejecta, for example $^{38}$Ar, $^{39}$K and $^{40}$Ca. This rate depends strongly on the resonance properties of low-lying $^{39}$Ca excited states, above the proton threshold. Since the reaction rate depends exponentially on the energies of the resonances, these energies will comprise a significant uncertainty contribution to an experiment-based reaction rate. Additional experiments are therefore needed to reduce the uncertainty in the energies of known levels, and to search for new and important $^{38}$K + p resonances.

The experiments at the (now closed) Maier-Leibnitz Laboratory (MLL) in Garching/ Germany and at TUNL/ USA aimed at a remeasurement of the $^{40}$Ca$(d,t)^{39}$Ca and a new measurement of the $^{39}$K$(^{3}He,t)^{39}$Ca reaction in the energy range of interest. When combined with a high-resolution magnetic spectrograph, such as a Q3D spectrograph at the MLL or Split-pole spectrograph at TUNL, excitation energies have been determined with typical uncertainties of $\approx$2--4 keV or smaller.



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\paragraph{Pseudo-bar Neutron Array}\label{sec:NA:n-array}
(Texas A\&M) Saint Mary's; {\intl USA}

%Apart from reaction rate measurements with astrophysical motivation  (see e.g. $^{22}$Ne$(^{6}Li,d)^{26}$Mg in Sec.~\ref{sec:NA:i}) also 
A new type of neutron detector array, TexNEUT, has been developed in collaboration with Texas A\&M which is planned to be brought to TRIUMF and coupled to the EMMA or IRIS facility for measurements (see Sec.~\ref{sec:NA:IRIS})

The detector consists of a large-area, position-sensitive array of organic p-Terphenyl scintillator which has shown excellent n/$\gamma$ pulse shape discrimination (PSD), fast timing, and high brightness. These properties make it an ideal fast neutron detector
\footnote{\href{https://doi.org/10.1016/j.nima.2015.04.038}{A. Sardet et al., Nucl. Instrum and Meth. A 792, 74 (2015)}}. The project developed a new method of coupling discrete p-Terphenyl cubes (see Fig.~\ref{Fig:NA:pseudo}) into a pseudo-bar geometry, with sensitivity to the cube of interaction. This enables the construction of large-area arrays with centimeter position sensitivity and PSD thresholds in the order of 100 keVee. A 128-bar array is presently under construction and will be used for indirect $(d,n)$ and neutron-decay experiments at the Cyclotron Institute at Texas A\&M and at TRIUMF. Initial work will center around an envisioned proof-of-principle experiment measuring the $^{26}$Al$(d,n)^{27}$Si reaction by coupling TexNEUT to either EMMA or IRIS. 

The medium-term plans are to develop a new, dedicated detector for neutron spectroscopy at TRIUMF. The envisioned detector will be constructed of p-Terphenyl using a similar pseudo-bar concept to TexNEUT. The applications of the array will be complimentary to existing neutron detectors at TRIUMF such as DESCANT. It will be targeted towards time-of-flight neutron spectroscopy with high resolution, high granularity, and n/$\gamma$ PSD. The array is expected to be sensitive to neutrons with energies of a few 100~keV up to 10~MeV. In addition to transfer reactions, the array will also be suitable for in-
flight neutron decay spectroscopy, $\beta$-delayed neutron spectroscopy, and direct measurements of astrophysical $(\alpha,n)$ or $(p, n)$ reactions. 


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\begin{figure}[htb!]
\centering
\includegraphics[width=0.6\columnwidth]{Figs/NA_pseudo-bar.png}
\caption{\label{Fig:NA:pseudo} Photograph of an un-wrapped (top) and wrapped (bottom) pseudo-bar consisting of six 1~cm$~\times$~1~cm$~\times$~1~cm cubes of the neutron scintillator p-Terphenyl. Light from either end of the bar is read out with photomultiplier tubes or silicon photomultipliers. A similar design is envisioned for the detector planned for TRIUMF.}
\end{figure}
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\subsection{Beyond the next five years}\label{sec:NA:beyond}
 
The main focus of the Canadian community for the time beyond 2027 will be without doubt the full exploitation of the new opportunities that the ARIEL project will bring. At the same time, FRIB in the USA will also be in full swing, and in Germany the first beams from the new FAIR facility will become available. Canadian nuclear astrophysicists will be involved in research programs at all of these new RIB facilities.
 
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\subsubsection{New setups at the ISAC facility}
One of the main goals for the existing ISAC facility will be to complement its already diverse selection of experimental setups with new detectors that complement the ongoing physics program and will make full use of the upcoming photofission beams from ARIEL.

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\paragraph{New detectors}
The pseudo-bar neutron array (see Sec.~\ref{sec:NA:n-array}) will allow a versatile program, e.g. making use of the intense re-accelerated beams of heavy nuclei provided by CANREB with dedicated $(d,n)$ experiments on rp-process nuclei. Coupling of the neutron detector to the new EXACT-TPC for nuclear structure would allow measurements on proton-dripline nuclei as well as three-body decays of neutron-unbound states with beam intensities that can be as low as a few hundred particles per second. Other experiments, like direct $(p,n)$ and $(\alpha,n)$ reactions important for core-collapse supernovae are envisioned by coupling the detector to EMMA. 

%\textbf{Add Total Absorption Spectrometer for GRIFFIN?}

%\paragraph{New X detector?}
%Another detector?
 
 %%%%%%%%%%%%%%%%%%
\paragraph{TRIUMF Storage Ring (TRISR)}\label{sec:NA:TRISR}
The by far largest future project in the nuclear astrophysics community will be the planned installation of a low-energy storage ring with a neutron generator target at the ISAC facility. Heavy ion storage rings connected to an ISOL facility provide a unique environment to carry out
nuclear physics experiments with stored radioactive beams due to the up to six orders of magnitude increased luminosity compared to ''one-time-pass" experiments. The installation of a low-energy storage ring at ISAC-I would create a worldwide unique facility and provide a valuable extension of TRIUMF's physics program by attracting new users.

About ten years ago, the CERN/ISOLDE facility had plans to re-use the \href{https://isolde.web.cern.ch/tsrisolde}{Test Storage Ring (TSR)} which was formerly installed at the Max-Planck Institute for Nuclear Physics in Heidelberg/ Germany \footnote{\href{https://doi.org/10.1140/epjst/e2012-01599-9}{M. Grieser et al., Eur. Phys. J. Spec. Topics 207, 1 (2012)}}. This project was deferred until after 2025. In the meantime a new design of a storage ring at ISOLDE, the ISR, has been brought up within the possible upgrades outlined in the EPIC project (Exploiting the Potential of ISOLDE at CERN) that was submitted to the European Strategy for Particle Physics update in December 2019. However, this new project is still missing approval from CERN and funding.

The TRISR at ISAC is building on these initial ideas but has many advantages. The storage ring will be designed in such a way that it will fit into the existing ISAC-I facility, without the need of a facility upgrade or an annex building (see Sec.~\ref{sec:isacupgrade} and Fig.~\ref{fig:facil_TriSR}). It will be a low-energy storage ring ($E\approx$0.1--10~MeV/u)
and the envisioned nuclear (astro)physics program will complement the existing program at ISAC. The TRISR will have an electron cooler (for decreasing the emittance of the beam) and potentially an internal gas (jet/droplet) target.

It is planned to couple a D-D neutron generator (e.g. the \href{https://phoenixwi.com/neutron-generators/dd-deuterium-deuterium-neutron-generators/}{Thunderbird compact generator from PHOENIX LLC} in Madison, WI/ USA) with the storage ring to allow --for the first time ever-- to measure direct neutron capture cross sections of short-lived radionuclides. The D+D fusion produces neutrons with an average energy of 2.45~MeV which would be moderated down to lower (astrophysically relevant) energies for capture reactions with the radioisotope beam circulating in the storage ring.

Neutron capture cross sections on radioactive nuclei are an important input parameter for all three heavy-element neutron capture processes ($s$, $r$, and $i$ process). Direct measurements were so far hampered by unavailability of proper macroscopic targets of both, the respective short-lived nuclei and neutrons. Neutron capture cross sections have been measured on longer-lived radioactive targets that could be produced in amounts in the order of a few $\mu$g (corresponding to $\approx$10$^{16}$ atoms). For shorter-lived nuclei, indirect (surrogate) methods with e.g. $(d,p)$ reactions have to be used instead (see the ongoing research program at EMMA, Sec.~\ref{sec:NA:emma}).

The plan is to submit an NSERC Project Grant in 2022 to initiate the machine study, define the physics program and the international collaboration, and prepare a Technical Design Report by 2024/25. This project with a cost of $\approx$C\$30--40 million would then be submitted as CFI proposal in 2025. If funding is successful, construction could start by 2026 and first physics experiments could be carried out by 2030/31.

Such a facility would be world-wide unique and open up a completely new research stream at ISAC. In addition, the facility is greatly benefiting from the higher intensities and cleaner neutron-rich beams in the ARIEL project. 

 
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\subsubsection{Theoretical Developments}
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\paragraph{Astrophysical Signatures}
aim to incorporate the effects found on neutrino oscillations and nucleon interactions directly into simulations of compact objects' evolution. The aim is also to develop Molecular Dynamic simulations of the accreting neutron star crust that allow a consistent study of neutron super
fluidity and exotic nuclear structure, e.g nuclear pasta, in abundance evolution of rp-process ashes. Furthermore, temperature effects on neutron captures will be studied, and $\alpha$ decay relevant for the rp- and r-processes. Those studies will provide a narrow set of reactions key in the synthesis of heavy elements that will guide experimental searches.

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\paragraph{\emph{Ab-initio} theories}
A future challenge for the {\em ab-initio} theory is to address nuclear deformation.
If successful, a comprehensive {\em ab-initio} description of the atomic nucleus applicable to light and medium mass nuclei will be achieved, with an increasing mass reach as the computing power increases to exascale and beyond. Further developments will allow to calculate processes with three charged particles in the final states, such as $^{11}$B$(p, \alpha\alpha)^4$He considered for aneutronic fusion energy generation. Nuclear quantum many-body problem will greatly benefit from the development of quantum computing capabilities. The implementation and development of quantum algorithms for {\em ab-initio} nuclear theories is also planned.

For the VS-IMSRG model the plan in the next decade and beyond is to extend the predictions to the driplines. A possibility is also the exploration of superheavy elements and the search for a potential island of stability around $A$$\approx$300.



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\subsection{Summary}
The Canadian nuclear astrophysics community has evolved enormously over the past decade and has taken a leading role in the worldwide research program. The extent of this chapter and the diversity of the experiments described here displays the popularity of this interdisciplinary field. 
The big question for the next decade and beyond is \textit{``What is the role of radioactive nuclei in shaping the visible matter in the universe?"}, and it will be driven by the new research possibilities enabled by the new generation of radioactive beam facilities. The main experimental setups are already installed and running, ready to take beam and to push the limits of nuclear astrophysical knowledge further out.

The domestic nuclear astrophysics program at TRIUMF-ISAC will benefit twofold from the upcoming ARIEL era: on one hand due to cleaner radioactive, neutron-rich beams for studies related to the rapid and intermediate neutron capture process; on the other hand due to the increase in available beamtime that will allow longer experiments to investigate astrophysical reaction rates with smaller cross sections or lower intensities. 

The next 5 years will see a multitude of additional (auxiliary) detectors being built to complement the already ongoing physics program. The next ''large" multi-million (Canadian) Dollar facility in this decade that is being discussed by the community might be the TRIUMF Storage Ring with a neutron generator target (see Sec.~\ref{sec:isacupgrade}). With the proper investment in additional HQP and faculty positions these efforts will lead to a further growth and strengthen the Canadian leadership in this field. 