M. J. G. Borge

We report the decisive resolution of the ground state spin and parity of the proton–drip line nucleus Al 22 , a prime candidate for a proton halo. The resolution stems from the first β -delayed charged particle emission experiment in the gas stopping area at the Facility for Rare Isotope Beams (FRIB), leveraging high-intensity, low-energy beams extracted from the Advanced Cryogenic Gas Stopper (ACGS). The pristine beam quality from FRIB and the ACGS enabled a sensitive particle identification technique using thin silicon detectors, allowing for the suppression of the dominant proton background and the first observation of the weak β -delayed α transition from the isobaric analog state in Mg 22 to the Ne 18 ground state. This observation uniquely fixes the Al 22 ground state as 4 + . The valence proton is confined by a dominant d -wave centrifugal barrier which, combined with the Coulomb repulsion, hinders the tunneling required for halo formation despite the exceptionally low proton separation energy of Al 22 .

A search for a β -delayed fission ( β DF ) decay branch of isomerically pure samples of Au g 178 and Au m 178 was performed at the ISOLDE-CERN facility. Two complementary detection systems capable of registering α decays and fission fragments, the ISOLDE Decay Station and the ASET (Alpha SETup), were used. Despite very high statistics of produced Au 178 nuclei, no fission fragments were detected. Upper limits of β DF probabilities of P β DF ( Au g 178 ) < 1.11 ( 2 ) × 10 − 8 and P β DF ( Au m 178 ) < 9.7 ( 2 ) × 10 − 9 were determined. Corresponding lower limits of β DF partial half-lives were deduced as well, and the results are discussed in the context of experimental systematics of β DF in the neutron-deficient region of the nuclear chart.

New measurements of the β decay of Ga 64 and Ga 66 have been carried out using total absorption spectroscopy at CERN-ISOLDE. The purpose of the study was to determine whether systemic effects such as have affected previous measurements and also determine the degree of isospin mixing in these proton-rich nuclei. Our results show that the β strength distribution of Ga 64 was previously underestimated, while that of Ga 66 agrees well with previous high-resolution measurements. The results allowed us to determine the amount of isospin mixing in the 0 + → 0 + transitions to the ground states of the daughter nuclei. From the extracted log f t values, we determined the isospin mixing parameter α for the two cases. They were found to be consistent with values for similar transitions in other nuclei. Ga 64 exhibits the largest amount of isospin mixing in such nuclei. These findings improve our understanding of beta decay and have implications for nuclear structure models and medical applications of Ga 66 in PET imaging.

This manuscript reports on the direct observation of a β -delayed two-neutron emission in a study of In 134 at the ISOLDE Decay Station using neutron spectroscopy. We also report on the first measurement in β − decay of the long-sought 13 / 2 + excited state in Sn 133 , attributed to be the neutron single-particle i 13 / 2 orbital. The observation of sequential neutron emission is used to extract the relative population of the i 13 / 2 state, which was found to be much smaller than the predictions of the statistical model. The experiment was possible because of the innovative use of a neutron array with neutron discrimination and interaction tracking capabilities. This is the first study of the details of the two-neutron emission for a nucleus, which belongs to the r -process path. Understanding β -delayed two-neutron emission probabilities is essential to validate models used in astrophysical r -process nucleosynthesis calculations. Observing two-neutron emissions in β − decay paves the way for new experiments to study energy and angular correlations for β -delayed multineutron emitters.

The changes in the mean-squared charge radii relative to Bi 126 209 ( δ 〈 r 2 〉 N , 126 ) and the magnetic dipole and electric quadrupole moments in Bi 214 − 218 have been measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). Magnetic moments of odd-odd bismuth nuclei have been analyzed by the additivity relation. Previous tentative spin-parity and configuration assignments based on the β -decay feeding patterns have been supported. A normal odd-even staggering in charge radii of bismuth isotopes with N > 126 has been observed. The new data for the δ 〈 r 2 〉 of bismuth isotopes allow a study of the isotonic dependencies in the charge radii, revealing jumps in δ 〈 r 2 〉 132 , 126 and δ 〈 r 2 〉 134 , 126 at Z = 84 . This pattern could be explained by a sudden onset of octupole deformation at N = 132 and 134 when going from polonium ( Z = 84 ) to astatine ( Z = 85 ).

An α - and β -decay study of a pure source of laser-ionized and mass-separated Au 182 ( Z = 79, N = 103) was carried out at the ISOLDE Decay Station at the ISOLDE-CERN facility. Detailed γ − γ analysis following EC / β + decay of Au 182 was performed, and the level scheme of daughter nuclide Pt 182 was considerably extended via the identification of 125 new levels and 336 new γ -ray transitions. The nonexistence of a relatively long-lived isomeric state in Au 182 and influence of the pandemonium effect on β -decay feeding intensities are discussed. Differences in feeding for two coexisting bands in Pt 182 were investigated. The α -decay scheme of Au 182 was extended and an α -decay branching ratio of 0.129 ( 11 ) % was measured. Hindrance factors for α -decay branches were calculated and I π = ( 1 + , 2 + , 3 + ) assignment for the Ir 178 ground state was proposed.

The process of β -delayed fission ( β DF ) of Fr 230 , 232 and Ac 230 , 232 , 234 was studied in an experiment performed at the ISOLDE facility at CERN. As no fission fragments were observed for any of the nuclei investigated, upper limits for their β DF probability ( P β DF ) were determined. The experimental results were compared with theoretical calculations that were first benchmarked on Tl 178 , 180 P β DF experimental values. The P β DF values were calculated using the code TALYS to which β -strength functions obtained from the D1M Gogny parametrization and from the Skyrme functional SKO' were given as input together with fission paths obtained with BSkG3 and BSk14 models. Sensitivity studies of different β -strength functions, and fission paths scaling on the P β DF values were conducted, suggesting a stronger dependence of the P β DF on the fission paths rather than on the β -strength function used. Published by the American Physical Society 2025