Beta-delayed neutron emission spectroscopy in the 78Ni region and development of YSO-based implantation detector

Decays of Cu isotopes provide a laboratory to study the properties for nearly doubly-magic nuclei with a significant neutron excess which are also relevant for the r-process models. The beta-decay properties of doubly-magic ⁷⁸Ni (N=50) are imprinted in the neighboring Cu decay, nuclei with a single proton outside the Z=28 core. The investigated isotopes ^79,80,81Cu are strong beta-delayed neutron precursors, suggesting that their decay strength distribution lies above neutron separation energies in ^79,80,81Zn daughters.

For decay studies, a segmented scintillator YSO (Y₂SiO₅ : Ce doped) based implantation detector was developed at the University of Tennessee, Knoxville. The detector is compact in structure and offers good spatial and timing resolution, crucial for ion-β correlations and time-of-flight (ToF) based β-delayed neutron emission spectroscopy, respectively. The detector was employed as a part of the BRIKEN neutron counter at the Radioactive Isotope Beam Factory (RIBF) at RIKEN Nishina Center in Japan, aiming to measure one- and two- neutron emission probabilities (P _n,2n ). for nuclei around the ⁷⁸Ni region. Another variant of the detector with a more advanced design was used along with VANDLE (Versatile Array for Neutron Detection at Low Energy) to conduct spectroscopy of β-delayed neutrons in the same region.

Reconstruction of the feeding intensities requires analysis of neutron-gamma cascades, as is suggested by the statistical model. This work reports the first direct measurements of β-decay strength to neutron-unbound states in the decay of ^{81, 80, 79}Cu. The results from the experiment are compared to shell-model calculations with various sets of single-particle energies and residual interactions. Finally, predictions for ⁷⁸Ni decay are made based on the model best-describing ⁷⁹Cu.