We measured the β-delayed neutron emission from ^25F for the first time at the Facility for Rare Isotope Beams (FRIB). Using combined neutron and γ-ray detector systems of the FRIB Decay Station Initiator (FDSi), we observed β-decay transitions populating neutron unbound states between 4.2 and 8 MeV in ^25Ne. The experimental results led to the revision of the β-decay half-life and β-delayed neutron-emission probability of ^25F. The β-decay strength distribution of ^25F extracted from the data agrees with the shell-model predictions using the USDB and SDPF-M effective interactions. This result indicates that the spherical neutron N = 16 shell gap persists in ^25F and ^25Ne.

The beta decay of O-24 was investigated at NSCL/MSU using a combination of neutron and gamma -ray spectroscopy. For the first time, the beta-delayed neutron energy spectrum of O-24 was measured, revealing three intensely populated, isolated neutron-unbound states in F-24. This allowed for the extraction of the decay strength in F-24 up to 6.2 MeV. A comprehensive comparison of the experimental results with various nuclear theories, ranging from the empirical shell model to the most advanced ab initio calculations, was conducted. While most theoretical predictions align with the experimental data for low-lying states, discrepancies arise at higher excitation energies. In the transition from O-24 to F-24, shell model calculations using the empirical USDB interaction predicted the structure of both nuclei without invoking the need for a stronger proton-neutron tensor force, which was postulated for the neighboring isotone F-25.

The first complete measurement of the β-decay strength distribution of _{17}^{45}Cl_{28} was performed at the Facility for Rare Isotope Beams (FRIB) with the FRIB Decay Station Initiator during the second FRIB experiment. The measurement involved the detection of neutrons and γ rays in two focal planes of the FRIB Decay Station Initiator in a single experiment for the first time. This enabled an analytical consistency in extracting the β-decay strength distribution over the large range of excitation energies, including neutron unbound states. We observe a rapid increase in the β-decay strength distribution above the neutron separation energy in _{18}^{45}Ar_{27}. This was interpreted to be caused by the transitioning of neutrons into protons excited across the Z=20 shell gap. The SDPF-MU interaction with reduced shell gap best reproduced the data. The measurement demonstrates a new approach that is sensitive to the proton shell gap in neutron rich nuclei according to SDPF-MU calculations.

We observed a new isomeric gamma transition at 168 keV in $^{36}$Mg, with a half-life of T$_{1/2}$=[130-500]$(\pm40)(^{+800}_{-20})_{sys}$ ns. We propose that the observed transition de-excites a new 0$^+$ isomeric state and populates the previously known first 2$^+$ state. The existence of this isomer is consistent with the predictions of the large-scale shell model calculations of $^{36}$Mg using the sdpf-u-mix interaction. The observed excitation energy of the second 0$^+$ state is caused by the small energy separation between two prolate-deformed configurations where the intruder configuration corresponds to two neutron excitations from the {\it sd} to the {\it pf} shell. Within this interpretation, $^{36}$Mg becomes the crossing point between nuclei in which ground state deformed/superdeformed configurations are caused by the dominance of N=20 intruders ($^{32,34}$Mg) and nuclei where deformed configurations are associated with N=28 intruders ($^{38}$Mg and beyond). We found the lack of three-body monopole corrections in other effective interactions results in a predominance of N=20 intruder configurations past $^{38}$Mg incompatible with our observation. We conclude that $^{36}$Mg bridges the N=20 and N=28 islands of inversion, forming the so-called Big Island of Deformation.

We observed a new isomeric gamma transition at 168 keV in $^{36}$Mg, with a half-life of T$_{1/2}$=[130-500]$(\pm40)(^{+800}_{-20})_{sys}$ ns. We propose that the observed transition de-excites a new 0$^+$ isomeric state and populates the previously known first 2$^+$ state. The existence of this isomer is consistent with the predictions of the large-scale shell model calculations of $^{36}$Mg using the sdpf-u-mix interaction. The observed excitation energy of the second 0$^+$ state is caused by the small energy separation between two prolate-deformed configurations where the intruder configuration corresponds to two neutron excitations from the {\it sd} to the {\it pf} shell. Within this interpretation, $^{36}$Mg becomes the crossing point between nuclei in which ground state deformed/superdeformed configurations are caused by the dominance of N=20 intruders ($^{32,34}$Mg) and nuclei where deformed configurations are associated with N=28 intruders ($^{38}$Mg and beyond). We found the lack of three-body monopole corrections in other effective interactions results in a predominance of N=20 intruder configurations past $^{38}$Mg incompatible with our observation. We conclude that $^{36}$Mg bridges the N=20 and N=28 islands of inversion, forming the so-called Big Island of Deformation.

The level structure of 36Al has been studied via β decay of 36Mg at the Facility for Rare Isotope Beams (FRIB) and the National Superconducting Cyclotron Laboratory (NSCL). A long-lived isomer in 36Al was identified which decays by β to an excited state of 36Si. The ground state and the isomeric state of 36Al were found to populate different energy levels of 36Si. The results from the two data sets in the present work complement each other. Configuration interaction calculations performed with the FSU shell-model Hamiltonians provide reasonable descriptions to the experimental observations and offer insight into future improvements of the theoretical interpretation.1 MoreReceived 29 March 2023Accepted 21 July 2023DOI:https://doi.org/10.1103/PhysRevC.108.014329©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayIsomer decaysNuclear structure & decaysShell modelProperties20 ≤ A ≤ 38Nuclear Physics

The level structure of 36Al has been studied via n decay of 36Mg at the Facility for Rare Isotope Beams (FRIB) and the National Superconducting Cyclotron Laboratory (NSCL). A long-lived isomer in 36Al was identified which decays by n to an excited state of 36Si. The ground state and the isomeric state of 36Al were found to populate different energy levels of 36Si. The results from the two data sets in the present work complement each other. Configuration interaction calculations performed with the FSU shell-model Hamiltonians provide reasonable descriptions to the experimental observations and offer insight into future improvements of the theoretical interpretation.

Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ rays in coincidence with ^{32}Na nuclei. This is the only known microsecond isomer (1  μs≤T_{1/2}<1  ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to ^{32}Mg, ^{32}Mg+π^{-1}+ν^{+1}. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ^{32}Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2^{+} state at 885 keV and a low-lying shape-coexisting 0_{2}^{+} state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in ^{32}Na: a 6^{-} spherical shape isomer that decays by E2 or a 0^{+} deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.

Background: The appearance of isomeric states in neutron-rich nuclei can provide a wealth of information on the underlying nuclear configurations as a progression is made from stable to exotic nuclei. These states can be characterized by measuring the half-lives and transition probabilities associated with their deexcitations. Recent studies on neutron-rich Si isotopes near N = 20 and approaching N = 28 have revealed states with intruder configurations resulting from multiple-particle, multiple-hole excitations across the closed shell gaps. These intruder states appear at low excitation energies along the Si isotopic chain, pointing to a diminished shell gap at N = 28. Purpose: The experiment was set up to directly measure the half-lives of isomeric states in neutron-rich nuclei populated via /3 decay. Methods: 37Al and 38Al isotopes were produced in the projectile fragmentation reaction of a 48Ca primary beam at NSCL. The isotopes were implanted into a CeBr3 detector, populating excited states of interest in 37Si following their /3- and /3-n decay, respectively. Ancillary arrays of HPGe and LaBr3(Ce) detectors were used to detect /3-delayed gamma rays. The half-lives of populated isomeric states were measured using the /3 - gamma timing method. Results: The half-life of the 68-keV (7/2-1 ) state in 37Si was measured for the first time as 9.1(7) ns, while that of the 156-keV (3/2-1) state was 3.20(4) ns, consistent with a previously reported value. The reduced transition probabilities and transition matrix elements associated with the gamma-ray decay from these excited states to the (5/2-1 ) ground state were extracted and agree with results from shell model calculations. Conclusions: The characterization of the (7/2-1) and (3/2-1) states in 37Si validates shell model predictions of low-lying nanosecond isomers in neutron-rich odd -A Si isotopes approaching the N = 28 shell gap.

The isomeric content of a Cl-34 beam produced in the intermediate-energy projectile fragmentation of a 150 MeV/u Ar-36 beam on a 3 mm-thick Be target was studied. beta-delayed gamma-ray spectroscopy was used to measure the population of Cl-34 fragments in the ground vs. isomeric states at zero degrees relative to the incoming primary beam for four different momentum settings of the fragment separator near the predicted central velocity of these fragments, as well as, at two non-zero-degree settings for one momentum setting. Of the settings explored, which excluded rigidities within 0.5% of the value predicted to maximize total Cl-34 yield due to unreacted primary beam, the maximum rate for the production of Cl-34m was found at a rigidity setting 0.75% below the predicted peak Cl-34 yield. The maximum population of the isomeric state relative to the ground state was observed at a rigidity 1.25% below the predicted maximum Cl-34 yield. Studies such as this are important in generating the understanding needed for producing isomer-enriched rare-isotope beams.

New half-lives for exotic isotopes approaching the neutron drip-line in the vicinity of N∼28 for Z=12-15 were measured at the Facility for Rare Isotope Beams (FRIB) with the FRIB decay station initiator. The first experimental results are compared to the latest quasiparticle random phase approximation and shell-model calculations. Overall, the measured half-lives are consistent with the available theoretical descriptions and suggest a well-developed region of deformation below ^{48}Ca in the N=28 isotones. The erosion of the Z=14 subshell closure in Si is experimentally confirmed at N=28, and a reduction in the ^{38}Mg half-life is observed as compared with its isotopic neighbors, which does not seem to be predicted well based on the decay energy and deformation trends. This highlights the need for both additional data in this very exotic region, and for more advanced theoretical efforts.

Precise neutron-energy measurements are required to probe the nuclear structure effects of neutron-rich nuclei, where a-delayed neutron emission becomes a dominant decay mode. The Neutron dEtector with Xn Tracking (NEXT) array has been designed and constructed to measure a-delayed neutrons with better energy resolution. The new design localizes the neutron interaction position by optically segmenting the detector along the direction of the neutron flight path, reducing the associated uncertainties in the neutron time-of-flight measurements. This significantly improves the energy resolution without losing the necessary detection efficiency. The proof-ofprinciple and efficiency measurements showed promising results. This article details the implementation of the neutron tracking capability of NEXT array in time-of-flight measurements.

An efficient neutron detection system with good energy resolution is required to correctly characterize decays of neutron-rich nuclei where β-delayed neutron emission is a dominant decay mode. The Neutron dEtector with Xn Tracking (NEXT) has been designed to measure β-delayed neutron emitters. By segmenting the detector along the neutron flight path, NEXT reduces the associated uncertainties in neutron time-of-flight measurements, improving energy resolution while maintaining detection efficiency. Detector prototypes are comprised of optically separated segments of a neutron-gamma discriminating plastic scintillator coupled to position-sensitive photomultiplier tubes. The first performance studies of this detector showed that high intrinsic neutron detection efficiency could be achieved while retaining good energy resolution. The results from the efficiency measurements using neutrons from direct reactions are presented.

The neutron detector NEXT will allow higher accuracy studies for beta-delayed neutron emission, while maintaining neutron-gamma discrimination. NEXT utilizes thin, segmented, inorganic scintillators which are paired with photosensitive devices to increase detection efficiency for energy measurement and tracking capabilities. NEXT is currently in a prototype phase and is continuously being modeled with GEANT4 based simulation software, NEXTSim. Neutrons and gamma-rays with energies ranging from 100 keV to 10 MeV have been simulated and show consistent results in regards to scattering patterns and energy resolution within NEXT. Sample simulation outputs will be shown and described in this work. This work is funded by NSF grant NSF-1919735, sub-grant A20-0254-S001.

Fast timing detectors are an essential element in the experimental setup for time-of-flight (ToF) mass measurements of unstable nuclei. We have upgraded the scintillator detectors used in experiments at the National Superconducting Cyclotron Laboratory (NSCL) by increasing the number of photomultiplier tubes that read out their light signals to four per detector, and characterized them in a test experiment with 48Ca beam at the NSCL. The new detectors achieved a time resolution (σ) of 7.5 ps. We systematically investigated different factors that affect their timing performance. In addition, we evaluated the ability of positioning the hitting points on the scintillator using the timing information and obtained a resolution (σ) below 1 mm for well-defined beam spots.