First half-life measurement of a low-lying isomer in 37Si

Background: The appearance of isomeric states in neutron-rich nuclei can provide a wealth of information on the underlying nuclear configurations as a progression is made from stable to exotic nuclei. These states can be characterized by measuring the half-lives and transition probabilities associated with their deexcitations. Recent studies on neutron-rich Si isotopes near N = 20 and approaching N = 28 have revealed states with intruder configurations resulting from multiple-particle, multiple-hole excitations across the closed shell gaps. These intruder states appear at low excitation energies along the Si isotopic chain, pointing to a diminished shell gap at N = 28. Purpose: The experiment was set up to directly measure the half-lives of isomeric states in neutron-rich nuclei populated via /3 decay. Methods: 37Al and 38Al isotopes were produced in the projectile fragmentation reaction of a 48Ca primary beam at NSCL. The isotopes were implanted into a CeBr3 detector, populating excited states of interest in 37Si following their /3- and /3-n decay, respectively. Ancillary arrays of HPGe and LaBr3(Ce) detectors were used to detect /3-delayed gamma rays. The half-lives of populated isomeric states were measured using the /3 - gamma timing method. Results: The half-life of the 68-keV (7/2-1 ) state in 37Si was measured for the first time as 9.1(7) ns, while that of the 156-keV (3/2-1) state was 3.20(4) ns, consistent with a previously reported value. The reduced transition probabilities and transition matrix elements associated with the gamma-ray decay from these excited states to the (5/2-1 ) ground state were extracted and agree with results from shell model calculations. Conclusions: The characterization of the (7/2-1) and (3/2-1) states in 37Si validates shell model predictions of low-lying nanosecond isomers in neutron-rich odd -A Si isotopes approaching the N = 28 shell gap.

Automated summary

The appearance of isomeric states in neutron-rich nuclei provides valuable information on nuclear configurations. The study reveals the presence of intruder states in neutron-rich Si isotopes, indicating a diminished shell gap at N = 28.