[en] Nuclear reactions on unstable fission products are of interest to nuclear non-proliferation efforts and basic science. While these reactions have historically been extremely difficult to measure, new experimental facilities are beginning to make beams of fission products available for the first time, enabling exciting experiments. The opening of this new area of the nuclear chart for measurements presents the opportunity to test, refine, and expand theories developed to explain behavior closer to stability, deepening our knowledge of the fundamental physics at play. We will present a summary of our white paper on the work needed, and the investments required to enable this work, to make measurements of nuclear reactions away from stability and the theoretical developments required to understand the underlying physics.

[en] Understanding neutron-induced reaction rates on rare isotopes is important in fission and nucleosynthesis processes and applications in nuclear energy and forensics and stewardship science. Informing these rates requires indirect methods and rare isotope beams. The Surrogate Reaction Method (SRM) [1], where beams interact with light-element targets, has been demonstrated [2] as a valid method to inform neutron-induced reaction rates. In the SRM the same compound nucleus (CN) can be formed as in the neutron-induced reaction and differences, in particular entrance spin-parity distributions, can be accounted for with nuclear reaction calculations. To date most of the SRM applications for (n,γ) reactions have involved measuring discrete γ-ray transitions in an established level scheme, to determine the gamma-decay probabilities of the CN. Low beam intensities of rare isotope beams require high efficiency, high resolution arrays of gamma-ray detectors to detect the discrete transitions. However, even the state-of-the-art γ-ray detector arrays are only about 15% efficient, with efficiency strongly dependent on γ-ray energy. A particular challenge is when the final nucleus is odd-odd, where not only may little be known about the level scheme, but the decay of the compound nucleus is likely to fragment into many paths. Our collaboration has recently developed the techniques to measure the γ-ray decay probability of a final nucleus in a surrogate (d,p) reaction, without requiring discrete γ-ray transitions. The Oak Ridge Rutgers University Barrel Array (ORRUBA) of position sensitive silicon strip detectors was coupled to the S800 spectrograph at NSCL to inform 84Se(n,γ) rates with the SRM (d,p) reaction [3]. Beam-like reaction products were measured in the focal plane of the S800 in coincidence with (d,p) reaction protons. By measuring the S800 focal plane energy loss as a function of time of flight, the 85Se recoils could be separated from the un-reacted 84Se beam, with about 35% efficiency. The 84Se from neutron emission from the 85Se CN has sufficiently different kinematics that these ions are well separated from the 85Se desired product and can be blocked in the focal plane, along with most of the unreacted 84Se beam. By careful measurement of the proton singles in the 84Se(d,p) reaction, the (n,γ) cross sections as a function of neutron energy can be informed. The collaboration is scheduled to measure the (d,pγ) reaction with ORRUBA coupled to GRETINA and the S800 in April 2024 with 80Ge and 75Ga FRIB beams. These measurements would inform (n,γ) rates important for nucleosynthesis processes intermediate between slow and rapid neutron capture. This talk would present the experimental techniques used in the earlier 84Se measurement of ORRUBA +S800 and how (n,γ) rates can be informed, as well as preliminary results from the Spring 2024 FRIB measurements. Work supported in part by U.S. Department of Energy National Nuclear Security Administration and Office of Science and National Science Foundation. [1] J.E. Escher et al., Rev. Mod. Phys. 84, 253 (2012) [2] A. Ratkiewicz et al., Phys. Rev. Lett. 122, 052502 (2019). [3] H.E. Sims, PhD Dissertation Rutgers University (2020) and to be published.

[en] The international monitoring system exists to verify compliance with the terms of the comprehensive test ban treaty. About 10% of the member stations will be capable of detecting radioxenon, which can be produced in nuclear detonations or through civilian processes. We have studied the activation of radioxenon by the prompt, intense spectrum of 14-MeV neutrons produced at the National Ignition Facility. While 14-MeV neutrons are not currently a significant contributor to the production of radioxenon, we find that radioxenon produced through activation of environmental xenon by 14-MeV neutrons would be distinguishable from activation by nuclear tests. (author)

[en] A Gamma-ray spectroscopic study of excited states of isobaric multiplets has been performed in recent years, with a view to gaining a quantitative understanding of energy differences between excited states in terms of a range of Coulomb and other isospin breaking phenomena. Recently, the experimental programme has been augmented by a study of isobaric analogue states of mirror nuclei populated in mirrored fragmentation reactions. In this presentation, recent results on the T = 3/2 analogue states in the T_z = ± 3/2 mirror pair ⁵³Ni/⁵³Mn will be summarised. In this work, further strong evidence is found for the need to include an anomalous isospin-breaking two-body matrix element for angular-momentum couplings of J = 2, in addition the expected Coulomb contribution, in order to account for the experimental data.

[en] We report on the first in-beam γ-ray spectroscopy study of the very neutron-rich nucleus ⁴⁶S. The N=30 isotones ⁴⁶S and ⁴⁸Ar were produced in a novel way in two steps that both necessarily involve nucleon exchange and neutron pickup reactions ⁹Be(⁴⁸Ca,⁴⁸K)X followed by ⁹Be(⁴⁸K,⁴⁸Ar+γ)X at 85.7 MeV/u midtarget energy and ⁹Be(⁴⁸Ca,⁴⁶Cl)X followed by ⁹Be(⁴⁶Cl,⁴⁶S+γ)X at 87.0 MeV/u midtarget energy, respectively. The results are compared to large-scale shell-model calculations in the sd-pf shell using the SDPF-NR effective interaction and Z-dependent modifications.

[en] The high-fluence neutron spectrum produced by the National Ignition Facility (NIF) provides an opportunity to measure the activation of materials by fast-spectrum neutrons. A new large-volume gas-cell diagnostic has been designed and qualified to measure the activation of gaseous substances at the NIF. This in-chamber diagnostic is recoverable, reusable and has been successfully fielded. Data from the qualification of the diagnostic have been used to benchmark an Monte Carlo N-Particle Transport Code simulation describing the downscattered neutron spectrum seen by the gas cell. We present early results from the use of this diagnostic to measure the activation of "n"a"tXe and discuss future work to study the strength of interactions between plasma and nuclei.

[en] Gamma decays have been observed for the first time in the T_z = -3/2 nucleus ⁵³Ni. This represents the first gamma-spectroscopy of a T_z = -3/2 nucleus heavier than A = 33. The nucleus was produced via a two-step fragmentation process, along with its mirror ⁵³Mn. Differences in excitation energy between isobaric analogue states have been calculated and a preliminary interpretation attempted; shell model calculations are required to further understand these results. This work represents the first study of isobaric analogue states via mirrored fragmentation reactions and demonstrates the power of this new technique

[en] GODDESS is a coupling of the charged-particle detection system ORRUBA to the gamma-ray detector array Gammasphere. This coupling has been developed in order to facilitate the high-resolution measurement of direct reactions in normal and inverse kinematics with stable and radioactive beams. GODDESS has been commissioned using a beam of ¹³⁴Xe at 10 MeV/A, in a campaign of stable beam measurements. The measurement demonstrates the capabilities of GODDESS under radioactive beam conditions, and provides the first data on the single-neutron states in ¹³⁵Xe, including previously unobserved states based on the orbitals above the N=82 shell closure.

[en] We report on the first measurement of the absolute transition strength B(E2;0₁⁺→2₁⁺) in the self-conjugate nucleus ⁶⁸Se. It is found that the 0₁⁺→2₁⁺ transition displays a strength similar to that for the triaxial ⁶⁴Ge nucleus, in sharp contrast to the much stronger collectivity observed for the oblate ⁷²Kr nucleus. Shape evolution along the N=Z line from zinc to strontium is analyzed through beyond-mean-field calculations using the D1S force. Over this narrow mass region the nuclear structure shows clear-cut transition from prolate to oblate and back to prolate, with ⁶⁸Se standing as pivotal between triaxial and oblate deformations.

[en] Isospin-breaking effects have been studied for the first time in T=(3/2) isobaric analog states. Gamma decays have been observed from T_z=-(3/2) nuclei, ⁴⁹Fe and ⁵³Ni, presented here in new level schemes, and mirror energy differences have been computed following observation of analog states in ⁴⁹V and ⁵³Mn, respectively. Shell-model calculations in the fp shell are in good agreement with the data and reveal the importance of non-Coulomb isospin-breaking effects in T=(3/2) isobaric analog states. A two-step fragmentation process was developed to allow access to highly proton-rich nuclei and to produce each member of a mirror pair via mirrored fragmentation of a ⁵⁶Ni secondary beam. This work represents the first study using this technique and demonstrates the power of this approach for future studies of isobaric analog states in very proton-rich systems.