Journal of Physics G: Nuclear and Particle Physics J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 (96pp) https://doi.org/10.1088/1361-6471/ac2827 International workshop on next generation gamma-ray source C R Howell1,2, M W Ahmed2,3,∗ , A Afanasev4 , D Alesini5, J R M Annand6, A Aprahamian7, D L Balabanski8, S V Benson9, A Bernstein10, C R Brune11, J Byrd12, B E Carlsten13, A E Champagne2,14, S Chattopadhyay15, D Davis2,16, E J Downie4, J M Durham13, G Feldman4, H Gao1,2, C G R Geddes17, H W Grießhammer4, R Hajima18 , H Hao1,2, D Hornidge19, J Isaak20, R V F Janssens2,14, D P Kendellen1,2, M A Kovash21, P P Martel22, U-G Meißner23 , R Miskimen24, B Pasquini25, D R Phillips11 , N Pietralla20, D Savran26, M R Schindler27, M H Sikora2,4, W M Snow28 , R P Springer1, C Sun17, C Tang29, B Tiburzi30 , A P Tonchev31, W Tornow3,18, C A Ur8, D Wang32, H R Weller1,2, V Werner20, Y K Wu1,2, J Yan1,2, Z Zhao33, A Zilges34 and F Zomer35 1 Department of Physics, Duke University, Durham, NC 27708, United States of America 2 Triangle Universities Nuclear Laboratory, Durham, NC 27708, United States of America 3 Department of Mathematics and Physics, North Carolina Central University, Durham, NC 27707, United States of America 4 Department of Physics, The George Washington University, Washington, D.C. 20052, United States of America 5 LNF-INFN, Via E Fermi 40, 00044 Frascati (Rome), Italy 6 SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, United Kingdom 7 Department of Physics, University of Notre Dame, Notre Dame, IN 46556, United States of America 8 Extreme Light Infrastructure Nuclear Physics (ELI-NP), Horia Hulubei National Institute for R & D in Physics and Nuclear Engineering (IFIN-HH), Magurele, Romania 9 Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, United States of America 10 Physics Department and Laboratory for Nuclear Science Massachusetts Institute of Technology, Cambridge, MA 02139, United States of America ∗Author to whom any correspondence should be addressed. 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Phys. 49 (2022) 010502 C R Howell et al 11 Department of Physics and Astronomy, Ohio University, Athens, OH 45701, United States of America 12 Argonne National Laboratory, Argonne, Illinois 60439, United States of America 13 Los Alamos National Laboratory, Los Alamos, NM 87545, United States of America 14 Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC, United States of America 15 Fermi National Accelerator Laboratory, Batavia, IL 60510, United States of America 16 Department of Physics, North Carolina State University, Raleigh, NC 27695, United States of America 17 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States of America 18 National Institutes for Quantum and Radiological Science and Technology, Tokai, Naka, Ibaraki 3191106, Japan 19 Mount Allison University, Sackville, New Brunswick E4L1E6, Canada 20 Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany 21 Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, United States of America 22 Institut für Kernphysik, University of Mainz, D-55099 Mainz, Germany 23 Helmholtz-Institut für Strahlen-und Kernphysik and Bethe Center for Theoretical Physics, Universität Bonn, D-53115 Bonn, Germany 24 University of Massachusetts, Amherst, MA 01003, United States of America 25 University of Pavia, Pavia, Italy 26 GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany 27 Department of Physics and Astronomy, University of South Carolina, Columbia, SC, United States of America 28 Department of Physics, Indiana University, Bloomington, IN 47408, United States of America 29 Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China 30 City College of New York, New York, NY, United States of America 31 Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States of America 32 Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China 33 University of Virginia, Charlottesville, VA, United States of America 34 Institut für Kernphysik, Universität zu Köln, Zülpicher Straße 77, D-50937 Köln, Germany 35 LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France E-mail: mahmed2@nccu.edu Received 13 May 2021, revised 11 June 2021 Accepted for publication 20 September 2021 Published 2 December 2021 Abstract A workshop on The Next Generation Gamma-Ray Source sponsored by the Office of Nuclear Physics at the Department of Energy, was held Novem- ber 17-19, 2016 in Bethesda, Maryland. The goals of the workshop were to 2 mailto:mahmed2@nccu.edu http://crossmark.crossref.org/dialog/?doi=10.1088/1361-6471/ac2827&domain=pdf&date_stamp=2021-11-25 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al identify basic and applied research opportunities at the frontiers of nuclear physics that would be made possible by the beam capabilities of an advanced laser Compton beam facility. To anchor the scientific vision to realisti- cally achievable beam specifications using proven technologies, the workshop brought together experts in the fields of electron accelerators, lasers, and optics to examine the technical options for achieving the beam specifications required by the most compelling parts of the proposed research programs. An interna- tional assembly of participants included current and prospective γ-ray beam users, accelerator and light-source physicists, and federal agency program man- agers. Sessions were organized to foster interactions between the beam users and facility developers, allowing for information sharing and mutual feedback between the two groups. The workshop findings and recommendations are summarized in this whitepaper. Keywords: gamma-ray, Compton scattering, nuclear astrophysics, nuclear structure, hadronic parity violation, low-energy QCD, nuclear theory (Some figures may appear in colour only in the online journal) 1. Executive summary The photon is a theoretically well-understood probe for investigating the structure of matter over a wide range of distance and energy scales. The angular-momentum selectivity and uncon- strained transfer of isospin in most gamma-ray (γ-ray) induced reactions enable highly precise strategic investigations of nuclear and nucleon structure and of collective motion responses of the internal degrees of freedom associated with electric charge and current distributions. In addition, high-energy photons are well suited for non-intrusive material analysis applica- tions in areas such as homeland security, nuclear security, structural integrity assessments, and medical diagnostics. Bremsstrahlung radiation produced by an electron beam incident on a high-Z target has been the workhorse photon-beam source used in nuclear physics research and applications for over a century. The energy spectrum of a bremsstrahlung γ-ray beam enables measure- ments of nuclear structure and reaction dynamics over a continuous energy range in a single experiment. Tagged bremsstrahlung sources provide the capability of associating the energy of the photon inducing a reaction with the photons detected in the final state. Also, coherent bremsstrahlung beams produced using a crystal target offer researchers a partially polarized γ-ray beam at energies near the end point of the bremsstrahlung spectrum. The scientific insight provided by research conducted using these sources is impressive, and facilities with advanced bremsstrahlung photon sources continue to be used in highly productive research programs. However, the continuous energy nature of bremsstrahlung γ-ray beams, which facil- itates survey measurements over a broad energy range, also has the adverse effect of creating backgrounds that limit the sensitivity of the experiments. These backgroundsmake it difficult to investigate nuclear phenomena with extremely low cross sections or to perform measurements on targets with small sample sizes, as is the case for isotopes with low natural abundances. The nearly monoenergeticγ-ray beams produced by laser Compton scattering offer an alter- native to bremsstrahlung beams, providing an enhanced signal-to-background ratio in basic and applied research, and reducing the radiation exposure in material analysis applications. In addition, laser Compton γ-ray beams can be produced with a beam polarization greater than 3 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al 95% for both linear and circular polarization. Linearly polarized laser Compton beams enable unambiguous determinations of the spin and parity of excited nuclear states, and beams with both linear and circular polarization are used in studies of the spin structure of nucleons. Over the last three decades, laser Compton γ-ray beam facilities have provided intense polarized and nearly mono-energetic γ-ray beams for research programs in basic and applied nuclear physics. These facilities include GRAAL at the European Synchrotron Radiation Facil- ity in Grenoble, LEGS at Brookhaven National Laboratory, LEPS at the SPring-8 facility, NewSUBARU at the University of Hyogo, and HIγS at the Triangle Universities Nuclear Lab- oratory. However, HIγS and LEPS are the only facilities worldwide that are operated with nuclear physics as the primary research focus. These facilities will soon be joined by two more that are currently under construction, ELI-NP in Romania and the γ-ray beam line at the Shang- hai Synchrotron Radiation Facility (SSRF) in China. These facilities will produce γ-ray beams with energies below 12 MeV (ELI-NP), and below 20 MeV and above 300 MeV (SSRF). Given that the time span between initial planning and the end of construction of an accelerator-driven light source is about a decade, consideration for next-generation laser Comp- ton γ-ray sources should start now. Technological advances in electron accelerators, lasers, and optics made during the last decade create new options for producing intense polarized γ-ray beams with narrow energy widths at beam energies from around 1 MeV to several GeV, the energies relevant to nuclear-physics research. A workshop on The Next Generation Gamma- Ray Sources, sponsored by the Office of Nuclear Physics at the Department of Energy, was held November 17–19, 2016 in Bethesda, Maryland. The goals of the workshop were to iden- tify basic and applied research opportunities at the frontiers of nuclear physics that would be made possible by the beam capabilities of an advanced laser Compton beam facility. To anchor the scientific vision to realistically achievable beam specifications using proven technologies, the workshop brought together experts in the fields of electron accelerators, lasers, and optics to examine the technical options for achieving the beam specifications required by the most compelling parts of the proposed research programs. An international assembly of participants included current and prospective γ-ray beam users, accelerator and light-source physicists, and federal agency program managers. Sessions were organized to foster interactions between the beam users and facility developers, allowing for information sharing and mutual feedback between the two groups. The workshop findings and recommendations are summarized below. 1.1. Findings of topical working groups The topical sessions at the workshop focused on five research areas: low-energy quantum chromodynamics (QCD), nuclear structure, nuclear astrophysics, fundamental symmetries, and applications. The advanced accelerator and light source technologies working group rec- ommended laser Compton scattering inside an optical cavity as the primary technology for the next generation γ-ray sources. The working group concluded that two γ-ray beam facil- ities are needed to meet the beam requirements of the research presented in the topical ses- sions at the workshop (1) a medium-energy source with γ-ray beams in the range of 25 and 400 MeV, and (2) a low-energy source capable of delivering γ-ray beams from 1.5 to about 30 MeV. The topical sessions at the workshop focused on five main subjects: nucleon struc- ture and low-energy QCD, nuclear structure, nuclear astrophysics, fundamental symmetries, and applications. Probing nucleon structure requires medium-energy photons, i.e., γ-ray beams with energies from about 60 to 350 MeV, while the four other research topics would use γ-ray beams at energies below about 20 MeV. Descriptions of the research opportunities for all five are in the science sections of this document. Summaries of the findings of the topical working groups are given below. 4 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al 1.1.1. Medium-energy NGLCGS facility. Understanding the emergence of hadron structure and the nuclear force in terms of QCD are key questions at the frontier of nuclear physics. These phenomena are a consequence of quarks and gluons interacting at confinement-scale distances, where color forces are strong. The beams at a medium-energy NGLCGS facility will enable measurements that uniquely probe hadron structure and hadronic interactions in this non-perturbative regime of QCD. The experimental program will investigate LEQCD phe- nomena with unprecedented precision in the photon energy range from about 60 MeV to the nucleon-to-delta(1232) transition. The initial research program at such a facility will two main components: (1) Compton scattering to investigate nucleon structure and (2) photopion pro- duction to investigate the QCD origins of isospin symmetry breaking in the strong nuclear force. Both program components will require high-density polarized targets. • Low-energy QCD and nucleon structure: the beam capabilities at a medium-energy NGLCGS will enable high-precision measurements of the scalar and spin nucleon polar- izabilities by Compton scattering from unpolarized and polarized targets. Such measure- ments, together with advances in calculations using lattice QCD (LQCD) and QCD-based effective field theories, will explore the QCD origin of nucleon structure associated with the collective response of nucleons to electromagnetic impulses with unprecedented sen- sitivity. For example, the experimental programs at the NGLCGS facility are proposed to improve the statistical accuracy of the nucleon spin polarizability measurements by more than a factor of 10 from current values and to impact the dynamical scaler polarizabilities of the neutron and proton with precision from energies below to above the pion production threshold. • Low-energy QCD: photopion production: the high intensity and high energy resolution of beams at a medium-energyNGLCGS facility will enable determination of electromagnetic s-wave and p-wave amplitudes to the π0 production with sufficiently high precision to observe charge-symmetry violation. Photoproduction of pions at energies near the reaction threshold provides mechanisms for investigation of the QCD origin of breaking of isospin symmetry in strong nuclear interactions. 1.1.2. Low-energy NGLCGS facility. The beams at low-energy NGLCGS facilities will enable high-accuracy measurements in nuclear structure, nuclear astrophysics, fundamental symme- tries, and γ-ray beam applications in nuclear and homeland security. The research opportunities in each area are summarized below: • Nuclear structure: the beams at an advanced γ-ray source will enable systematic studies of weak collective dipole and quadrupole nuclear excitations with unprecedented precision. Such studies will provide nuclear structure details and information about the symmetry energy of the nuclear equation of state (EOS) that are difficult to obtain by other means. The high γ-ray beam intensities will enable mapping of states accessed through M1 tran- sitions from the ground state in nuclei with a level of detail and breadth that will contribute to modeling of coherent neutrino-nucleus scattering and to calculating nuclear matrix ele- ments for neutrinoless double-beta decay. Also, a next-generation γ-ray beam facility will enable new exclusive measurements of photodisintegration of few-nucleon systems with a precision that provides sensitivity to three-nucleon interactions. • Nuclear astrophysics: new γ-ray beam capabilities will enable measurements that con- tribute broadly to open questions in nuclear astrophysics, questions such as big-bang nucleosynthesis, helium burning in massive stars, and synthesis of heavy nuclei. One of the most important reactions in stellar modeling is 12C(α, γ)16O. The rate of this reaction relative to the carbon forming reaction 3α→ 12C determines the fate of massive stars. 5 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al The measurement of the rate of the 16O(γ,α)12C reaction at energies approaching the temperatures at the core of stars is a grand challenge in nuclear astrophysics. • Fundamental symmetries: the beams at an advanced low-energy γ-ray beam facility will enable measurements of parity violating photodisintegration of few-nucleon systems. In particular, a measurement of parity violation (PV) in deuteron photodisintegration near threshold is sensitive to a nucleon–nucleon (NN) PV amplitude that is not accessible using other systems. Such measurements sample the short-range part of the NN inter- action, providing unique quantities for comparison with calculations using lattice gauge theory. • Applications: the intense mono-energetic γ-ray beams at low-energy NGLCGS facilities will enable photonuclear reaction measurements important for technologies and tech- niques used in homeland security and nuclear safeguards. Programs to develop field- deployable system will benefit by having a target areas equipped for evaluating concepts for γ-ray beam interrogation of cargo, nuclear fuel, and special assemblies at these facil- ities. The new beam capabilities at advanced γ-ray sources will also create opportunities for applications in medicine. 1.2. Findings of advanced accelerator and light-source technologies working group Details of the various technology options considered are presented in the chapter on accelerator concepts for NGLCGS facilities. A summary of the findings of the working group is below. It is unlikely that a single γ-ray beam source can meet the requirements of both the low- energy (Eγ < 20 MeV) and medium-energy (Eγ > 60 MeV) parts of the field as described in the working group summaries above and in the science sections of this document. In the options discussed, the γ rays are produced by Compton scattering of electrons from photons in an optical cavity that is pumped with an external laser. Two options for the electron beam accelerators for the low-energy γ-ray source were considered: a storage ring and an energy- recovery linac with superconducting RF (SRF) cavities. For the medium-energyγ-ray source, a storage ring was the primary option. There is confidence that a high-quality electron beam with low emittance and low energy spread can be maintained in modern storage-ring lattices, thereby enabling production of γ-ray beams with low energy spread. The new facility construction cost of the storage-ring option for either a low-energy or medium-energy next-generation Compton γ-ray source will be about $150M. The working group cautions that this estimate is extremely uncertain; it is intended only to set the scale within about a factor of two. For the low-energy sources, less expensive options, such as upgrades to existing facilities, were also discussed. 1.3. Recommendations The working groups through consensus make the following recommendations. The order of this listing is not prioritized. • High intensity γ-ray beams with circular and linear polarizations will be produced at next-generation γ-ray sources by Compton scattering of photons from relativistic elec- trons inside a high finesse optical cavity. The optical cavity will be pumped by a laser system with high precision control of beam polarization. The electron beam accelerator will use proven technologies, either a storage ring or an energy-recovery linac. The main technological challenge is the production of reliable optical cavities with the technical specifications required for the next-generation γ-ray sources. The highest priority R & D work for the next-generation γ-ray source should be the development of high finesse optical cavities and the associated laser and optical systems. 6 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al It is important for this work to include testing and optimization of the cavity under γ-ray production conditions. • For photopion production experiments, small angle electron scattering (virtual photon tagging) with intense electron beams was discussed as an alternative to measurements using tagged bremsstrahlung sources or γ-ray beams produced by Compton scattering. This alternative technique would be implemented with high-current electron beams, pos- sibly in a storage ring, and with thin targets that allow detection of the low-energy charged particles produced in the reaction. This method promises to be much more effective than conventional photon tagging techniques. R & D should be supported to develop an alternative to Compton scattering for pro- ducing γ rays with energies above the pion-production threshold. A system for vir- tual photon tagging in small-angle electron scattering is a promising candidate for such studies. • Low-energy QCD phenomena will be explored at NGLCGS facilities with unprecedented precision at energies from below the pion-production threshold through the delta(1232) resonance region. The core experimental programs will involve Compton scattering to study the spin-dependent electromagnetic response of nucleons and near-threshold photo- pion production. Both programs require polarized beams and targets. The measurements enabled by NGLCGS facilities, together with advances in calculations using LQCD and QCD-based effective field theories, will explore the QCD origin of nucleon structure and charge-symmetry breaking. Investments in polarized targets are needed to prepare for experiments at the NGLCGS facilities. Investments in nuclear theory are needed to support the planning and analysis of low- energy QCD experiments at the NGLCGS facilities. • To ensure full realization of the scientific potential of an NGLCGS it is crucial that a strong theory effort in this area be maintained as the machine concept is developed and imple- mented. This will facilitate planning for experiments that optimally realize the scientific goals articulated in this document and continue the strong tradition of synergy between theory and experiment in low- and medium-energy photonuclear physics. Mechanisms that will ensure there is a strong international theory community working on this physics that is fully engaged include workshops with small lead times and durations of up to a month and partial support for postdoctoral researchers and/or graduate students work- ing on theory projects related to the NGLCGS. In addition, computing resources can help address the challenge of solving QCD in this regime and contribute to the 2015 long-range plan’s recommendation of ‘new investments in computational nuclear theory that exploit US leadership in high-performance computing’. • The 12C(α, γ)16O reaction helps regulate the efficiency of helium burning in massive stars and ultimately determines the mass of the iron core in the incipient supernova. The uncer- tainty in the measured cross section for this reaction substantially limits our understanding of the late stages of the life of massive stars and the details of the nucleosynthesis under the explosive conditions of supernovae. The beams at NGLCGS facilities will enable measure- ments of the 16O(γ,α)12C reaction that determine the reaction rate of α-particle capture on 12C at center-of-mass energies lower than have been achieved with other techniques. Measurements of angular distributions require thin targets to allow detection of the α par- ticles along with charged-particle detectors with wide angle coverage. Options include 7 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 1. Schematic diagram for a coherent theoretical treatment of nuclear systems starting from high energies (at the bottom), where perturbative QCD can be applied, and ending with low-energy nuclear phenomena, where mean-field potential models are most efficient. Adapted from [18]. CC BY 4.0. time projection chambers with optical and charge readout and silicon strip detectors with thin solid or gas targets. Investments in active targets, such as time projection chambers, are needed to carry out the highest impact nuclear astrophysics measurements at NGLCGS facilities. 2. Introduction Over the last decade, substantial progress has been made in developing formalisms and com- putational methods that contribute to theoretically coherent descriptions of nuclear phenomena with origins in QCD. An ultimate goal would be to describe nuclear matter, over wide distance and energy scales, with a QCD Lagrangian. Achieving this aim will likely require the effort of generations of scientists, as is often the case for grand challenges in science and technol- ogy. Figure 1 shows a schematic diagram of a plausible hierarchy for organizing the theoretical treatments of nuclear systems spanning a variety of phenomena. The scheme starts at high ener- gies, where the most fundamental degrees of freedom are quarks and gluons, and progressively evolves in complexity. This diagram is intended only to represent gross features that should be included in a coherent picture of strongly interacting matter. At the top of the diagram, mean- field potentials that describe nuclear structure properties, the collective motion of nuclei, and nuclear reactions should be derived from residual strong interactions between nucleons. Ab initio calculations of the structure of light nuclei [1–4] and few-nucleon reaction dynamics [5] enable refinement of two-nucleon and multi-nucleon interactions using effective degrees of freedom. Current theoretical tools for describing the strong nuclear force (two- and three- nucleon interactions) include semi-empirical potential models (see, e.g., references [6–10]), effective field theory (EFT) formulations of two-nucleon (2N) and three-nucleon (3N) interac- tions [11–14], and LQCD calculations of few-nucleon systems (see, e.g., references [15, 176]). Descriptions of the collective properties of nucleons in terms of effective field theories (see, e.g., references [11, 12]) and LQCD (see, e.g., references [17, 176]) are steps toward bridg- ing gaps between QCD and theoretical treatments of few-nucleon systems. The remainder of this chapter introduces photon beams and their applications in nuclear science. References are provided in the later chapters, where these subjects are developed in detail. 8 https://creativecommons.org/licenses/by/4.0/ J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Photon beams are a highly-selective probe of the electric-charge and magnetism distribu- tions of nuclei and nucleons. Measurements of photon scattering and photon-induced reactions provide information on the collective response of the internal degrees of freedom of compos- ite nuclear objects, as depicted in figure 1. For almost a century, bremsstrahlung γ-ray beams have been the workhorse for investigating nuclear and nucleon structure and for measuring photon-induced nuclear reactions. These beams have the advantage of allowing measurements to be performed over a broad energy range in a single experiment, thereby providing informa- tion about the energy dependence of phenomena. However, the continuous energy nature of bremsstrahlung γ-ray beams has the disadvantage of limiting measurement sensitivity due to backgrounds created by the photons with energies outside the region of interest. A comple- mentary tool is monoenergetic γ-ray beams, such as those created by laser Compton photon sources. The narrow energy bandwidth of laser Compton γ-ray beams enhances the signal-to- background ratio in comparison to what is achievable using bremsstrahlung beams. In addition, the high beam polarization available for both linear and circular polarization in the γ-ray beams produced by laser Compton photon sources enables unambiguous determination of the spin and parity of excited nuclear states and facilitates studies of the spin structure of nucleons. Over the last several decades, laser Compton γ-ray beam facilities have provided intense polarized and nearly mono-energetic γ-ray beams for research programs in basic and applied nuclear physics. The facilities include GRAAL at the European Synchrotron Radiation Facility in Grenoble, LEGS at Brookhaven National Laboratory, LEPS at the SPring-8 facility, NewSUB- ARU at the University of Hyogo, and HIγS at the Triangle Universities Nuclear Laboratory. New laser Compton γ-ray beam facilities under construction include ELI-NP in Romania and the γ-ray beam line at the SSRF in China. The current generation of laser Compton γ-ray beam sources are based mostly on either a single-pass light pulse from an external laser scat- tered off a single-pass electron beam bunch or a single-pass laser pulse scattered off electron beam bunches circulating in a storage ring. The technique of intracavity Compton scattering of electrons circulating in a storage ring that is employed at HIγS is indicative of the technologies to be used in next-generation laser Compton γ-ray sources. Combining the high beam current of storage rings with the high photon density inside optical cavities can potentially produce monoenergetic γ-ray beams with intensities more than three orders of magnitude higher than HIγS, which is currently the most intense laser Compton source in the world. This whitepaper describes research opportunities that would be opened up by the γ-ray beam capabilities at next-generation laser Compton γ-ray beam facilities. Its content is based mainly on discussions and findings of The International Workshop on the Next Generation Laser Compton Gamma-Ray Beam Facility that was held November 17–19, 2016 in Bethesda, MD. The workshop brought together both researchers from the international low-energy and medium-energy nuclear physics communities, along with accelerator physicists and experts in optics. The accelerator and optics experts anchored this exercise into the constraints of beam performance parameters achievable with technologies that can be implemented in the coming decade. For both practical reasons and scientific considerations, the upper energy reach of the γ-ray source was limited to 500 MeV. Within these boundaries, the broad areas of nuclear physics considered correspond to the upper three panels of figure 1. It is probably fair to state that most participants began the workshop with the belief that one laser Compton source would be capable of serving both low-energy experiments, with γ-ray beam energies (Eγ) below about 20 MeV, and medium-energyexperiments, with Eγ > 60 MeV. The questions addressed in low- energy and medium-energy γ-ray experiments have considerable intellectual overlap and the intense, monoenergetic γ-ray beams of an NGLCGS would facilitate progress in both regimes. However, the γ-ray source considerations mean that research at low and medium energies likely must be carried out at two different facilities. For this reason, the discussion of the research 9 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 2. Illustration of the research areas impacted by advanced γ-ray source technolo- gies. The upper half of the graphics represent the main research at γ-ray beam energies from about 60 MeV to 350 MeV. The lower half represents the research areas covered at energies below 20 MeV. opportunities created by laser Compton γ-ray sources is organized by the two energy ranges of the facilities presented at the workshop, Eγ � 20 MeV and 60 MeV � Eγ � 350 MeV. The areas explored in each energy region are illustrated in figure 2. While low-energy QCD is mentioned explicitly only in the upper half of the circle it is also strongly connected to the proposed research on PV in hadronic systems and the studies of few-nucleon systems and light nuclei that compare ab initio calculations to data. Indeed, since all nuclear dynamics is underpinned by QCD low-energy QCD in fact pervades all the opportunities that would be created by an NGLCGS. These include PV in hadronic systems and studies of few-nucleon systems and light nuclei using ab initio calculations. Details of the opportunities are presented in the topical chapters that follow. A synopsis of each topical discussion is given below. The main opportunities at energies below 20 MeV are presented in section 3 and are in the areas of nuclear structure (including photon-induced fission), nuclear astrophysics, and PV in few-nucleon systems and light nuclei. Studies of collective modes of excitation can pro- vide information about short-range correlations between nucleons in nuclei and can reveal features of the nuclear EOS. The giant dipole resonance (GDR) dominates the nuclear col- lective response and is well understood. The rotational (rigid rotor) and vibrational (scissor) modes at energies below about 4 MeV have been extensively studied and are described by well-established models. The nuclear excitations on the low-energy tail of the GDR, above about 5 MeV, and below the particle separation energy are not well characterized. Much of the difficulty in understanding the nature of the excitations in this energy region is associ- ated with distinguishing between effects due to the GDR and those due to other mechanisms. The dipole excitation strength in excess of the GDR tail is referred to as the pygmy dipole resonance (PDR). The generally accepted mechanism for the PDR is the vibration of a neu- tron skin off an isoscalar core. If this picture is correct, a study of its strength as a function of isospin and A should provide information about the density dependence of the symmetry energy term in the nuclear EOS. The beam intensities available at the next-generation laser Compton γ-ray sources will enable nuclear resonance florescence (NRF) measurements on nuclei at extremes of isospin where the natural abundance is low and consequently target material is 10 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al sparse. In addition, the beam at next-generation laser Compton γ-ray sources will enable thor- ough measurements of the M1 γ-ray strength in nuclei. Special attention will be given to nuclei in the mass vicinity of isotopes used in double-beta decay experiments. The beams at the next- generation laser Compton γ-ray source will also enable photon-induced fission measurements at energies near threshold, where the cross section is low, and the study of low-yield frag- ments. Such experiments will provide new information about the potential energy surface that governs the evolution of the fission process. Examples of the initial nuclear structure research made possible by the NGLCGS facilities are described in section 3.1. The beams at the next-generation laser Compton γ-ray sources will enable strategic mea- surements of a variety of NRF and photon-induced nuclear reactions that contribute to model- ing p-process, s-process and r-process nucleosynthesis. However, the highest impact contribu- tion to nuclear astrophysics will be in determining the reaction rate for alpha-particle capture by carbon via the 12C(α, γ)16O reaction in massive stars during the alpha-burning stage. The goal for all measurements of this reaction is to determine the cross section at a center-of-mass energy of 300 keV. In γ-ray beam facilities, the time reversed reaction 16O(γ,α)12C will be measured, and detailed balance will be applied. This measurement will be the flagship nuclear astrophysics experiment at advanced γ-ray sources, and, as such, should be measured using a variety of techniques. Nuclear reaction rate measurements enabled by the NGLCGS facilities that are at key points in stellar nucleosynthesis networks are discussed in section 3.2. In the area of fundamental symmetries, the primary focus will be on studying PV in hadronic systems. Such PV is a measure of the weak interaction inside systems of strongly interacting particles. The ultimate goal is to measure PV in photodisintegration reactions of few-nucleon systems, such as 2H and 3He. Because these measurements must be performed near the reaction threshold energy, and because the parity violating asymmetry is extremely small (∼10−7), these experiments make the most stringent demands on the source-performance parameters. That is, a γ-ray source that delivers beam intensities an order of magnitude below what is required for these experiments will enable all the nuclear structure and nuclear astrophysics discussed in this whitepaper. Measurements of parity violating photoabsorption asymmetries on parity doublets in light nuclei will be used to assess beam and instrumentation asymmetries in the early stage of developing the capabilities for performing 10−7 asymmetry measurements. For this purpose, nuclei with photoabsorption asymmetries of 10−4 to 10−3 will be selected. Hadronic PV (HPV) studies using photon-induced reactions are described in section 3.3. In addition to the basic science research enabled by the NGLCGS facilities, these γ-ray sources will provide capabilities that support technology and techniques development work for nuclear security. How such facilities might be used in efforts to advance γ-ray beam interrogation systems is outlined in section 3.4. QCD is the fundamental theory of the strong nuclear force. When written in terms of quark and gluon degrees of freedom, it is deceptively simple. Indeed, QCD’s asymptotic freedom guarantees that this form of the theory describes strong interactions at sufficiently high energy. However, in that regime, QCD does not bind quarks and gluons into neutrons and protons. The task of fully and rigorously deriving the presence and properties of neutrons, protons, and nuclei from the Standard Model (SM) remains a grand challenge for physics, requiring us to understand the emergence of new degrees of freedom as the theory becomes strongly coupled. Without such an understanding, treatments of the nuclear force based on the fundamental the- ory will remain elusive, and so too will the ability to model nuclei in a reliable manner that is grounded in QCD. High-intensity beams of polarized γ rays in the energy range of 60–300 MeV provide a unique opportunity to test our understanding of the emergence of neutron and proton structure from the SM. In this regime, a description of experimentally observed phenomena in terms of 11 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al neutrons and protons and their low-lying excitations is efficient, thanks to the chiral symmetry of QCD. The initial research opportunities in nucleon structure and low-energy QCD at NGLCGS facilities are described in section 4. One of the main goals at these energies will be to map out the spin-independent and spin-dependent polarizabilities starting at energies below the pion production threshold, extending over the pion production threshold energy, and contin- uing through the delta(1232) region. These measurements will be carried out using Compton scattering from unpolarized and polarized targets. The other major aim in the medium-energy domain is precision near-threshold pion photoproduction data. Such data would provide the opportunity to extract the s-wave π0p scattering length and so gain a new window on the QCD origin of charge-symmetry breaking. In the limit of vanishing up-, down-, and strange-quark masses, the QCD Lagrangian admits a global chiral symmetry: SU(3)L × SU(3)R. This symmetry is broken spontaneously, which implies the existence of eight pseudoscalar massless Goldstone bosons. Furthermore, since the quark masses are finite (but small), these Goldstone bosons acquire a small mass and are identified with the pions, kaons, and etas. The interaction of these Goldstone bosons with them- selves or with matter fields such as nucleons is weak. This allows for a systematic low-energy expansion in terms of small momenta and quark masses: chiral perturbation theory. Chiral per- turbation theory (χPT) describes meson–meson interactions in this energy domain with high precision. It also describes the interactions of mesons with nucleons at very low energies. Incor- porating the Δ(1232) as an explicit degree of freedom in the theory yields a chiral EFT (χEFT) that describes processes at energies below the chiral symmetry breaking scale (Λ ≈ 1 GeV) via an expansion in a small, dimensionless parameter. This allows reliable quantification of residual theory uncertainties. Thus χEFT exploits the chiral symmetry of QCD in order to rig- orously connect QCD with the phenomenology of nuclear and particle physics. Because χEFT encodes the consequences of the SM for low-energy processes involving photons, pions, and nucleons, it can be used to test whether observed strong-interaction phenomena are consistent with the SM. In particular, χEFT allows us to elucidate the experimental consequences of the pattern of chiral-symmetry breaking in QCD. Accelerator concepts for the NGLCGS are described in section 5. Two options for the accel- erator configuration were discussed for energies below 20 MeV: an energy-recovery linac with SRF cavities or a storage ring. Only the storage-ring option was presented for energies above 60 MeV. In all source configurations, the Compton scattering occurs inside an optical cavity that is pumped by an external laser. The electron source, accelerator structures, and storage- ring lattices that meet the required technical specifications for source performance are robust and can be implemented to order. The situation is not as well established for the optical cav- ity. R & D is needed to develop an optical cavity design that can meet the power, stability, and reliability requirements of the next-generation laser Compton γ-ray source. 3. Nuclear physics research and applications with gamma-ray beams below 20 MeV 3.1. Nuclear structure The landscape for the response of nuclei to electromagnetic radiation as a function of excitation energy is shown schematically in figure 3. The electric GDR is the dominant collective response of nuclei. This mode can be modeled as the oscillation of the proton matter distribution against the neutron distribution. In the classical interpretation, the width of the cross-section enhance- ment associated with the resonance provides information about the damping forces, i.e., the 12 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 3. Diagram of the landscape of the response of nuclei to photon absorption. viscosity of the nuclear matter. The nature of the nuclear response at excitation energies below the GDR around the particle in thresholds is still rather unclear and is the topic of consider- able theoretical and experimental work [19]. The excitation in this region has been observed to be largely electric dipole (E1). This additional structure below the GDR has been shown to be a common feature in the E1 response of atomic nuclei, and is generally referred to as the PDR. The E1 response offers the possibility of studying the EOS of neutron matter [20–25]. Also the γ-ray absorption cross section in the PDR energy region influences nuclear reaction rates in stars [26–29]. In deformed nuclei, the collective nuclear dipole response at excitation energies below the PDR region is mainly due to magnetic dipole (M1) transitions, which can be described as a scissor motion. The (γ, γ ′) reaction, which is referred to as nuclear reso- nance fluorescence (NRF), is the most effective approach for probing these features of nuclear structure. Nuclear fission is a complex process in which the collective motion of nucleons results in a strong change in the shape of the nucleus, ultimately leading to a breakup into fragments. Photon-induced fission can provide unique insight into the evolution of the potential energy surface during the fission process as it progresses from photon absorption through to the scis- sion point. Measurements of fission product yields of isotopes with different half lives provide information on the evolution of the nuclear deformation and breakup of the nucleus during the fission process. The high intensity, nearly monoenergetic, and linearly polarized photon beams at a next- generation laser Compton γ-ray beam facility will enable high-precision photonuclear reaction measurements. Research opportunities for NRF and photofission measurements are described in this section. 3.1.1. Nuclear resonance fluorescence. The nuclear resonance fluorescence (NRF) reaction is one of the work horses in the investigation of the dipole response of atomic nuclei. The method enables the extraction of intrinsic properties of excited states such as spin, parity and transition widths in a model independent way. Furthermore, due to the low momentum transfer of photons, primary dipole transitions are induced, i.e., in even–even nuclei states with J = 1± are populated. This makes NRF an excellent reaction to systematically study the phenomena in the dipole response of nuclei, such as the PDR and the scissors mode over a large range of nuclei. 13 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 4. Example of γγ coincidence spectroscopy. The spectra show projections of LaBr3 × LaBr3 matrices after gating on transitions from the first and second 2+ states in 76Ge. The corresponding level scheme is given on the right-hand side. The scheme is an example for any dipole excited state with arbitrary intermediate levels between the J = 1 excited state and the ground state. Thus far, mostly the decay branch back to the ground state as been investigated with the NRF reaction. However, the detailed decay pattern of excited states is often connected to important details in its nuclear structure. The decay widths to the individual lower-lying excited states or the ground state are directly linked to the corresponding transition matrix elements. Thus, the individual decay channels are sensitive to different components of the nuclear-state wave function. For example, in the case of transitions to lower-lying excited states, the de-excitation takes place via a different component in the wave function than the excitation from the ground state. Therefore, the observation of these transitions and the determination of the branching ratios reveal important experimental information that tests modern nuclear-structure models. The method of γγ coincidences in the spectroscopy of the decay of excited states has been proven to be a powerful tool for determining even small branching ratios to excited low-lying states [30, 31]. The principle is illustrated in the left part of figure 4. After excitation by pho- toabsorption, the high-lying state at excitation energy Ex may de-excite either directly to the ground state with width Γ0 or via an intermediate state with width Γi. By detecting the two emitted γ rays in coincidence, even small branching ratios Γi/Γ0 can be determined with good precision, since the background is strongly suppressed. This technique allows the branching of dipole-excited states to individual lower-lying levels to be determined, thus greatly aiding in building up complex level and decay schemes. An example is shown in figure 4. The right-hand side of the figure provide, the level structure for a measurement on 76Ge, where the decay of the 1+ scissors mode has been investigated [32]. The 1+ state at 3.763 MeV is populated by photo-excitation. Gating on the decays of the 2+1 and 2+ 2 states, as shown on the left side of the figure, allows the order of the γ-ray transitions in the decay to be determined. 14 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al The combination of large-volume LaBr3 and high-resolution HPGe detectors opens the way for γγ-coincidence measurements with sufficient statistical accuracy to track γ-ray decay cas- cades. High-resolution HPGe detectors allow for a separation of individual γ-ray transitions from dipole excited states in regions where the level density is not yet too high. Even in regions with a high level density, individual states can sometimes have substantial excitation strength and are, thus, separable from the otherwise rather continuous spectrum. This provides a way to normalize spectra to complementary data sets obtained using bremsstrahlung beams. Neverthe- less, most dipole-excitation strength in regions of high level density is so strongly fragmented that individual states cannot be resolved. In this case, the large-volume LaBr3 detectors have a significant advantage over HPGe detectors because of their higher detection efficiency. This high efficiency allows the fragmented strength in the beam region to be separated from the background and enables integrated branching strengths to be determined. In deformed nuclei, where the first-excited-state energies are very low, decays to the ground state and the first excited state are close in energy, posing a challenge to the accelerator team to deliver beams with high intensity and low energy spread to the target. Deconvolution methods have recently been developed and applied to aid in extracting decays out of the detector response (figure 5), see e.g. [33, 34]. With the combination of HPGe and LaBr3 detectors, the high efficiency of the latter pro- vides sufficient statistics to observe branching decays from higher-lying states, either in low- resolution LaBr3 or even in high-resolution HPGe spectroscopy, as demonstrated in [30, 31]. Especially at low energies, electric quadrupole excitation probabilities can be sufficiently large for observation, and can be separated from the dipole strength making use of angular distribu- tion and correlation measurements using arrays of HPGe and LaBr3 detectors. In certain cases even the mixing ratio of M1/E2 transitions can be measured. This recently facilitated a first measurement of the isovector quadrupole strengths in 156Gd [35], for example. The capabilities of the next-generation laser Compton γ-ray source combined with modern γ-ray detectors will enable measurements that address a broad range of physics through photo- excitation studies. The topics include multi-phonon structures such as quadrupole–octupole coupled states and the search for rotational isovector scissors excitations; the investigation of the PDR along with its structural evolution and fingerprints in going from spherical to deformed regions; and detailed spectroscopic studies of the M1 spin-flip resonance. Therefore, the physics topics to be addressed embrace shape coexistence, octupole correlations, isovec- tor excitations, neutron skins, and photon strength functions (PSFs), often with strong rele- vance for the fields of nuclear astrophysics and weak-interaction physics such as neutrinoless double-beta decay and neutrino scattering. 3.1.1.1. Shape coexistence and link with physics with radioactive beams In the context of nuclear structure studies, a direct link between research carried out with γ-ray beams and with radioactive beams should be briefly discussed. Shell structure is a cornerstone in the description of atomic nuclei as many-body quantum systems. The first data on neutron-rich nuclei far from stability have provided evidence that shell structure evolves with neutron excess. For example, some well-known magic numbers disappear while others appear. Progress in the description of these exotic nuclei is due in no small part to the development of new experimental tech- niques that have gone hand in hand with the introduction of new theoretical concepts. It has been shown that effective single-particle energies are significantly modified in neutron-rich systems through the action of the monopole component of the proton–neutron tensor force [36]. Furthermore, multi-particle multi-hole excitations trigger shell evolution as a function of spin and excitation energy [37]. In this picture, the occupation of specific deformation-driving orbitals leads to changes in nuclear shapes and to the possibility of shape coexistence. The 15 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 5. HPGe (top) and LaBr3 (bottom) spectra from singles γ-ray spectroscopy of 128Te at a beam energy of 6.4 MeV. The black histograms show the actual spectra, and the red ones are the result of the deconvolution, which includes correcting for the detector response. The beam profile is indicated by a dashed curve in the upper panel. Reproduced with permission from [33]. region between Z = 20, N = 20 40Ca and Z = 28, N = 50 78Ni provides a good illustration of the synergy between experiments at the national user facilities and measurements with γ- ray beams. For example, triple shape coexistence has recently been discovered in neutron-rich 66,68,70Ni [38]. These 0+ states are the result of multi-particle multi-hole excitations. Some of these levels, in particular those associated with prolate deformation, are understood as proton excitations that are predicted to be present in the stable 60,62,64Ni as well [38], and the evolu- tion of these states’ excitation energy with N is a matter of much theoretical debate. The issue can be addressed with intense γ-ray beams: in a first phase, all the low-spin excitations can be mapped out. Following this discovery phase, information on the wave function of the states will be obtained from state lifetime measurements, and from the determination of decay branch- ing ratios. Furthermore, excitations built on the excited 0+ levels will be delineated and their properties characterized. Other regions of the nuclear chart lend themselves to similar studies. Specifically, shape changes and shape coexistence phenomena, driven by multi-particle multi- hole excitations are predicted to occur in the Sr–Mo–Zr region as well as in the vicinity of doubly-magic 208Pb. 16 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 6. Schematic diagram of nuclear excitations involving multiple phonons and illustrating mixed symmetry and octupole excited states. 3.1.1.2. Multi-phonon and rotational low-lying states At low energies, in spherical or near- spherical nuclei, excited states are often formed by phonon excitations. For example, the first 2+ state is identified with a quadrupole phonon, and the first 3− state with an octupole phonon. Such phonons can be coupled, leading to multiplets of multi-phonon states built from like phonons, or from mixtures of, e.g., quadrupole and octupole phonons. As such, the quadrupole–octupole coupled quintuplet of states, 1− . . . 5− contains an E1-excited 1− state which typically is the first excited 1− state, located at roughly the sum energy of its constituents and carrying most of the E1 strength at low energies. (See the schematic diagram of γ-ray transitions associated with multiple-phonon de-excitation is given in figure 6.) In addition to excitations where protons and neutrons move in phase, there are so-called mixed-symmetry states where at least one pair of protons/neutrons has a different phase [39]. This leads to addi- tional states, such as a quadrupole-excited one-phonon mixed-symmetric 2+ ms state. Again, the phonon coupling between the latter and the symmetric 2+ 1 phonon state leads to a multiplet of mixed-symmetry states, one of which is an M1-excited 1+ state, which is typically the strongest M1 excitation at low energies of roughly 3 to 4 MeV. In rotational nuclei, this state evolves into the well-known scissors mode, where valence protons and neutrons are counter-oscillating in a scissors-like fashion. Different mechanisms forming E1 and M1 excited states at such low energies can, however, compete with one another. For example, spin-flip excitations, where a nucleon is moved from an �± 1/2 orbital to its �∓ 1/2 partner, can generate M1 excitation strengths similar to those of the scissors mode. In the E1 sector, the generation of E1 strength in valence spaces containing either no or a very limited number of opposite-parity orbitals is under much discussion, and there are other possibilities, such as the formation of alpha clusters (see below). Therefore, it is not only important to locate those dipole excited states and to determine their parity and decay strengths, it is also crucial to measure their decay pattern, which contains important information 17 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al on their underlying structure. These tasks are uniquely facilitated by the capabilities at a next- generation laser Compton γ-ray source for measuring decay branches, parities, and relative excitation strengths. Furthermore, in rotational nuclei, band structures are expected on J = 1 band-heads such as the scissors-mode state. Indeed, in recent work [35], a first candidate for the long-sought rotational 2+ sc member of the scissors-mode band has been discovered. In addition, measurements of angular correlations made possible by arrays of γ-ray detectors with large angle coverage, high detection efficiency, and high energy resolution used in combination with linearly polarized monoenergetic γ-ray beams will enable measurements of multipole-mixing ratios. These, in turn, can reveal absolute isovector E2 strengths. High quality data of this type are crucial in constraining effective charges in nuclear dynamics calculations. Another aspect of dipole excited states has recently been found and is connected to weak- interaction physics. While the strong interaction dominates in nuclei, nuclei are also labora- tories for studying the weak interaction through the possibility of β or ββ decay. Of special interest for many experiments, and for the characterization of neutrinos in general, is the search for neutrinoless double-beta (0νββ) decay. The existence of this decay mode would identify the neutrino as a Majorana particle, one which is its own anti-particle. It would provide information about the absolute mass of the neutrino involved in the decay through the ratio of the measured decay rate and the calculated nuclear matrix element for the decay. Since many candidate iso- topes (mother and/or daughter) for 0νββ decay occur in mass regions near transitions from spherical to deformed shapes, their model description is challenging. Furthermore, isovector parameters are seldom known. NRF measurements can be used to extract information on phe- nomena such as shape coexistence and mixed-symmetry states from the observation of the scissors mode and its decays, especially its decays to the potentially mixed-configuration 0+ 1,2 states (see reference [40]). 3.1.1.3. The E1 pygmy dipole resonance. The strongest and best-known E1 mode in atomic nuclei is the GDR. It is a broad resonance structure peaking well above the particle-separation threshold energy. Therefore, in NRF experiments, only the low-energy tail of the GDR below the particle threshold energy is directly observable. Often, this tail is parameterized by a Lorentzian, eventually with modifications such as temperature dependence, e.g., parameteri- zations that directly use the so-called PSFs. PSFs have a direct relation to nuclear astrophysics, since they give a handle on the balance between particle-capture and photodisintegration pro- cesses in stellar nuclear synthesis. Over the last decade, observations have given rise to addi- tional strength, on top of the low-energy tail of the GDR. This added strength is the so-called PDR [19]. This strength, typically a few percent of the GDR strength, appears around the neutron-separation threshold. It would directly influence PSFs and would, therefore, impact nucleosynthesis, which ultimately shapes the abundance patterns of elements [41]. Various studies of the PDR have involved magic or near-magic spherical nuclei. Usually, the origin of the PDR is thought to be an oscillation of a neutron skin against the proton–neutron saturated core. This picture is supported by several microscopic calculations, showing an excess of neutron transition densities near the nuclear surface. However, other possibilities exist. For example, the occurrence of alpha clusters on the surface, which would yield enhanced E1 strength, has been suggested [42]. Additional problems lie in the differentiation of PDR strength from the low-energy tail of the GDR. Alpha scattering seems to be a possible method for accessing this problem [43–45], because, compared to NRF data, low-energy E1 strength appears to split into isoscalar and isovector parts. Other challenges lie in the determination of the total strength. For example, proton- scattering data partly point to significantly more observed E1 strength than photon-scattering data [46]. This may, at least in part, be due to branching transitions from dipole-excited states 18 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al and many weak unresolved transitions, which would have been missed in earlier NRF experi- ments using bremsstrahlung beams. At the next-generation γ-ray source, the focus will be on identifying the strength distribution (state-by-state or average), measuring the parities as well as branching behavior of the excited states. From these data, more stringent conclusions can be drawn on the nature of the PDR, its overall strength, and therefore also on the determination of PSFs. In addition, the Brink hypothesis (the excitations that are built on the ground state should also be built on any excited state), which is part of the basis of the statistical model, can be tested in a model independent way using γ–γ decay spectroscopy, as has been recently demonstrated [47]. A further challenge lies in the deformation degree of freedom. In the end, most nuclei on the chart of the nuclides are deformed, but our information on the evolution of the PDR toward deformed nuclei is sparse at best. Therefore, the experimental program should extend to address this problem. Indications of a potential splitting of the PDR into two parts with different K quantum number have recently been observed in proton-scattering on 154Sm [48]. This would be similar to the splitting of the GDR with respect to the two (or even three) axes of the nuclear body. However, the high degree of fragmentation in deformed nuclei renders a state-by-state analysis meaningless. Therefore, measurements that track the cascade of γ-ray decays are most useful. In addition, the very low energy of the first excited state in deformed nuclei requires a reduced energy spread of the photon beam compared to existing facilities in order to enable a clean separation of the decay intensity to the first excited state and the ground state. Such measurements over sizable isotope chains will be made possible by the beam capabilities at the next-generation laser Compton γ-ray beam facilities. 3.1.1.4. Alpha-cluster excitations. Below the particle-separation threshold, two underlying structures of E1 excitations have been intensively studied in the last decade, the octupole modes [49, 50] and the PDR [19]. For both modes, a non-uniform distribution of protons and neutron generates E1 transitions at lower energies. As pointed out in earlier sections, these two modes could have substantial impact on nuclear structure models and nuclear synthesis calculations. Another mode giving rise to enhanced E1 strength at lower energies is the oscillation of an alpha cluster relative to the remaining bulk nuclear matter [42]. For light nuclei, alpha clus- tering is well-established [51], and its implications for the E1 strength have been discussed. Also, in heavy isotopes such as 212Po, strong indications of a 208Pb + α system have been observed [52]. Recently, the existence of alpha clusters in heavy nuclei was supported by adding four-particle correlations to shell-model calculations [53]. Because the formation of alpha clusters provides interesting insights into the formation of bosonic clusters in strongly coupled fermionic systems, identifying new signatures of alpha clustering in heavier nuclei is of general scientific interest. Several enhanced E1 transitions were observed in the neodymium chain by (γ, γ′) exper- iments up to 4 MeV [54–56]. To shed light on the origin of the low-lying J = 1− states, spdf interacting-boson-model (IBM) calculations have been performed [57]. It has been pro- posed that the p-boson is related to α-cluster configurations. By identifying the basis states by means of their boson contents |[ns], [np], [nd], [n f ]〉, the ratio np/n f permits an assignment of quadrupole–octupole or p-boson character to be associated with the excited states. The results of these calculations suggest the presence of alpha-cluster modes occurring at the surface of nuclei with atomic masses just above magic numbers. For example, in IBM calculations for 144Nd, a remarkable increase of p-boson contributions has been observed for energies up to the neutron-separation threshold. 3.1.1.5. The M1 spin-flip resonance. Although, overall, much less M1 strength is expected in the region where the PDR occurs, there should be significant strength due to the spin-flip 19 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Table 1. Suggested beam parameters for NRF experiments. Parameter Value Energy 2–20 MeV Flux (γ/s) 109 at 1% FWHM Polarization Linear Diameter 10 mm on target Beam repetition rate Few MHz Beam pulse width <1.0 ns resonance. The exact energy of this resonance depends on the underlying shell structure, but typically it coincides with the onset of the major E1 strength distribution. Knowledge about this M1 resonance is sparse, and present parameterizations are based on little available data. The challenges at hand are the identification of M1 strength within the sea of E1 excited states. In cases of low fragmentation, this can be done with a state-by-state analysis of excited-state parities, but at high fragmentation, one has to choose an average approach. The best way to filter out M1 strength in those energy regions is the use of photon-scattering with a linearly polarized beam, such as those available at laser Compton γ-ray sources. The impact of such higher-lying M1-excited states has different facets. On the one hand, they will provide constraints to microscopic calculations, since the occurrence of spin-flip tran- sitions involving particles moving between �± 1/2 partner orbitals, usually across a major nuclear shell. In addition, states excited by a spin-flip transition are those which are popu- lated through Gamov–Teller transitions in β decay. On the other hand, such 1+ states will directly influence PSFs in even–even nuclei, since decays between 1− (GDR/PDR) states and the spin-flip states are allowed, changing the overall shape of the PSFs. More reliable data on the M1 response can, therefore, serve as a test of parity asymmetry. (Usually the same number of positive and negative parity states is assumed.) Again, M1 strength, this time at energies above the scissors mode, can prove important for neutrino physics. Specifically, there are detectors, such as the molybdenum-based MOON detector [58], which are in the planning and construction phases and are expected to serve, not only for the detection of potential 0νββ decay events, but also for the direct detection of neutri- nos through inverse ‘β decay’ by the capture of neutrinos. Another facet of neutrino detection lies in the excitation of 1+ states by neutrinos [59]. Hence, the M1 response of relevant nuclei plays an important role in the characterization of such detectors and also in rate estimates using nuclear models such as the quasiparticle random-phase approximation. The beam requirements for carrying out the nuclear structure research using NRF described in this section are given in table 1. 3.1.2. Photon-induced nuclear fission. Nuclear fission is a highly exothermic and strongly collective nu clear process in which most of the energy is released through the kinetic energy of the ejected fission fragments. The evolution of a fissioning system proceeds from the initial impact of the incident particle through the intermediate saddle point(s), then through scission, and finally to the configuration of separated fission fragments. This evolution is governed by a multi-dimensional potential-energy surface (PES) and by the shell structure of the fragments. Development of reliable theoretical models of nuclear fission is important for basic research and for the development of nuclear energy and nuclear security technologies. For these pur- poses, data for different types of observables are needed, including fission product yields over wide ranges of masses and half lives, as well as the kinetic energy and angular distributions of the emitted fragments and neutrons. 20 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al To first approximation, the fission barrier of a heavy nucleus can be studied by measuring the fission probability as a function of the excitation energy. By comparing the experimentally determined fission probabilities to the results obtained with the WKB approximation, the shape of the fission barrier (its height and curvature parameter) can be determined. Significant progress has been made in studying the multiple-humped fission barrier land- scape of actinides, where a strongly deformed deep third minimum in the potential landscape was established [60, 61]. This is illustrated in figure 7, where horizontal dashed lines show transmission resonances in the superdeformed (SD) second minimum and in the hyperde- formed (HD) third minimum. One expects that the HD minimum in a cluster description consists of a rather spherical 132Sn-like component with magic neutron and proton numbers N = 82 and Z = 50. Recent theoretical results [62, 63] highlight the role of shell corrections in the prominence of the third minima for thorium and light uranium isotopes. There is a clear trend suggesting that lower N values produce larger neutron shell corrections and, thus, more prominent third minima. This is in stark disagreement with experimental results. Using the 231Pa(3He, df ) reaction, Csige et al [64] studied sub-barrier fission in 232U and inferred a fission barrier with a well-formed third minimum that does not agree with the theoretical predictions. Pronounced third minima have also been inferred to exist through observation of fine struc- ture in the cross section of transfer-reaction-induced fission or through sub-barrier photofis- sion cross-section measurements. There is particular interest in identifying resonances in the third minimum, where very large deformations cause the GDR to split into two components with a low-lying oscillation along the long symmetry axis with a typical excitation energy of 4–5 MeV. It is expected that these resonances may have a significantly enhanced γ-ray width Γγ of about 100 eV in the population of the third minimum, while, for the second minimum, one expects a γ-ray width of only about 1 eV. E1 resonances excited from the ground state will have about the same absolute excitation strength as resonances in the third minimum, but resonances in the third minimum will be much broader, with a total width of about 1 keV. Photofission measurements enable the selective investigation of extremely deformed nuclear states in the light actinides and can be utilized to better understand the landscape of the multiple-humped PES of these nuclei. The selectivity of these measurements stems from the low and reasonably well-defined amount of angular momentum transferred during the pho- toabsorption process. High-resolution studies can be performed on the mass, atomic number, and kinetic energy distributions of the fission fragments following the decay of well-defined initial states in the first, second and third minima of the PES in the region of the light actinides. The beams available at the next-generation laser Compton source facilities will enable high fidelity studies of the PES using transmission resonance spectroscopy, as well as studies of heavy clusterization in the actinides, and of rare fission processes such as ternary fission. Each of these opportunities is discussed in this section. 3.1.2.1. Transmission resonance spectroscopy in photofission. The approach to investigating extremely deformed collective and single-particle nuclear states of the light actinides is based on the observation of transmission resonances in the prompt fission cross section. Observing transmission resonances as a function of the excitation energy allows the identification of the excitation energies of the SD and HD states. Moreover, the observed states can be ordered into rotational bands, with moments of inertia proving that the underlying nuclear shape of these states is indeed an SD or an HD configuration. For the identification of the rotational bands, the spin information can be obtained by measuring the angular distributions of the fis- sion fragments. Furthermore, the PES of the actinides can be parameterized very precisely by analyzing the overall structure of the fission cross section, and by fitting it with the results of nuclear reaction code calculations. 21 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 7. Schematic overview of the multiple-humped fission barrier in actinide iso- topes, along with the corresponding nuclear shapes. The lower part shows a cut through the potential energy surface along the fission path, revealing an SD second minimum at an axis ratio of 2:1 and an HD third minimum at an axis ratio of 3:1. This figure, includ- ing the energies and locations of the saddle points and minima were reprinted from [60], Copyright (2002), with permission from Elsevier. In the upper part, the corresponding nuclear shapes are displayed as a function of the quadrupole and octupole degrees of freedom. So far, transmission resonances in the light actinides have been studied primarily in light- particle-induced nuclear reactions. These studies do not benefit from the same selectivity found in photonuclear excitation and, consequently, they are complicated by a statistical population of the states in the second (and third) minimum. These measurements have also suffered from a dominant prompt-fission background. By contrast, the next-generation laser Compton γ-ray sources will enable the identification of sub-barrier transmission resonances with very low integrated cross sections down to Γσ ≈ 0.1 eVb and in so far unexamined nuclei. The narrow energy bandwidth of the next generation γ-ray sources will enable a significant reduction of the presently dominant background from non-resonant processes. Besides exploring the level structure in the second and third minima of the fission barrier of the light actinides, the harmonicity of the potential barrier can also be examined, and the parameters of the fission barrier can be extracted. Such fission barrier parameters are crucial inputs for cross-section calculations in the thorium–uranium fuel cycle of fourth-generation nu clear power plants. The selectivity of the photofission measurements allows high-resolution investigation of fission resonances in photofission in the second and third minima of the fission barrier. Detailed studies of SD and HD states via transmission resonance spectroscopy is rel- evant also for achieving much cleaner energy production by an efficient transmutation of the long-lived, and most hazardous radioactive components of nuclear waste, and by controlling the fission process through using entrance channels via HD states. 22 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Table 2. Suggested beam parameters for the photofission research program. Parameter Value Energy 5–20 MeV Flux (γ/s) 109 at 3% FWHM Polarization Linear and circular Diameter 10 mm on target Beam repetition rate Few MHz Beam pulse width <1.0 ns Macropulse width 1 μs to 10 s Macropulse repetition rate 0.1 Hz to 500 kHz 3.1.2.2. Photofission as a probe of heavy clusterization in the actinides. Theoretical consider- ations suggest that in a cluster description, the HD configuration of a light actinide consists of a spherical 132Sn-like component with magic neutron and proton numbers N = 82 and Z = 50, respectively, complemented by an attached, elongated second cluster of nucleons. Since the fission-product mass distribution is distinctly determined by the configuration at the scission point, and the third minimum is very close to the scission configuration, it is expected that the mass distribution, following the decay of an HD nucleus, will exhibit a pronounced asymmet- ric structure. However, such a dramatic effect of the shell structure has not been observed so far due to the fact that available studies used particle-induced fission. Brilliant quasi-monoenergeticγ-ray beams will enable high-resolution investigations of the mass, atomic number, and kinetic energy distributions of the fission fragments following the decay of states in the first, second and third minima of the PES in the region of the light actinides. In these measurements, the heavy clusterization and the predicted cold valleys of the fission potential can be studied for the first time. Data on the heavy cluster formation will provide valuable information on the fission dynamics. 3.1.2.3. Investigation of rare fission modes: ternary photofission. So far, information on ternary and more exotic fission modes has been deduced from neutron-induced and spontaneous fission experiments. Ternary particles are released very close to the scission point, thus providing valuable information on both the scission-point configuration of the fissioning nucleus and the dynamics of the fission process. However, ternary photofission has never been studied due to the very low cross section of the reaction channel. In such studies, the geometry can be fixed by using polarized γ-ray beams, which is a clear advantage over neutron-induced or spontaneous fission experiments. Moreover, by employing quasi-monoenergeticγ-ray beams, excitation-energy correlations of the ternary fission process can be explored with good resolution. These experiments investigate open problems such as the mechanism of ternary particle emission, the role of the deformation energy and of the spectroscopic factor, and the possible formation of heavier clusters. It will be interesting to measure light-particle decay in photofission and to search for the predicted enhanced α decay of SD and HD states of the light actinides. The availability of brilliant quasi-monoenergetic γ-ray beams will make it possible to study the angular distribution of the ternary particles. These, in turn, will provide important spectroscopic information on the fissioning system. The beam requirements to carry out the photofission research program described in this section are given in table 2. 23 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Table 3. Suggested beam parameters for the nuclear astrophysics program. The beam parameters required to carryout the 16O(γ, α)12C measurements are the most demanding in this research area. In addition to the listed beam attributes, another important require- ment is that the beam on target have a very small bremsstrahlung component, in order to minimize backgrounds caused by γ-ray-induced particle production. Parameter Value Energy 2–20 MeV Flux (γ/s) 1011 at 1% FWHM Polarization Linear and circular Diameter 10 mm on target Beam repetition rate Few MHz Beam pulse width <1.0 ns 3.2. Nuclear astrophysics Many questions in astrophysics require a detailed understanding of stars and stellar properties, thus challenging stellar models to become more sophisticated, quantitative, and realistic in their predictive power. This in turn requires more detailed input, such as thermonuclear reaction rates and opacities, and a concerted effort to validate models through systematic observations. Consequently, the study of nuclear reactions in the Universe remains at the forefront of nuclear physics and astrophysics research. Nuclear reactions generate the energy in stars and are responsible for the synthesis of the elements. When stars eject part of their matter through various means, they enrich the inter- stellar medium with their nuclear ashes and thereby provide the building blocks for the birth of new stars, of planets, and of life itself. Element synthesis and nuclear energy generation in stars are the two primary research topics in nuclear astrophysics. Both require accurate knowledge of charged-particle- and neutron-induced nuclear reactions that take place in the hot stellar plasma. It is remarkable how the quantum mechanical nature of atomic nuclei influence the macroscopic properties of stars. The DOE report The 2015 Long Range Plan for Nuclear Science: Reaching for the Horizon organizes nuclear astrophysics into five broad topical areas: (1) the origin of the elements, (2) the life of stars, (3) the death of stars, (4) the matter of neutron stars, and (5) connections: dark matter, QCD phase diagram, weak interactions and neutrinos. The beams available at the next- generation laser Compton γ-ray source facilities will enable measurements that contribute to the first four topical areas. The main opportunities are for cross-section measurements of (γ, γ′) NRF processes and (γ, particle) reactions. The NRF measurements provide important informa- tion for determining PSFs, γ-ray transition probabilities, and nuclear structure spectroscopic information, all of which are inputs to nuclear astrophysics reaction-network calculations. The (γ, particle) reaction measurements provide data that are important input for γ-ray-induced reactions on stable nuclei in stars and also for the time reverse of particle capture on unstable nuclei. These measurements are particularly relevant for p-process, s-process, and r-process nucleosynthesis. The most important contribution to nuclear astrophysics that will come from opportunities created by the beam capabilities of a next-generation laser Compton γ-ray source is the measurement of the 16O(γ,α)12C reaction cross section as a means for determining the cross section for the 12C(α, γ)16O reaction at center-of-mass energies important for carbon burning in massive stars using time reversal invariance. Some examples of research opportu- nities at the next-generation laser Compton γ-ray source are outlined below. Table 3 gives the suggested beam parameters for the nuclear astrophysics program. 24 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al 3.2.1. The origin of the elements: p-process and s-process nucleosynthesis. Models of p- process and s-process nucleosynthesis require reliable (γ, n), (γ, p) and (γ,α) reaction cross sections on hundreds of stable and unstable nuclei. The proton-rich nuclei cannot be produced by neutron capture reactions. Complete network calculations on p-process nucleosynthesis include several hundred isotopes and the corresponding reaction rates. Theoretical predictions of the rates, normally in the framework of the Hauser–Feshbach (HF) theory, are necessary for modeling the nucleosynthesis reaction network. The reliability of these calculations should be tested experimentally, especially at rate-limiting paths in the network. Different approaches are available and necessary to improve the experimental data base for the p-process. The (γ, n) cross sections in the energy regime of the GDR have already been measured extensively (see, e.g., [65]). More recently, substantial effort has be devoted to using beams with a continuous bremsstrahlung spectrum to determine the reaction rates without any assumptions on the shape of the cross section’s energy dependence in the astrophysically relevant energy region, close to the reaction threshold [66–68]. A determination of the reaction rates by an absolute cross section measurement is also possible using monoenergetic photon beams produced by a laser Compton γ-ray source [69]. The beam capabilities at next-generation laser Compton γ-ray sources will open the possibility of higher accuracy (γ, n) cross-section measurements and measurements on nuclei with low natural abundances, for which the amount of target material will be small. In contrast, the experimental knowledge about the (γ, p) and (γ,α) reactions in the corre- sponding Gamow window is smaller. In fact, the experimental data is based on the observation of the time reversal (p, γ) and (α, γ) cross sections, respectively [70–74] for the proton-rich nuclei with mass numbers around 100. Due to the difficulties concerning the experimental accessibility of the (γ,α) reaction rates, a method using elastic α scattering has been estab- lished [75, 76]. It would be a tremendous improvement in the quality of the database to measure these rates directly using photon beams. Having a significant impact on this database will require a program of systematic measurements on a broad range of nuclei. This type of program would be made possible by a next-generation laser Compton γ-ray source. The heavy elements above the so-called iron peak are mainly produced in neutron capture processes: the r process (r: rapid neutron capture) deals with high neutron densities, well above 1020 cm−3, and temperatures of around 2 to 3 GK. It is thought to occur in explosive scenarios such as supernovae [78, 79] and was recently verified in neutron star merger via gravitational wave observations [80]. In contrast, the average neutron densities during s-process nucleosyn- thesis (s: slow neutron capture) are rather small (around 108 cm−3), so that the neutron capture rateλn is normally well below the β-decay rateλβ , and the reaction path is therefore close to the valley of β stability [81–83]. However, during the peak neutron densities, branching occurs at unstable isotopes with half-lives as low as several days. The half-lives of these branch points are normally known with high accuracy, at least under laboratory conditions, and rely on theory only for the extrapolation to stellar temperatures [84]. However, their neutron capture cross sections are accessible to direct experiments only in special cases. For this reason, nuclear astrophysics reaction-network simulations currently rely heavily on HF model calculations of the critical neutron capture cross sections. The difficulty is that there are many examples where the results of HF model calculations differ substantially from measurements (see, e.g., refer- ence [85]). Thus, experimental constraints on the theoretical predictions of these branch points are needed. Laser Compton γ-ray sources enable measurements of the inverse (γ, n) reaction, which can also provide information about the stellar enhancement factors (SEFs). The SEF accounts for the difference in the neutron-capture cross section measured in the laboratory and the effective cross section in the stellar environment. Because nuclei in stars spend much of the time in excited states, capture reactions mostly occur on excited states, not the ground state. 25 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 8. The s-process reaction path in the Ni–Cu–Zn region. During He-core burning (yellow), the temperature and neutron density are low compared to those during C-shell burning (red). Therefore, the unstable nucleus 63Ni either experiences decay or radiative neutron capture and acts as a branch-point nucleus. The isotopic abundance pattern of copper and zinc depends strongly on the stellar neutron-capture rate of 63Ni. Reprinted figure with permission from [77], Copyright (2013) by the American Physical Society. In addition to determining the cross sections and the SEFs directly from (γ, n) cross-section measurements, the SEFs can be calculated using PSFs determined from NRF measurements. The importance of each nucleus and reaction in the s-process network must be analyzed on its own merits. An example of the type of information that can be obtained using linearly polar- ized mono-energetic photon beams is illustrated in the proposed NRF measurement on 64Ni at HIγS. The 64Ni nucleus is the product of neutron capture on 63Ni, which is a branch-point nucleus. Figure 8 illustrates the situation during He-core burning (yellow) and C-shell burning (red) of a massive star, where the weak component of the s-process originates [86]. As the branching is very sensitive to the 63Ni(n, γ) cross section, a measurement was performed at CERN using a radioactive 63Ni target [77, 87]. However, the reaction rate in the hot environment of the stellar plasma differs significantly from the measured value because excited states are populated [88]. In the case of 63gsNi(n, γ), the stellar rate is still around 90% at He-core burning temperatures, but it drops to around 40% at the higher temperature in the C-shell burning phase [77]. HF model calculations are required to account for the stellar enhancement [89]. These calcula- tions rely on the PSF in 64Ni, which can be deduced from photoabsorption cross sections and the decay properties of low-spin states. The linearly polarized monoenergetic beams available at laser Compton γ-ray sources enable model-independent determination of photoabsorption cross sections as a function of the excitation energy, and consequently, a determination of the PSFs (see, e.g., [30, 33, 34, 90, 91]). 3.2.2. The life of stars: the 12C(α, γ) reaction cross section. Late-stage red giant stars produce energy in their interiors via helium burning. In first generation stars, helium burning proceeds through the 3α process and then 12C(α, γ)16O, while in later generation stars, α captures also involve the various CNO seed nuclei. In both cases, the 12C(α, γ)16O reaction helps to regulate the efficiency of helium burning in massive stars (those with masses greater than the suns). It also determines core mass, temperature and density during the latter stages of stellar evolution, and ultimately the mass of the iron core in the incipient supernova. In addition, the carbon-to- oxygen ratio (C/O) influences the abundances of elements produced in the ensuing explosion. After several decades of effort, the uncertainty in the rate of the α-particle capture reaction on 12C is still large enough to substantially limit our understanding of the latter stages of stellar evolution. To resolve this problem, the astrophysical S factor for the 12C(α, γ)16O reaction must be determined to an accuracy better than about 10% in the Gamow window (Ecm = 300 keV), which is indicated by the vertical band in figure 9 [92, 93]. 26 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 9. Plot of total S-factor data (filled-in circles) [94] for 12C(α, γ)16O compared with E1 (open triangles) and E2 (open squares) γ-ray measurements [95] and the Ex = 6.05 MeV cascade data (open circles) [96]. The solid line represents the sum of the single amplitudes of an R-matrix fit [97], while the dotted and dashed lines are the E1 and E2 amplitudes, respectively. In addition, the R-matrix fit of [96] to their cascade data is shown as the dot-dashed line. The latter component is not included in the sum and might explain the high yield in the S-factor data between the resonances. Reproduced from [98]. © IOP Publishing Ltd. All rights reserved. As shown in figure 9, the cross section has been measured at various levels of precision down to a center-of-mass (cm) energy of 1.2 MeV; then it must be extrapolated down to 300 keV, the energy needed for stellar reaction-rate calculations. One of the major uncer- tainties in performing the extrapolation arises from the presence of several resonances which contribute to the cross section at energies around Ecm = 0.75 MeV. Above Ecm = 1 MeV, the elastic scattering and capture reactions are dominated by a broad 1− resonance at an exci- tation energy in 16O of Ex = 9.59 MeV (Ecm = 2.43 MeV) and a narrow 2+ state at Ex = 9.85 MeV (Ecm = 2.70 MeV). However, a 1− state at Ex = 7.12 MeV, just 42 keV below threshold, determines the capture cross section in the astrophysically relevant energy region, including its interference with the higher lying 1− and 2+ states. In addition, broad high-lying states and direct processes produce a coherent background that affects the energy dependence of the cross section and thereby the extrapolation. Direct measurements of the 12C(α, γ)16O cross section at energies below Ecm = 2 MeV have been attempted for over 30 years. The major difficulty encountered in these experiments is the intense neutron background arising from the 13C(α, n) reaction. This background tends to swamp the γ-ray detector. The γ-ray beams produced by laser Compton sources have a narrow energy spread and thus offer an alternative technique for measuring the cross section for α capture on 12C at energies important to astrophysics as discussed above. The principle of detailed balance allows the determination of the (α, γ) cross section from the measurement of the cross section for the time reversed (γ,α) reaction. An added advantage of using the (γ,α) reaction over the direct reaction is that the cross section in the gamma window is enhanced by 27 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 10. Plot of the cross section of the 16O(γ,α)12C reaction as a function the system center-of-mass energy (bottom axis) and the incident γ-ray beam energy (top axis). The green hash shows the region where data have been collected at HIγS using the optical time-projection chamber (TPC). The blue filled bar is the lowest energy region experi- mentally accessible at HIγS using current target and detector technologies. The energy region indicated by the red brick filled area is what would be possible with an intensity upgrade of HIγS. about a factor of 50 due to detailed balance. This experimental concept has been demonstrated at HIγS using a CO2 gas-filled optical TPC [99, 100]. The calculated cross section for the 16O(γ,α)12 reaction is shown in figure 10. An upgrade of HIγS to increase the γ-ray beam flux on target by a factor of about fifty in the energy region important for this reaction, would enable meaningful measurements in the red shaded energy range (〈Ecm〉 = 1.3 MeV) in the figure. The next-generation laser Compton sources could potentially deliver more than an additional factor of 500 in beam flux on target relative to HIγS, thereby enabling measurements down to about 〈Ecm〉 = 700 keV. This measurement should be performed at next-generation laser Compton γ-ray source facilities using a variety of experimental techniques, such as gas TPCs (both optical and charge readout), silicon strip detectors with thin targets, and total cross-section measurements using super-heated high-purity water detectors (e.g., as in reference [101]). 3.2.3. The death of stars: r-process nucleosynthesis. More than half of the heavy nuclei with A > 120 are produced by r-process nucleosynthesis. The r-process involves nuclear reactions driven by rapid neutron capture, where the neutron capture rate is faster than the competing beta decay. The likely environments for the r-process are type-II supernovae and merging neutron stars. Simulations of r-process nucleosynthesis require reliable (n, γ) reaction cross sections on hundreds of stable and unstable nuclei. Theoretical predictions of the rates, normally in the framework of HF theory, are necessary for modeling neutron capture on unstable nuclei. The reliability of these calculations should be tested experimentally, especially at rate-limiting paths in the network. The (γ, n) time-reversed photodisintegration reaction offers a mechanism 28 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al Figure 11. Constraints of the symmetry energy at saturation J and slope parameter L, obtained from a comparison of relativistic nuclear energy density functional results and data on anti-analog giant dipole resonance [109] and IVGQR [104] excitation energies in 208Pb; the dipole polarizability αD of 208Pb [110]; and the PDR energy-weighted strength in 68Ni [111] and 130,132Sn [107]. Reprinted figure with permission from [112], Copyright (2014) by the American Physical Society. for determining the neutron capture cross section and level density of radioactive nuclei. Next- generation laser Comptonγ-ray sources will enable measurements on isotopes with low natural abundances and thus very limited sample sizes. 3.2.4. Neutron stars: the EOS of neutron-rich matter. Obtaining information about the detailed structure of the crust of a neutron star is an important open challenge in astrophysics. The crust is composed of non-uniform neutron-rich solid matter that is about 1 km thick and located above a liquid core [102, 103]. The inner crust comprises the region from the density at which neutrons drip from nuclei to the inner edge separating the solid crust from the homogeneous liquid core. While the density at which neutrons drip from nuclei is well determined, the transition density at the inner edge is much less certain because of insufficient knowledge of the EOS of neutron-rich nuclear matter. Measurements of collective responses of neutron- rich nuclei provide constraints on the nuclear EOS. The monoenergetic and linearly polarized beams at laser Compton γ-ray sources enable unique measurements of dipole and quadrupole excitations. Those most relevant to exploring the EOS are studies of the PDR on nuclei at the neutron rich end of an isotope chain using NRF and determination of the centroid energy and width of the isovector giant quadrupole resonance (IVGQR) via Compton scattering [104]. The underlying structure of the PDR is often interpreted as an oscillation of less bound valence neutrons forming a neutron skin [105, 106]. However, the true nature of the PDR is a matter of ongoing discussion. Furthermore the PDR and the E1 strength in general have a direct con- nection to the neutron skin of nuclei and the symmetry energy of nuclear matter [107, 108] (figure 11). 29 J. Phys. G: Nucl. Part. Phys. 49 (2022) 010502 C R Howell et al 3.3. Hadronic parity violation The capabilities of a next generation laser Compton γ-ray source, combined with current and expected advances in theory and lattice calculations, provide the opportunity to make signif- icant