Using the time-of-flight technique, we measured the beta-delayed neutron emission of ^{132}Cd. From our large-scale shell model (LSSM) calculation using the N^{3}LO interaction [Z. Y. Xu et al., Phys. Rev. Lett. 131, 022501 (2023)PRLTAO0031-900710.1103/PhysRevLett.131.022501], we suggest the decay is dominated by the transformation of a neutron in the g_{7/2} orbital, deep below the Fermi surface, into a proton in the g_{9/2} orbital. We compare the beta-decay half-lives and neutron branching ratios of nuclei with Z<50 and N≥82 obtained with our LSSM with those of leading "global" models such as finite-range droplet model (FRDM). Our calculations match known half-lives and neutron branching ratios well and suggest that current leading models overestimate the yet-to-be-measured half-lives. Our model, backed by the ^{132}Cd decay data presented here, offers robust predictive power for nuclei of astrophysical interest such as r-process waiting points.

We investigated decays of ^{51,52,53}K at the ISOLDE Decay Station at CERN in order to understand the mechanism of the β-delayed neutron-emission (βn) process. The experiment quantified neutron and γ-ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the βn process originate from the structureless "compound nucleus." The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied β decay due to insufficient excitation energy and level densities in the neutron emitter. In the ^{53}K βn decay, we found a preferential population of the first excited state in ^{52}Ca that contradicted Bohr's hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of "compound-nucleus" decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy.

Background: The nuclear structure of low-lying excited states in I-139, a neutron-rich nucleus with Z = 53 and N = 86 above Sn-132 and in the proximity of A similar to 140, is investigated. Purpose: The decay scheme for I-139 is obtained after the first beta-decay measurement of Te-139. Transitions in I-138 are detected after the beta-delayed neutron emission of Te-139. The Pn ratio is investigated based on the gamma-ray emissions. Methods: beta-delayed. -ray spectroscopy is employed to study excited states in I-139, populated in the decay of a mass-separated beam of Te-139, produced in the in-flight fission of U-235 on a Be-9 target. Results: The new level scheme of I-139 with 26 new transitions, established for the first time in beta decay, is reported. Two new transitions are observed also in I-138. The beta-delayed neutron emission probability P-n of Te-139 is deduced to be 17.6(48)%. New and/or more restrictive spin/parity assignments of states in the daughter I-138 nucleus are reported as well. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell-model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

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 beta -decaying states of 70 , 72 Co were studied at the National Superconducting Cyclotron Laboratory using the VANDLE neutron time -of -flight array. The (6 - , 7 - ) beta -decaying state in 70 Co is near -spherical with a lifetime of 113 +/- 7 ms, and the low -spin (1 + , 2 + ) beta -decaying state is postulated to be the prolate deformed ground state with a lifetime of 508 +/- 7 ms. Both decay predominantly to the bound states of 70 Ni. For the first time neutron -emissions from neutron unbound states from both the (6 - , 7 - ) and (1 + , 2 + ) beta decays were measured. Even with the low statistics data, we were able to disentangle the neutron emission from both decays, which enabled a determination of beta -decay strength above the neutron separation energy of 70 Ni. Neutron emission probabilities were measured to be 7 . 1 +/- 1 . 5% and 9 . 4 +/- 1 . 7%, respectively, for the (6 - , 7 - ) and (1 + , 2 + ) decays. The decay pattern of the 70 Co is driven by neutron f 5 / 2 to proton f 7 / 2 Gamow-Teller transformation. The observed population of neutron unbound states is attributed to the conversion of p 1 / 2 and p 3 / 2 neutrons to p 3 / 2 and p 1 / 2 protons excited across the Z = 28 closed shell.

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.

The magnetic dipole and the spectroscopic quadrupole moments of the nuclear ground states in the odd-mass nickel isotopes 59−67Ni have been determined using collinear laser spectroscopy at the CERN-ISOLDE facility. They are compared to ab initio valence-space in-medium similarity renormalization group (VS-IMSRG) calculations including contributions of two-body currents as well as to shell-model calculations. The two-body-current contributions significantly improve the agreement with experimental data, reducing the mean-square deviation from the experimental moments by a factor of 3 to 5, depending on the employed interaction. For all interactions, the largest contributions are obtained for the 52− (72−) isotopes 65Ni (55Ni), which is ascribed to the high angular momentum of the f orbitals. Our results demonstrate that the inclusion of two-body-current contributions to the magnetic moment in an isotopic chain of complex nuclei can be handled by the VS-IMSRG and can outperform phenomenological shell-model calculations using effective g-factors in the nickel region.

Background: The nuclear structure of low-lying excited states in 139 I, a neutron-rich nucleus with Z = 53 and N = 86 above 132 Sn and in the proximity of A similar to 140, is investigated. Purpose: The decay scheme for 139I is obtained after the first /3-decay measurement of 139 Te. Transitions in 138I are detected after the /3-delayed neutron emission of 139 Te. The Pn ratio is investigated based on the gamma-ray emissions. Methods: /3-delayed gamma-ray spectroscopy is employed to study excited states in 139 I, populated in the decay of a mass-separated beam of 139 Te, produced in the in-flight fission of 235U on a 9 Be target. Results: The new level scheme of 139I with 26 new transitions, established for the first time in /3 decay, is reported. Two new transitions are observed also in 138 I. The /3-delayed neutron emission probability Pn of 139 Te is deduced to be 17.6(48)%. New and/or more restrictive spin/parity assignments of states in the daughter 138I nucleus are reported as well. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell-model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

Collinear laser spectroscopy measurements were performed on 68-74Ge isotopes (.. = 32) at ISOLDE-CERN, by probing the 4..24..2 3.. 1. 4..24..5.. 3.... 1 atomic transition (269 nm) of germanium. Nuclear charge radii are determined via the measured isotope shifts, revealing a larger local variation than the neighboring isotopic chains. Nuclear density functional theory with the Fayans functionals Fy(...,HFB) and Fy(IVP), and the SV-min Skyrme describes the experimental data for the differential charge radii.....2. and charge radii.. c within the theoretical uncertainties. The observed large variation in the charge radii of germanium isotopes is better accounted for by theoretical models incorporating ground state quadrupole correlations. This suggests that the polarization effects due to pairing and deformation contribute to the observed large odd-even staggering in the charge radii of the Ge isotopic chain.

Collinear laser spectroscopy was performed on the isomer of the aluminium isotope $^{26m}$Al. The measured isotope shift to $^{27}$Al in the $3s^{2}3p\;^{2}\!P^\circ_{3/2} \rightarrow 3s^{2}4s\;^{2}\!S_{1/2}$ atomic transition enabled the first experimental determination of the nuclear charge radius of $^{26m}$Al, resulting in $R_c$=\qty{3.130\pm.015}{\femto\meter}. This differs by 4.5 standard deviations from the extrapolated value used to calculate the isospin-symmetry breaking corrections in the superallowed $\beta$ decay of $^{26m}$Al. Its corrected $\mathcal{F}t$ value, important for the estimation of $V_{ud}$ in the CKM matrix, is thus shifted by one standard deviation to \qty{3071.4\pm1.0}{\second}.

The decay properties of ^133In were studied in detail at the ISOLDE Decay Station (IDS). The implementation of the Resonance Ionization Laser Ion Source (RILIS) allowed separate measurements of its 9/2^+ ground state (^133gIn) and 1/2^- isomer (^133mIn). With the use of β-delayed neutron and γ spectroscopy, the decay strengths above the neutron separation energy were quantified in this neutron-rich nucleus for the first time. The allowed Gamow-Teller transition 9/2^+→7/2^+ was located at 5.92 MeV in the ^133gIn decay with a logft = 4.7(1). In addition, several neutron-unbound states were populated at lower excitation energies by the First-Forbidden decays of ^133g,mIn. We assigned spins and parities to those neutron-unbound states based on the β-decay selection rules, the logft values, and systematics.

Isomeric states were observed in nuclei produced in an experiment at the RIKEN Nishina Center Radioactive Isotope Beam Factory following the in-flight fission of a 345 MeV/nucleon ^238 U beam. Isomers reported in nuclei spanning a predicted prolate-oblate shape change boundary, ^111 Zr ( E=283.1 keV; τ =0.326(63) s), ^112 Nb ( E=44.2 keV; τ =0.094(26) s), ^113 Nb ( E=135.4 keV; τ =0.846(80) s), and ^115 Mo ( E=198.6 keV; τ =63(4) s), are compared to potential-energy surface calculations which gave a selection of low-lying configurations for each nucleus. Tentative assignments of ground and excited states were made based on energy similarities to the calculations, reduced transition probabilities of the decays, and constraints of transition multipolarities from γ -ray coincidence measurements. These assignments are suggestive of significant deformation being persistent for N>70 in this region. In addition, isomers in ^108 Nb, ^109 Nb, ^113 Tc, ^117 Ru, ^119 Ru, ^120 Rh, and ^122 Rh, not spanning the prolate-oblate transition discussed, are presented.

The decay properties of $^{133}$In were studied in detail at the ISOLDE Decay Station (IDS). The implementation of the Resonance Ionization Laser Ion Source (RILIS) allowed separate measurements of its $9/2^+$ ground state ($^{133g}$In) and $1/2^-$ isomer ($^{133m}$In). With the use of $\beta$-delayed neutron and $\gamma$ spectroscopy, the decay strengths above the neutron separation energy were quantified in this neutron-rich nucleus for the first time. The allowed Gamow-Teller transition $9/2^+\rightarrow7/2^+$ was located at 5.92 MeV in the $^{133g}$In decay with a logft = 4.7(1). In addition, several neutron-unbound states were populated at lower excitation energies by the First-Forbidden decays of $^{133g,m}$In. We assigned spins and parities to those neutron-unbound states based on the $\beta$-decay selection rules, the logft values, and systematics.

The β decays from both the ground state and a long-lived isomer of ^{133}In were studied at the ISOLDE Decay Station (IDS). With a hybrid detection system sensitive to β, γ, and neutron spectroscopy, the comparative partial half-lives (logft) have been measured for all their dominant β-decay channels for the first time, including a low-energy Gamow-Teller transition and several first-forbidden (FF) transitions. Uniquely for such a heavy neutron-rich nucleus, their β decays selectively populate only a few isolated neutron unbound states in ^{133}Sn. Precise energy and branching-ratio measurements of those resonances allow us to benchmark β-decay theories at an unprecedented level in this region of the nuclear chart. The results show good agreement with the newly developed large-scale shell model (LSSM) calculations. The experimental findings establish an archetype for the β decay of neutron-rich nuclei southeast of ^{132}Sn and will serve as a guide for future theoretical development aiming to describe accurately the key β decays in the rapid-neutron capture (r-) process.

The $\ensuremath{\beta}$ decay of $^{142}\mathrm{Te}_{90}$ to $^{142}\mathrm{I}_{89}$ was investigated for the first time. The parent nucleus was produced by the in-flight fission of a $^{238}\mathrm{U}$ beam with an energy of 345 MeV per nucleon, impinging on a $^{9}\mathrm{Be}$ target at the Radioactive Isotope Beam Factory of RIKEN. Excited states in $^{142}\mathrm{I}$ were established by $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray spectroscopy. The observed $({1}^{+})$ states in $^{142}\mathrm{I}$ could be interpreted to be predominantly the $\ensuremath{\nu}0{h}_{9/2}\ensuremath{\bigotimes}\ensuremath{\pi}0{h}_{11/2}$ configuration formed by a Gamow-Teller transition between a neutron in the $0{h}_{9/2}$ orbital and a proton in the $0{h}_{11/2}$ orbital. Additional features of the $({1}^{+})$ states are discussed by comparing with neighboring heavier isotones, such as $^{144}\mathrm{Cs}$ and $^{146}\mathrm{La}$. In the context of deformed shell-model calculations, the $({1}_{1}^{+})$ state is closely related to the $\ensuremath{\nu}[5,3,2]3/2\ensuremath{\bigotimes}\ensuremath{\pi}[5,5,0]1/2$ configuration, which may be related to the weak Gamow-Teller transition strength.

Models of the beta-delayed neutron emission (beta n) assume that neutrons are emitted statistically via an intermediate compound nucleus post beta decay. Evidence to the contrary was found in an In-134 beta-decay experiment carried out at ISOLDE CERN. Neutron emission probabilities from the unbound states in Sn-134 to known low-lying, single-particle states in Sn-133 were measured. The neutron energies were determined using the time-of-flight technique, and the subsequent decay of excited states in Sn-133 was studied using gamma-ray detectors. Individual beta n probabilities were determined by correlating the relative intensities and energies of neutrons and gamma rays. The experimental data disagree with the predictions of representative statistical models which are based upon the compound nucleus postulate. Our results suggest that violation of the compound nucleus assumption may occur in beta-delayed neutron emission. This impacts the neutron-emission probabilities and other properties of nuclei participating in the r-process. A model of neutron emission, which links the observed neutron emission probabilities to nuclear shell effects, is proposed.

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

Collinear laser spectroscopy was performed on the isomer of the aluminium isotope ^{26m}Al. The measured isotope shift to ^{27}Al in the 3s^{2}3p ^{2}P_{3/2}^{○}→3s^{2}4s ^{2}S_{1/2} atomic transition enabled the first experimental determination of the nuclear charge radius of ^{26m}Al, resulting in R_{c}=3.130(15) fm. This differs by 4.5 standard deviations from the extrapolated value used to calculate the isospin-symmetry breaking corrections in the superallowed β decay of ^{26m}Al. Its corrected Ft value, important for the estimation of V_{ud} in the Cabibbo-Kobayashi-Maskawa matrix, is thus shifted by 1 standard deviation to 3071.4(1.0) s.