[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 study of the nuclear physics properties which govern energy generation and nucleosynthesis in the astrophysical phenomena we observe in the universe is crucial to understanding how these objects behave and how the chemical history of the universe evolved to its present state. The low cross sections and short nuclear lifetimes involved in many of these reactions make their experimental determination challenging, requiring developments in beams and instrumentation. A selection of developments in nuclear astrophysics instrumentation is discussed, using as examples projects involving the nuclear astrophysics group at Oak Ridge National Laboratory. These developments will be key to the instrumentation necessary to fully exploit nuclear astrophysics opportunities at the Facility for Rare Isotope Beams which is currently under construction

[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] To make measurements with the intense (but often contaminated) radioactive beams available today, one often needs to identify the reaction products to determine the events of interest. The low energies required for many astrophysics measurements make impossible the use of traditional energy loss techniques, and additional constraints are required. We demonstrate a simple technique to measure the risetimes of silicon strip-detector signals and show partial discrimination can be obtained even at energies below 1 MeV/u.

[en] Neutron transfer (d,p) reactions have been measured with rare isotope beams of ¹³²Sn, ¹³⁰Sn and ¹³⁴Te accelerated to ∼4.5 MeV/u interacting with CD₂ targets. Reaction protons were detected in an early implementation of the ORRUBA array of position-sensitive silicon strip detectors. Neutron excitations in the 2f_7/2, 3p_3/2, 3p_1/2 and 2f_5/2 orbitals were populated.

[en] Measurements of the helium-cluster breakup and neutron removal cross-sections for neutron-rich Be isotopes ^10-12,14Be are presented. These have been studied in the 30 to 42 MeV/nucleon energy range where reaction measurements are proposed to be sensitive to the cluster content of the ground-state wave-function. These measurements provide a comprehensive survey of the decay processes of the Be isotopes by which the valence neutrons are removed revealing the underlying α-α core-cluster structure. The measurements indicate that clustering in the Be isotopes remains important up to the drip-line nucleus ¹⁴Be and that the dominant helium-cluster structure in the neutron-rich Be isotopes corresponds to α-Xn-α

[en] Background: Current nuclear structure models indicate that the shell structure near the valley of stability, with well-established shell closures at N=50, for example, changes in very neutron-rich nuclei far from stability. Single-particle properties of nuclei away from stability can be probed in single-neutron (d,p) transfer reactions with beams of rare isotopes. The interpretation of these data requires reaction theories with various effective interactions. Often, approximations made to the final bound-state potential introduce a large uncertainty in the extracted single-particle properties, in particular the spectroscopic factor. Purpose: Mitigate this uncertainty using a combined measurement method to constrain the shape of the bound-state potential and to reliably extract the spectroscopic factor. Methods: The ²H(⁸⁶Kr,p)⁸⁷Kr reaction was measured at 33 MeV/u at the National Superconducting Cyclotron Laboratory (NSCL) as a test of the combined method. The reaction protons were detected with the Oak Ridge Rutgers University Barrel Array (ORRUBA) of position sensitive silicon strip detectors, the first implementation of ORRUBA coupled to the S800 spectrograph with fast beams at NSCL. Results: These measurements at 33 MeV/u are combined with previous studies of the ⁸⁶Kr(d,p) reaction at 5.5 MeV/u to demonstrate a successful case of the combined method to constrain the shape of the single-particle potential and deduce asymptotic normalization coefficients and spectroscopic factors. In particular, the single-particle asymptotic normalization coefficient for the ground state of ⁸⁷Kr was constrained to b_d5/2=6.46${}_{\text{–<!-- ??? -->}0.57}^{+1.12}$ fm^–1/2, and therefore the deduced spectroscopic factor is S=0.44${}_{\text{–<!-- ??? -->}0.13}^{+0.09}$ with uncertainties dominated by experimental statistics. Conclusions: By combining measurements at two very different beam energies, single-particle asymptotic normalization coefficients, at least for low angular momentum transfers, can be constrained. Thus, spectroscopic factors can be deduced with uncertainties dominated by experimental uncertainties, rather than limited knowledge of bound-state potential parameters.

[en] The α+⁶He decay of ¹⁰Be has been studied via the ⁷Li(⁷Li,α ⁶He)α reaction at 58 MeV. The excitation energy of the ¹⁰Be nucleus was determined following the coincident detection of the α and ⁶He decay products. A study of the fragment angular correlations has provided a spin assignment consistent with the previously reported value of 3^- for the 10.15 MeV state in ¹⁰Be. A tentative assignment of (4⁺), 6⁺ is proposed for the 11.76 MeV state

[en] A new program to measure (d,p) reactions on rare isotopes of fission fragments has been established at Oak Ridge National Laboratory. Initial measurements on N=50 isotones and prospects for Z=50 experiments are reported

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