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The $\ensuremath{\gamma}$ process is an explosive astrophysical scenario, which is thought to be the primary source of the rare proton-rich stable $p$ nuclei. However, current $\ensuremath{\gamma}$-process models remain insufficient in describing the observed $p$-nuclei abundances, with disagreements up to two orders of magnitude. A sensitivity study has identified ${}^{111}\mathrm{In}$ as a model-sensitive $(\ensuremath{\gamma},p)/(\ensuremath{\gamma},n)$ branching point within the $\ensuremath{\gamma}$ process. Constraining the involved reaction rates may have a significant impact on the predicted $p$-nuclei abundances. Here we report on measurements of the cross sections for $^{102}\mathrm{Pd}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{103}\mathrm{Ag},\phantom{\rule{0.28em}{0ex}}^{108}\mathrm{Cd}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{109}\mathrm{In}$, and $^{110}\mathrm{Cd}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{111}\mathrm{In}$ reactions for proton laboratory energies 3--8 MeV using the high efficiency total absorption spectrometer and the $\ensuremath{\gamma}$-summing technique. These measurements were used to constrain Hauser-Feshbach parameters used in talys 1.9, which constrains the $^{111}\mathrm{In}(\ensuremath{\gamma},p)\phantom{\rule{0.16em}{0ex}}^{110}\mathrm{Cd}$ and $^{111}\mathrm{In}(\ensuremath{\gamma},n)\phantom{\rule{0.16em}{0ex}}^{110}\mathrm{Ag}$ reaction rates. The newly constrained reaction rates indicate that the $^{111}\mathrm{In}\phantom{\rule{4pt}{0ex}}(\ensuremath{\gamma},p)/(\ensuremath{\gamma},n)$ branching point occurs at a temperature of $2.71\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.28em}{0ex}}\mathrm{GK}$, well within the temperature range relevant to the $\ensuremath{\gamma}$ process. These findings differ significantly from previous studies and may impact the calculated abundances.

We introduce a new framework for studying clustering and for calculating alpha partial widths using ab initio wave functions. We demonstrate the formalism for $^{20}$Ne, by calculating the overlap between the $^{16}$O$+\alpha$ cluster configuration and states in $^{20}$Ne computed in the ab initio symmetry-adapted no-core shell model. We present spectroscopic amplitudes and spectroscopic factors, and compare those to no-core symplectic shell-model results in larger model spaces, to gain insight into the underlying physics that drives alpha-clustering. Specifically, we report on the alpha partial width of the lowest $1^-$ resonance in $^{20}$Ne, which is found to be in good agreement with experiment. We also present first no-core shell-model estimates for asymptotic normalization coefficients for the ground state, as well as for the first excited $4^{+}$ state in $^{20}$Ne that lies in a close proximity to the $^{16}$O$+\alpha$ threshold. This outcome highlights the importance of correlations for developing cluster structures and for describing alpha widths. The widths can then be used to calculate alpha-capture reaction rates for narrow resonances of interest to astrophysics. We explore the reaction rate for the alpha-capture reaction $^{16}$O$(\alpha,\gamma)^{20}$Ne at astrophysically relevant temperatures and determine its impact on simulated X-ray burst abundances.

Highly excited states in 156Gd, populated via the neutron pickup reaction 157Gd(3He,4He)156Gd, are investigated, and their spin–parity distribution P(Jπ,E) is examined. The cross section for one-neutron transfers to states above the neutron separation energy in 156Gd is calculated as coherent sum, using standard reaction codes that employ spherical basis states. Spectroscopic factors and form factors for the relevant states are obtained by expanding the deformed neutron wave functions in a spherical Sturmian basis. For the energy regime relevant to surrogate applications involving neutron absorption, 155Gd+n →156Gd⋆, the calculations show that the reaction 3He+157Gd →4He+156Gd⋆ induces a well-behaved formation probability P(Jπ,E) of approximately Gaussian shape. It is observed that the centroid and shape of the Gaussian distributions of the positive and negative parity states of the compound system can be significantly different from each other!

The neutron capture cross section for 90Zr(n, γ) has recently been determined using surrogate 92Zr(p, dγ) data and nuclear reaction theory (J.E. Escher et al., Phys Rev Lett 121:052501, 2018). That work employed an approximate fitting method based on Bayesian Monte Carlo sampling to determine parameters needed for calculating the 90Zr(n, γ) cross section. Here, we improve the approach by introducing a more sophisticated Markov-Chain Monte Carlo sampling method (W.K. Hastings, Biometrika 57:97, 1970). We present preliminary results.

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21–23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9–10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12–13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade.

We present the current development of a new ab initio approach for nuclear reactions that takes advantage of SU(3) symmetry and its relevant dynamics combined with the resonating group method. In this model, the structure of the clusters is based on the ab initio symmetry-adapted no-core shell model, which enables the description of spatially enhanced nuclear configurations. We will present the formalism that involves the expression of the norm kernels in the SU(3) symmetry-adapted basis, in addition to first results for the p-\({ \alpha }\), p-\({ { }^{ 16 } }\)O and p-\({ { }^{ 20 } }\)Ne scattering reactions.

A set of prompt partial $\ensuremath{\gamma}$-ray production cross sections from thermal neutron capture were measured for the $^{139}\mathrm{La}(n,\ensuremath{\gamma})$ reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a $^{\mathrm{nat}}\mathrm{La}_{2}{\mathrm{O}}_{3}$ target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute $^{140}\mathrm{La}$ cross sections were determined relative to the well-known comparator $^{35}\mathrm{Cl}(n,\ensuremath{\gamma})$ cross sections from the irradiation of a stoichiometric $^{\mathrm{nat}}\mathrm{LaCl}_{3}$ sample. The total cross section for radiative thermal neutron capture on $^{139}\mathrm{La}$ from the sum of experimentally measured cross sections observed to directly feed the $^{140}\mathrm{La}$ ground state was determined to be ${\ensuremath{\sigma}}_{0}^{\mathrm{expt}}=8.58(50)$ b. To assess completeness of the decay scheme and as a consistency check, the measured cross sections for transitions feeding the ground state from levels below a critical energy of ${E}_{c}=285$ keV were combined with a modeled contribution accounting for ground-state feeding from the quasicontinuum to arrive at a total cross section of ${\ensuremath{\sigma}}_{0}=9.36(74)$ b. In addition, a neutron-separation energy of ${S}_{n}=5161.001(21)$ keV was determined from a least-squares fit of the measured primary $\ensuremath{\gamma}$-ray energies to the low-lying levels of the $^{140}\mathrm{La}$ decay scheme. Furthermore, several nuclear structure improvements are proposed for the decay scheme. The measured cross-section and separation-energy results are comparable to earlier measurements of these quantities.

The neutron-capture reaction plays a critical role in the synthesis of the elements in stars and is important for societal applications including nuclear power generation and stockpile-stewardship science. However, it is difficult---if not impossible---to directly measure neutron capture cross sections for the exotic, short-lived nuclei that participate in these processes. In this Letter we demonstrate a new technique which can be used to indirectly determine neutron-capture cross sections for exotic systems. This technique makes use of the $(d,p)$ transfer reaction, which has long been used as a tool to study the structure of nuclei. Recent advances in reaction theory, together with data collected using this reaction, enable the determination of neutron-capture cross sections for short-lived nuclei. A benchmark study of the $^{95}\mathrm{Mo}(d,p)$ reaction is presented, which illustrates the approach and provides guidance for future applications of the method with short-lived isotopes produced at rare isotope accelerators.

The need for accurate nuclear reaction data of actinides is well documented and several initiatives from international organizations for improvement have been initiated in the past. This need, particularly in view of method development for non-destructive assay of nuclear waste, has generated a joint effort to use prompt and de- layed neutron activation techniques to enhance nuclear capture data of some long lived actinides such as 237 Np, 242 Pu and 241 Am in the frame of a multilateral cooperation. This research initiative is targeted to lay grounds for the development of a non-destructive active neutron interro- gation technique to quantify actinides in mixed waste and residues from decommissioning of nuclear installations for safe treatment and storage of such materials.

We present an algorithm to calculate non-relativistic partial-wave Coulomb functions in momentum space. The arguments are the Sommerfeld parameter η, the angular momentum l, the asymptotic momentum q and the 'running' momentum p, where both momenta are real. Since the partial-wave Coulomb functions exhibit singular behavior when p → q, different representations of the Legendre functions of the 2nd kind need to be implemented in computing the functions for the values of p close to the singularity and far away from it. The code for the momentum-space Coulomb wave functions is applicable for values of vertical bar eta vertical bar in the range of 10-1 to 10, and thus is particularly suited for momentum space calculations of nuclear reactions.

Obtaining reliable data for nuclear reactions on unstable isotopes remains an extremely important task and a formidable challenge. Neutron capture cross sections---crucial ingredients for models of astrophysical processes, national security applications, and simulations of nuclear energy generation---are particularly elusive, as both projectile and target in the reaction are unstable. We demonstrate a new method for determining cross sections for neutron capture on unstable isotopes, using $^{87}\mathrm{Y}(n,\ensuremath{\gamma})$ as a prototype. To validate the method, a benchmark experiment is carried out to obtain the known $^{90}\mathrm{Zr}(n,\ensuremath{\gamma})$ cross section analogously. Our approach, which employs an indirect (``surrogate'') measurement combined with theory, can be generalized to a larger class of nuclear reactions. It can be used both with traditional stable-beam experiments and in inverse kinematics at rare-isotope facilities.

We outline a new formalism that begins with first-principles structure calculations to describe alpha-clustering, and ultimately leads to a description of alpha-capture reaction rates and impacts on abundance patterns from x-ray burst (XRB) nucle-osynthesis. We utilize a symmetry-adapted basis, which allows us to extend traditional ab initio calculations into the larger model spaces needed for the development of collectivity and clustering in nuclei. In particular, the use of symplectic symmetry allows us to describe spatially expansive states in nuclei – including the Hoyle state of 12C, its 2+ excitation, and B(E2) transitions – with only one or a few basis configurations. For narrow resonances, coupling to the continuum is weak and the number of competing channels is greatly reduced, so most of the physics of the system is described through the overlap of a wave function for the complete A-particle system, computed with a single symplectic configuration (consisting of several hundreds of basis states), and a cluster basis for a single cluster partitioning. This proves to be a very powerful tool for estimating spectroscopic amplitudes, decay widths, and nuclear reaction rates, with the ability to push toward nuclear reactions involving exotic nuclei that cannot currently be measured. We show preliminary results for the 16O(α, γ)20Ne reaction rate, and consider the implications for abundance patterns determined from XRB nucleosynthesis simulations.

We present an account of the current status of the theoretical treatment of inclusive $(d,p)$ reactions in the breakup-fusion formalism, pointing to some applications and making the connection with current experimental capabilities. Three independent implementations of the reaction formalism have been recently developed, making use of different numerical strategies. The codes also originally relied on two different but equivalent representations, namely the prior (Udagawa-Tamura, UT) and the post (Ichimura-Austern-Vincent, IAV) representations. The different implementations have been benchmarked, and then applied to the Ca isotopic chain. The neutron-Ca propagator is described in the Dispersive Optical Model (DOM) framework, and the interplay between elastic breakup (EB) and non-elastic breakup (NEB) is studied for three Ca isotopes at two different bombarding energies. The accuracy of the description of different reaction observables is assessed by comparing with experimental data of $(d,p)$ on $^{40,48}$Ca. We discuss the predictions of the model for the extreme case of an isotope ($^{60}$Ca) currently unavailable experimentally, though possibly available in future facilities (nominally within production reach at FRIB). We explore the use of $(d,p)$ reactions as surrogates for $(n,\gamma)$ processes, by using the formalism to describe the compound nucleus formation in a $(d,p\gamma)$ reaction as a function of excitation energy, spin, and parity. The subsequent decay is then computed within a Hauser-Feshbach formalism. Comparisons between the $(d,p\gamma)$ and $(n,\gamma)$ induced gamma decay spectra are discussed to inform efforts to infer neutron captures from $(d,p\gamma)$ reactions. Finally, we identify areas of opportunity for future developments, and discuss a possible path toward a predictive reaction theory.

Accurate neutron-capture cross sections on unstable nuclei near the line of beta stability are crucial for understanding the s-process nucleosynthesis. However, neutron-capture cross sections for short-lived radionuclides are difficult to measure due to the fact that the measurements require both highly radioactive samples and intense neutron sources. Essential ingredients for describing the γ decays following neutron capture are the γ-ray strength function and level densities. We will compare different indirect approaches for obtaining the most relevant observables that can constrain Hauser-Feshbach statistical-model calculations of capture cross sections. Specifically, we will consider photon scattering using monoenergetic and 100% linearly polarized photon beams. Challenges that exist on the path to obtaining neutron-capture cross sections for reactions on isotopes near and far from stability will be discussed., SCOPUS: cp.p, ND 2016: International Conference on Nuclear Data for Science and Technology, info:eu-repo/semantics/published

Thermal neutron radiative capture cross sections σ0γ of 155,157Gd are determined by summing the transition cross sections feeding the ground states of the respective product nuclei. The transition ...

The Evaluated Gamma-ray Activation File (EGAF) is the result of a 2000–2007 IAEA Coordinated Research Project to develop a database of thermal, prompt γ-ray cross sections, σ γ , for all elemental and selected radioactive targets. No previous database of this kind had existed. EGAF was originally based on measurements using guided neutron beams from the Budapest Reactor on all elemental targets from Z=1–82, 90 and 92, except for He and Pm. The EGAF σ γ data were published in the Database of Prompt Gamma Rays from Slow Neutron Capture for Elemental Analysis [1]. An international collaboration has formed to continue the EGAF measurements with isotopically enriched targets, derive total radiative thermal neutron cross sections, σ 0 , extend the σ γ data from thermal to 20 MeV neutrons, compile a completed activation data file, improve sections of the Reference Input Parameter Library (RIPL) with more complete and up to date level and γ-ray data, evaluate statistical γ-ray data from reaction studies, and determine recommended neutron separations energies, S n , for atomic mass evaluations. A new guided neutron beam facility has become available at the Garching (Munich) FRM II Reactor, and high energy neutron experimental facilities are being developed by a Berkeley area collaboration where 5–33 MeV neutron beams are available at the LBNL 88” cyclotron, 2.5 and 14 MeV beams at the University of California, Berkeley neutron generator laboratory, and high flux, 10 27 – 33 n⋅cm ⋅ − 2 s − 1 , neutron pulses available from the LLNL National Ignition Facility (NIF).

Neutron capture on unstable nuclei is important in understanding abundances in r-process nucleosynthesis. Previously, the non-elastic breakup of the deuteron in the (d,p) reaction has been shown to provide a neutron that can be captured by the nucleus and the gamma-ray decay of the subsequent compound nucleus can be modelled to predict the gamma-ray decay of the compound nucleus in the (n,γ) reaction. Preliminary results from the 95 Mo(d,pγ) reaction in normal kinematics support the (d,pγ) reaction as a valid surrogate for neutron capture. The techniques to measure the (d,pγ) reaction in inverse kinematics have been developed.

The neutron-capture reaction is fundamental for identifying and analyzing the γ-ray spectrum from an unknown assembly because it provides unambiguous information on the neutron-absorbing isotopes. Nondestructive-assay applications may exploit this phenomenon passively, for example, in the presence of spontaneous-fission neutrons, or actively where an external neutron source is used as a probe. There are known gaps in the Evaluated Nuclear Data File libraries corresponding to neutron-capture γ-ray data that otherwise limit transport-modeling applications. In this work, we describe how new thermal neutron-capture data are being used to improve information in the neutron-data libraries for isotopes relevant to nonproliferation applications. We address this problem by providing new experimentally-deduced partial and total neutron-capture reaction cross sections and then evaluate these data by comparison with statistical-model calculations.

We studied the low-spin structure of odd-odd 88Y via (p,dγ) reactions on an 89Y target. The K150 Cyclotron at the Texas A&M University Cyclotron Institute was employed to provide a 28.5-MeV proton beam, and particle-γ and particle-γ-γ coincidence data were collected with the STARLiTeR array. Moreover, a number of new levels and γ rays have been observed below 2.5 MeV, while level and γ-ray energies as well as spin-parity assignments have been re-evaluated.

Nuclear reaction cross sections are important for a variety of applications in the areas of astrophysics, nuclear energy, and national security. When these cross sections cannot be measured directly or predicted reliably, it becomes necessary to develop indirect methods for determining the relevant reaction rates. The surrogate nuclear reactions approach is such an indirect method. First used in the 1970s for estimating ðn; fÞ cross sections, the method has recently been recognized as a potentially powerful tool for a wide range of applications that involve compound-nuclear reactions. The method is expected to become an important focus of inverse-kinematics experiments at rareisotope facilities. The present paper reviews the current status of the surrogate approach. Experimental techniques employed and theoretical descriptions of the reaction mechanisms involved are presented and representative cross section measurements are discussed.

The prospects for obtaining neutron capture cross sections indirectly from surrogate measurements are discussed. The surrogate reactions approach has been successfully employed to determine (n,f) cross sections from observed fission decays of compound nuclei created with the help of light-ion inelastic scattering or transfer reactions. The challenges encountered in applications of the method to capture reactions are considered. Case studies are presented that shed light on the accuracy to be expected from this approach, and ideas for improving the resulting cross sections are discussed.

Author(s): Sleaford, B.W. | Abstract: The neutron capture reaction is useful in identifying and analyzing the gamma-ray spectrum from an unknown assembly as it gives unambiguous information on its composition. This can be done passively or actively where an external neutron source is used to probe an unknown assembly. There are known capture gamma-ray data gaps in the ENDF libraries used by transport codes for various nuclear applications. The Evaluated Gamma-ray Activation file (EGAF) is a new thermal neutron capture database of discrete line spectra and cross sections for over 260 isotopes that was developed as part of an IAEA Coordinated Research Project. EGAF has been used to improve the capture gamma production in ENDF libraries. For medium to heavy nuclei the quasi continuum contribution to the gamma cascades is not experimentally resolved. The continuum contains up to 90percent of all the decay energy an is modeled here with the statistical nuclear structure code DICEBOX. This code also provides a consistency check of the level scheme nuclear structure evaluation. The calculated continuum is of sufficient accuracy to include in the ENDF libraries. This analysis also determines new total thermal capture cross sections and provides an improved RIPL database. For higher energy neutron capture there is less experimental data available making benchmarking of the modeling codes more difficult. We use CASINO, a version of DICEBOX that is modified for this purpose. This can be used to simulate the neutron capture at incident neutron energies up to 20 MeV to improve the gamma-ray spectrum in neutron data libraries used for transport modelling of unknown assemblies.

J. T. Burke, ∗ J. J. Ressler, J. E. Escher, N. D. Scielzo, I. J. Thompson, R. Henderson, J. Gostic, L. Bernstein, D. Bluel, M. Weideking, V. Meot, O. Roig, L. W. Phair, R. Hatarik, J. Munson, 4 C. Angell, B. Goldblum, C. W. Beausang, T. Ross, R. Hughes, M. Aiche, G. Barreau, N. Cappelan, S. Czajkowski, B. Hass, B. Jurado, L. Mathieu and I. Companis 8 Physics Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA Service de Physique Nucleaire, Commissariat a l’Energie Atomique, Bruyeres-le-Chatel, Arpajon, France Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA Nuclear Physics Department, University of Richmond, Richmond, Virginia 23173, USA Centre d’Etudes Nucleaires Bordeaux Gradignan, CNRS/IN2P3, Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France Centre d’Etudes Nuclaires Bordeaux Gradignan, CNRS/IN2P3, Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France Horia Hulubei National Institute for Physics and Nuclear Engineering, 077125 Bucharest-Magurele, Romania

We have measured partial γ-ray cross sections following neutron capture in enriched 151Eu and 153Eu targets at the cold-neutron-beam facility of the Budapest Research Reactor. The cross sections were standardized using a stoichiometric natEuCl 3 target with the well-known 1951-keV γ-ray cross section from the 35Cl(n,γ)36Cl reaction at the cold-neutron-beam facility of the Garching Research Reactor. The γ-ray cross sections were corrected for effective g-factors. These data were combined with the structural information of 152Eu and 154Eu given in the Evaluated Nuclear Structure Data File to produce capture γ-ray level schemes. The total radiative capture cross sections of the 151Eu(n,γ) 152 m 1 , g Eu and 153Eu(n,γ)154Eu reactions were determined by summing the experimental transition intensities from known levels with simulated intensities of transitions from higher excitations to the ground- or metastable-state. The individual 151Eu(n,γ) 152 m 1 Eu and 151Eu(n,γ)152gEu reaction cross sections disagree with values in the literature. However, the total cross section of the 151Eu(n,γ)152Eu reaction does agree with those values. Also, our deduced cross section for the 153Eu(n,γ)154Eu reaction closely follows the data in the literature. These results are supported by an earlier standardization experiment done at the Budapest Research Reactor using a target of Eu 2 O 3 solution in H 2 SO 4 .

Tungsten occurs naturally in five isotopic forms; four of them, 182 , 183 , 184 , 186 W, contribute significantly to the overall elemental abundance (with each contribution between 14 and 30 %), whereas 180 W only occurs at the 0.12 % level and is a minor isotope. Given its very low abundance, a precise measurement of the thermal neutron-capture cross section is extremely challenging. This work reports a new value of the thermal neutron-capture cross section from a direct 180 W(n, γ ) measurement using a guided-thermal beam at the Budapest Research Reactor, incident upon an 11.35 % enriched sample to induce prompt γ -ray activation within the sample. The thermal-capture cross section was determined as the sum of experimentally observed partial neutron-capture γ -ray cross sections feeding the ground state directly, and, the modeled contribution from the (unobserved) ground-state feeding predicted from statistical-model calculations using the Monte Carlo program DICEBOX. The preliminary value of the 180 W(n, γ ) thermal neutron-capture cross section is 20.5(42) b.

Semidirect capture, a two-step process that excites a giant-dipole resonance followed by its radiative de-excitation, is a dominant process near giant-dipole resonances, that is, for incoming neutron energies within 5–20 MeV. At lower energies such processes may affect neutron capture rates that are relevant to astrophysical nucleosynthesis models. We implement a semidirect capture model in the coupled-channel reaction code Fresco and validate it by comparing the cross section for direct-semidirect capture 208 Pb(n, γ ) 209 Pb to experimental data. We also investigate the effect of low-energy electric dipole strength in the pygmy resonance. We use a conventional single-particle direct-semidirect capture code Cupido for comparison. Furthermore, we present and discuss our results for direct-semidirect capture reaction 130 Sn(n, γ ) 131 Sn, the cross section of which is known to have a significant effect on nucleosynthesis models.

Cross sections for compound-nuclear reactions involving unstable targets are important for many applications, but can often not be measured directly. Several indi- rect methods have recently been proposed to determine neutron capture cross sections for unstable isotopes. We consider three approaches that aim at constraining statistical calculations of capture cross sections with data obtained from the decay of the compound nucleus relevant to the desired reaction. Each method produces this compound nucleus in ad ifferent manner (via a light-ion reaction, a photon-induced reaction, or β-decay) and requires additional ingredients to yield the sought-after cross section. We give a brief outline of the approaches and employ preliminary results from recent measurements to illustrate the methods. We discuss the main advantages and challenges of each approach.

The repulsive Coulomb force poses severe challenges when describing (d , p ) reactions for highly charged nuclei as a three-body problem. Casting Faddeev-AGS equations in a Coulomb basis avoids introducing screening of the Coulomb force. However, momentum space partial-wave t -matrix elements need to be evaluated in this basis. When those t -matrices are separable, the evaluation requires the folding of a form factor, depending on one momentum variable, with a momentum space partial-wave Coulomb function, which has a singular behavior at the external momentum q . We developed an improved regularization scheme to calculate Coulomb distorted form factors as the integral over the Coulomb function and complex nuclear form factors.

The surrogate reaction method is an indirect technique for determining cross sections for nuclear reactions that proceed through a well-defined compound nucleus. In this method, the same compound nucleus is produced by an alternate (“surrogate”) reaction and its decay products measured. The assumptions underlying the method are examined for the special case of 235 U( n , f ).

The effect of the production mechanism on the decay of a compound nucleus is investigated. The nucleus $^{90}\mathrm{Zr}$ was produced by three different reactions, namely $^{90}\mathrm{Zr}(p,{p}^{\ensuremath{'}})^{90}\mathrm{Zr}$, $^{91}\mathrm{Zr}(p,d)^{90}\mathrm{Zr}$, and $^{92}\mathrm{Zr}(p,t)^{90}\mathrm{Zr}$, which served as surrogate reactions for $^{89}\mathrm{Zr}(n,\ensuremath{\gamma})$. The spin-parity $({J}^{\ensuremath{\pi}})$ distributions of the states populated by these reactions were studied to investigate the surrogate reaction approach, which aims at indirectly determining cross sections for compound-nuclear reactions involving unstable targets such as $^{89}\mathrm{Zr}$. Discrete $\ensuremath{\gamma}$ rays, associated with transitions in $^{90}\mathrm{Zr}$ and $^{89}\mathrm{Zr}$, were measured in coincidence with light ions for scattering angles of ${25}^{\ensuremath{\circ}}--{60}^{\ensuremath{\circ}}$ and $^{90}\mathrm{Zr}$ excitation energies extending above the neutron separation energy. The measured transition systematics were used to gain insights into the ${J}^{\ensuremath{\pi}}$ distributions of $^{90}\mathrm{Zr}$. The $^{90}\mathrm{Zr}(p,{p}^{\ensuremath{'}})$ reaction was found to produce fewer $\ensuremath{\gamma}$ rays associated with transitions involving high spin states $(J=6--8\phantom{\rule{4.pt}{0ex}}\ensuremath{\hbar})$ than the other two reactions, suggesting that inelastic scattering preferentially populates states in $^{90}\mathrm{Zr}$ that have lower spins than those populated in the transfer reactions investigated. The $\ensuremath{\gamma}$-ray production was also observed to vary by factors of 2--3 with the angle at which the outgoing particle was detected. These findings are relevant to the application of the surrogate reaction approach.

A procedure for determining the effective thickness of non-uniform irregular-shaped samples via radiative capture is described. In this technique, partial γ-ray production cross sections of a compound nucleus produced in a neutron-capture reaction are measured using Prompt Gamma Activation Analysis and compared to their corresponding standardized absolute values. For the low-energy transitions, the measured cross sections are lower than their standard values due to significant photoelectric absorption of the γ rays within the bulk-sample volume itself. Using standard theoretical techniques, the amount of γ-ray self absorption and neutron self shielding can then be calculated by iteratively varying the sample thickness until the observed cross sections converge with the known standards. The overall attenuation, thus, provides a measure of the effective sample thickness illuminated by the neutron beam. This procedure is illustrated through radiative neutron capture using powdered oxide samples comprising enriched 186W and 182W from which their tungsten-equivalent effective thicknesses are deduced to be 0.077(3) mm and 0.042(8) mm, respectively.

Treating $(d,p)$ reactions in a Faddeev-AGS framework requires the interactions in the sub-systems as input. We derived separable representations for the neutron- and proton-nucleus interactions from phenomenological global optical potentials. In order to take into account excitations of the nucleus, excitations need to be included explicity, leading to a coupled-channel separable representation of the optical potential., 2 pages, 2 figures, 21st International Conference on Few-body Problems in Physics, May 18-22, 2015, Chicago, IL

The work of this collaboration during its existence is summarized. The mission of the TORUS Topical Collaboration was to develop new methods that advance nuclear reaction theory for unstable isotopes by using three-body techniques to improve direct reaction calculations. This multi-institution collaborative effort was and remains directly relevant to three areas of interest: the properties of nuclei far from stability, microscopic studies of nuclear input parameters for astrophysics, and microscopic nuclear reaction theory. The TORUS project focused on understanding the details of (d,p) reactions for neutron transfer to heavier nuclei. The bulk of the work fell into three areas: coupled channel theory, modeling (d,p) reactions with a Faddeev-AGS approach, and capture reactions.

An indirect method for determining cross sections of reactions proceeding through a compound nucleus is presented. Some applications of the Surrogate nuclear reaction approach are considered and challenges that need to be addressed are outlined.

An innovative method for indirectly determining reaction cross sections via Surrogate Nuclear Reactions is presented. Exploring indirect approaches for obtaining reaction cross sections is important since a large number of nuclear reactions relevant to astrophysics cannot be measured with currently available techniques. A program is outlined for developing a comprehensive framework for planing and interpreting experiments that can yield the cross sections of interest. The applications will focus on reactions involving unstable nuclei that play a key role in the production of the elements between iron and uranium.

Neutron-induced reaction cross sections are sometimes difficult to measure due to target or beam limitations. For two-step reactions proceeding through an equilibrated intermediate state, an alternate "surrogate reaction" technique [J.D. Cramer and H.C. Britt, Nucl. Sci. Eng. 41, 177 (1970), H.C. Britt and J.B. Wilhelmy, Nucl. Sci. Eng. 72, 222 (1979), W.Younes and H.C. Britt, Phys. Rev. C 67, 024610 (2003)] can be applicable, and is currently undergoing investigation at LLNL. Measured decay probabilities for the intermediate nucleus formed in a light-ion reaction can be combined with optical-model calculations for the formation of the same intermediate nucleus via the neutron-induced reaction. The result is an estimation for overall (n,γ/n/2n) cross sections. As a bench-mark, the reaction 92Zr(α, α ′ ), surrogate for n+91Zr, was studied at the A.W. Wright Nuclear Structure Laboratory at Yale. Particles were detected in the silicon telescope STARS (Silicon Telescope Array for Reaction Studies) and γ-ray energies measured with germanium clover detectors from the YRAST (Yale Rochester Array for SpecTroscopy) ball. The experiment and preliminary observations will be discussed.

One-nucleon transfer reactions, long used as a tool to study the structure of nuclei, are potentially valuable for determining reaction cross sections indirectly. This is significant, as many reactions of interest to astrophysics and other applications involve short-lived isotopes and cannot be measured directly. We describe a procedure for obtaining constraints for calculations of neutron capture cross sections using observables from experiments with transfer reactions. As a first step toward demonstrating the method, we outline the theory developments used to properly describe the production of the compound nucleus 88Y* via the one-nucleon pickup reaction 89Y(p,d)88Y* and test the description with data from a recent experiment. We indicate how this development can be used to extract the unknown 87Y(n,γ) cross section from 89Y(p,dγ) data. The example illustrates a more generally applicable method for determining unknown cross sections via a combination of theory and transfer (or inelastic scattering) experiments.

An important ingredient for applications of nuclear physics to e.g. astrophysics or nuclear energy are the cross sections for reactions of neutrons with rare isotopes. Since direct measurements are often not possible, indirect methods like $(d,p)$ reactions must be used instead. Those $(d,p)$ reactions may be viewed as effective three-body reactions and described with Faddeev techniques. An additional challenge posed by $(d,p)$ reactions involving heavier nuclei is the treatment of the Coulomb force. To avoid numerical complications in dealing with the screening of the Coulomb force, recently a new approach using the Coulomb distorted basis in momentum space was suggested. In order to implement this suggestion, one needs to derive a separable representation of neutron- and proton-nucleus optical potentials and compute their matrix elements in this basis., Comment: submitted to proceedings of the 21st International School on Nuclear Physics, Neutron Physics and Applications and International Symposium on Exotic Nuclei (ISEN-2015), September 9-11, 2015, Varna, Bulgaria. arXiv admin note: text overlap with arXiv:1410.1227

Low-spin single quasineutron levels in $^{153}\mathrm{Gd}$ and $^{157}\mathrm{Gd}$ have been studied following the $^{154}\mathrm{Gd}$($p,d\ensuremath{-}\ensuremath{\gamma}$)$^{153}\mathrm{Gd}$ and $^{158}\mathrm{Gd}$($p,d\ensuremath{-}\ensuremath{\gamma}$)$^{157}\mathrm{Gd}$ reactions. A combined Si telescope and high-purity germanium array was utilized, allowing $d\text{\ensuremath{-}}\ensuremath{\gamma}$ and $d\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence measurements. Almost all of the established low-excitation-energy, low-spin structures were confirmed in both $^{153}\mathrm{Gd}$ and $^{157}\mathrm{Gd}$. Several new levels and numerous new $\ensuremath{\gamma}$ rays are observed in both nuclei, particularly for ${E}_{x}\ensuremath{\ge}1$ MeV. Residual effects of a neutron subshell closure at $N=64$ are observed in the form of a large excitation energy gap in the single quasineutron level schemes.

The $^{240}\mathrm{Am}(n,f$) cross section has been measured for the first time above 4 MeV, using the surrogate-ratio method over the neutron energy range of 200 keV to 14 MeV. The reactions $^{243}\mathrm{Am}(p,tf$) and $^{238}\mathrm{U}(p,tf$), which proceed through the fissioning excited nuclei $^{241}\mathrm{Am}$* and $^{236}\mathrm{U}$*, were used as surrogates for the desired $^{240}\mathrm{Am}(n,f$) and $^{235}\mathrm{U}(n,f$) reactions. The experiment was fielded using the STARLiTeR detector system with a recently commissioned VME-based data acquisition system. The 38.4-MeV proton beam used in these measurements was provided by the K150 cyclotron at the Texas A Cyclotron Institute. The measured $^{240}\mathrm{Am}(n,f$) cross section disagrees with many of the most recent evaluations, and a reevaluation is recommended.