The paper reports a new measurement of beta-delayed neutron emission from the neutron-rich nucleus cadmium-132 (132Cd). Beta-delayed neutron emission happens when a nucleus first undergoes beta decay (a neutron turns into a proton while emitting an electron) and the daughter nucleus is left in such a high-energy state that it promptly ejects a neutron. The authors find that 132Cd populates only neutron-unbound states, and they deduce a neutron-branching ratio close to 100% — in other words, nearly every beta decay of 132Cd is followed by neutron emission.

The experimental work was done at the ISOLDE facility at CERN. The team produced 132Cd by firing 1.4 GeV protons onto a uranium carbide (UCx) target heated to about 2200 °C, and then selected cadmium ions using laser ionization and a high-resolution separator. The ions were implanted on a moving tape at the ISOLDE Decay Station and observed with four high-purity germanium gamma detectors and a dedicated neutron time-of-flight array called VANDLE. The setup collected about 2.2×10^5 decays. Neutron energies were obtained from time-of-flight measurements; the neutron spectrum shows a strong concentration of intensity near 2 MeV.

To interpret the data, the authors used a large-scale shell-model (LSSM) calculation with the N3LO nuclear interaction. The calculation reproduces the experimental decay-strength distribution and suggests the decay is dominated by a transformation of a neutron in the g7/2 single-particle orbital deep below the Fermi surface into a proton in the g9/2 orbital. In plain terms, the most likely beta transition moves a neutron from an inner neutron shell into a proton state that sits at a different energy, producing daughter states high enough to emit neutrons.

This result matters for nuclear astrophysics. The rapid neutron-capture process (r-process) that builds many of the heavy elements in the universe follows paths that pass near nuclei with N≈82 neutrons. The beta-decay half-lives and the probabilities to emit neutrons after decay affect the flow of material and the final abundance pattern. The authors compared their LSSM predictions for half-lives and neutron branching ratios in nearby nuclei (atomic number Z<50 and neutron number N≥82) with those from widely used “global” models such as the Finite-Range Droplet Model (FRDM). Their shell-model calculations match known data well and suggest that some global models overestimate half-lives and underestimate neutron emission probabilities in this region; the paper reports cases where the LSSM half-lives can be up to about two times shorter than FRDM predictions.