A leading challenge of nuclear-structure research is to understand the properties of nuclides of extreme isospin. Experiments at radioactive-ion-beam facilities, such as the Facility for Rare Isotope Beams in the US, may answer key questions that address diverse topics including fundamental nuclear physics, stellar nucleosynthesis and nuclear applications. The neutron-rich Mo–Ru–Pd (Z = 42 – 46) nuclides are hypothesised to exhibit triaxial-oblate deformation. We performed an experiment with the Facility for Rare Isotope Beams Decay Station initiator (FDSi) to study the structure and decay properties of nuclides in this region. Over 100 different nuclides have been identified in a preliminary analysis of the data. This work presents a first look at several examples between Rb (Z = 37) and Ag (Z = 47). Performance of the FDSi and methods developed to measure ground-state and excited-state lifetimes are presented, and plans for future work are also discussed.

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.

Polonium isotopes having two protons above the shell closure at Z=82 show a wide variety of low-lying, high-spin isomeric states across the whole chain. The structure of neutron-deficient isotopes up to ^{210}Po  (N=126) is well established as they are easily produced through various methods. However, there is not much information available for the neutron-rich counterparts for which only selective techniques can be used for their production. We report on the first fast-timing measurements of yrast states up to the 8^{+} level in ^{214,216,218}Po isotopes produced in the β^{-} decay of ^{214,216,218}Bi at ISOLDE, CERN. In particular, our new half-life value of 607(14) ps for the 8_{1}^{+} state in ^{214}Po is nearly 20 times shorter than the value available in the literature and comparable with the newly measured half-lives of 409(16) and 628(25) ps for the corresponding 8_{1}^{+} states in ^{216,218}Po, respectively. The measured B(E2;8_{1}^{+}→6_{1}^{+}) transition probability values follow an increasing trend relative to isotope mass, reaching a maximum for ^{216}Po. The increase contradicts the previous claims of isomerism for the 8^{+} yrast states in neutron-rich ^{214}Po and beyond. Together with the other measured yrast transitions, the B(E2) values provide a crucial test of the different theoretical approaches describing the underlying configurations of the yrast band. The new experimental results are compared to shell-model calculations using the KHPE and H208 effective interactions and their pairing-modified versions, showing an increase in configuration mixing when moving toward the heavier isotopes.

Beta-delayed proton and gamma emission in the decay of ^21 Mg has been measured at ISOLDE, CERN with the ISOLDE Decay Station (IDS) set-up. The existing decay scheme is updated, in particular what concerns proton transitions to excited states in ^20 Ne. Signatures of interference in several parts of the spectrum are used to settle spin and parity assignments to highly excited states in ^21 Na. The previously reported β p α branch is confirmed. A half-life of 120.5(4) ms is extracted for ^21 Mg. The revised decay scheme is employed to test mirror symmetry in the decay and to extract the beta strength distribution of ^21 Mg that is compared with theory.

The changes in the mean-squared charge radius of Tlg209 (N=128) and Tlm207 (N=126) relative to Tl205 have been measured for the first time using the in-source laser resonance-ionization spectroscopy technique with the Laser Ion Source and Trap (LIST) at ISOLDE (CERN). The application of the LIST suppresses the dominant background from isobaric francium isotopes and allows access to thallium nuclides with A⩾207. The characteristic kink in the charge radii at the N=126 neutron shell closure, as well as the odd-even effect similar to that in the adjacent bismuth, lead, and mercury isotopic chains, have been observed. The self-consistent theory of finite Fermi systems based on the energy density functional by Fayans reproduces the behavior of charge radii in these isotopic chains near N=126. The comparison with calculations in the framework of the relativistic mean field (RMF) approach is also presented. In the case of the Fayans functional it is a specific form of pairing interaction with the dependence on the density gradient that is essential to provide agreement with the experimental charge radii. In particular, the kink is reproduced without the inversion of g9/2 and i11/2 neutron single-particle states, which is a prerequisite to correctly describe the kink in the RMF models. Published by the American Physical Society 2024

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.

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 excited structure of the single-hole nucleus 131 Sn populated by the beta - decay of 131 In was investigated in detail at the ISOLDE facility at CERN. This new experiment took advantage of isomeric purification capabilities provided by resonant ionization, making it possible to independently study the decay of each isomer for the first time. The position of the first-excited nu h 11 / 2 neutron-hole state was confirmed via an independent mass spectroscopy experiment performed at the Ion Guide Isotope Separator On-Line facility at the University of Jyv & auml;skyl & auml;. The level scheme of 131 Sn was notably expanded with the addition of 31 new gamma-ray transitions and 22 new excited levels. The gamma-emitting excited levels above the neutron separation energy in 131 Sn were investigated, revealing a large number of states, which in some cases decay by transitions to other neutron-unbound states. Our analysis showed the dependence between the population of these states in 131 Sn and the beta-decaying 131 In state feeding them. Profiting from the isomer selectivity, it was possible to estimate the direct beta feeding to the 3/2+ / 2 + ground and 11/2- / 2 - isomeric states, disentangling the contributions from the three indium parent states. This made possible to resolve the discrepancies in log ft for first-forbidden transitions observed in previous studies, and to determine the beta-delayed neutron decay probability (Pn) P n ) values of each indium isomers independently. The first measurement of subnanosecond lifetimes in 131 Sn was performed in this work. A short T 1 / 2 = 18(4)-ps value was measured for the 1/2+ / 2 + neutron single-hole 332-keV state, which indicates an enhanced l-forbidden M 1 behavior for the nu 3 s - 1 1/2 / 2 -> nu 3 d - 13 / 2 transition. The measured half-lives of high-energy states populated in the beta decay of the (21/2+) / 2 + ) second isomeric state ( 131 m 2 In) provided valuable information on transition rates, supporting the interpretation of these levels as core-excited states analogous to those observed in the doubly-magic 132 Sn.

The 9 decays of 87 Br, 88 Br, and 87 Kr were measured with the Modular Total Absorption Spectrometer (MTAS) at Oak Ridge National Laboratory's Holifield Radioactive Ion Beam Facility (HRIBF). Both bromine isotopes are 9-delayed neutron emitters that have large cumulative fission yields and were identified as top-priority cases for total absorption study by the Nuclear Energy Agency in 2007. Our investigations corroborate that the decay schemes of 87 Br and 88 Br suffer from the so-called pandemonium effect. Unique MTAS properties enable direct neutron measurements. We present MTAS-derived 9-delayed neutron spectra, 9-delayed neutron emission probabilities of P n ( 87 Br) = 2.36(24)%, and P n ( 88 Br) = 6.4(6)%, and the 9-delayed neutron transitions intensity of 4(2)% to the first excited 87 Kr state, populated in the 9-neutron decay of 88 Br. Incorporating new data into calculations of the electromagnetic decay heat component emitted during thermal neutron fission of 235U and 239 Pu improves agreement with experimental data up to approximately 80 s after fission. The estimation of the nuclear reactor nu e flux results in changes of up to 1% in the expected nu e interactions with the detector material for 235 U, 238 U, 239 Pu, and 241 Pu.

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 excited structure of the single-hole nucleus Sn131 populated by the β− decay of In131 was investigated in detail at the ISOLDE facility at CERN. This new experiment took advantage of isomeric purification capabilities provided by resonant ionization, making it possible to independently study the decay of each isomer for the first time. The position of the first-excited νh11/2 neutron-hole state was confirmed via an independent mass spectroscopy experiment performed at the Ion Guide Isotope Separator On-Line facility at the University of Jyväskylä. The level scheme of Sn131 was notably expanded with the addition of 31 new γ-ray transitions and 22 new excited levels. The γ-emitting excited levels above the neutron separation energy in Sn131 were investigated, revealing a large number of states, which in some cases decay by transitions to other neutron-unbound states. Our analysis showed the dependence between the population of these states in Sn131 and the β-decaying In131 state feeding them. Profiting from the isomer selectivity, it was possible to estimate the direct β feeding to the 3/2+ ground and 11/2− isomeric states, disentangling the contributions from the three indium parent states. This made possible to resolve the discrepancies in logft for first-forbidden transitions observed in previous studies, and to determine the β-delayed neutron decay probability (Pn) values of each indium isomers independently. The first measurement of subnanosecond lifetimes in Sn131 was performed in this work. A short T1/2=18(4)−ps value was measured for the 1/2+ neutron single-hole 332-keV state, which indicates an enhanced l-forbidden M1 behavior for the ν3s1/2−1→ν3d3/2−1 transition. The measured half-lives of high-energy states populated in the β decay of the (21/2+) second isomeric state (In131m2) provided valuable information on transition rates, supporting the interpretation of these levels as core-excited states analogous to those observed in the doubly-magic Sn132. Published by the American Physical Society 2024

A detailed β-decay study of the low- and high-spin states in Bi216 has been performed at the ISOLDE Decay Station at the CERN-ISOLDE facility. In total, 48 new levels and 83 new transitions in the β-decay daughter Po216 were identified. Shell-model calculations for excited states in Bi216 and Po216 were performed using the H208 and the modified Kuo-Herling particle effective interactions. Based on the experimental observations and the shell-model calculations, the most likely spin and parity assignments for the β-decaying states in Bi216 are (3−) and (8−), respectively. Published by the American Physical Society 2024

The magnetic dipole moments (μ) of 209Tlg (N=128) and 207Tlm (N=126) have been measured for the first time using the in-source laser resonance-ionization spectroscopy technique with the Laser Ion Source and Trap (LIST) at ISOLDE (CERN). The application of the LIST suppresses the usually overwhelming background of the isobaric francium isotopes and allows access to heavy thallium isotopes with A⩾207. The self-consistent theory of finite Fermi systems based on the energy density functional by Fayans et al. well describes the N dependence of μ for 1/2+ thallium ground states, as well as μ for the 11/2− isomeric states in europium, gold and thallium isotopes. The inclusion of particle-vibration coupling leads to a better agreement between the theory and experiment for μ(Tlg, Iπ=1/2+). It is shown that beyond mean-field contributions to μ cannot be neglected at least for thallium isotopes with Iπ=1/2+.

The excited structure of A = 128 isobars populated in the beta decay of Cd-128 has been investigated by means of high-resolution gamma-spectroscopy and fast-timing measurements. The experiment was performed at the ISOLDE facility at CERN profiting from the production of intense and pure Cd beams by means of a temperature-controlled quartz transfer line, capable of suppressing surface-ionized species. The production yields and purity of Cd beams are presented. Results on sub-nanosecond lifetimes for excited states in In-128 and Te-128 are discussed.

The presence of a neutron halo in 15C has been demonstrated in several reaction experiments at intermediate energies. In the present study, the dynamical effects of this structure are observed for the first time at Coulomb barrier energies in the 15C + 208Pb quasi-elastic scattering at Elab=65 MeV, measured at the HIE-ISOLDE facility, CERN using the high-granularity detector array GLORIA. A combined continuum discretised coupled channels and coupled reaction channels calculation describes the data well and significant coupling effects due both to breakup and single-neutron stripping are identified.

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.

beta-delayed gamma-neutron spectroscopy has been performed on the decay of A=84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of 3He neutron counters (BRIKEN). beta-2n-gamma events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the gamma branching ratios were obtained for these isotopes, and the neutron emission probabilities (P-xn) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimental branching ratios. We found that the P-1n and P-2n values are sensitive to the nuclear level densities of 1n daughter nuclei and showed that the statistical model reproduced the P-2n/P-1n ratio better when experimental levels plus shell-model level densities fit by the Gilbert-Cameron formula were used as the level-density input. We also showed the neutron and gamma branching ratios are sensitive to the ground-state spin of the parent nucleus. Our statistical model analysis suggested J <= 3 for the unknown ground-state spin of the odd-odd nucleus Ga-86, from the I gamma(4(+)-> 2(+))/I-gamma(2(+)-> 0(+)) ratio of Ga-84 and the P-2n/P-1n ratio. These results show the necessity of detailed understanding of the decay scheme, including data from neutron spectroscopy, in addition to gamma measurements of the multineutron emitters.

Neutron-rich lanthanides were produced via in-flight fission of a U-238 primary beam at the RIBF, RIKEN Nishina Center to measure half-lives (T-1/2) and beta-delayed neutron emission probabilities (P-n) in order to constrain r-process abundance calculations. Pm159-166, Sm161-168, Eu165-170, and Gd167-172 ions were implanted in the Advanced Implantation Detector Array (AIDA), and beta-delayed neutrons and gamma-rays were detected by the surrounding detector array (BRIKEN). For the validation of T-1/2 values derived from implantation-beta (i-beta) time correlations, gamma-spectroscopic methods were used as well. The experimental results of the beta-delayed gamma-spectroscopy of Pm-162 are presented here as an example. A half-life value from gamma-decay curves was derived with a comparable uncertainty to the result from the i-beta method, and a mean value well within the 1 sigma range.