Gamma-ray fast-timing coincidence measurements from the 18O+18O fusion-evaporation reaction using a mixed LaBr3-HPGe array.
We report on a gamma-ray coincidence analysis using a mixed array of hyperpure germanium and cerium-doped lanthanum tri-bromide (LaBr3:Ce) scintillation detectors to study nuclear electromagnetic transition rates in the pico-to-nanosecond time regime in 33,34P and 33S following fusion-evaporation reactions between an 18O beam and an isotopically enriched 18O implanted tantalum target. Energies from decay gamma-rays associated with the reaction residues were measured in event-by-event coincidence mode, with the measured time difference information between the pairs of gamma-rays in each event also recorded using the ultra-fast coincidence timing technique. The experiment used the good full-energy peak resolution of the LaBr3:Ce detectors coupled with their excellent timing responses in order to determine the excited state lifetime associated with the lowest lying, cross-shell, Iπ=4- "intruder" state previously reported in the N=19 isotone 34P. The extracted lifetime is consistent with a mainly single-particle M2 multipolarity associated with a f7/2→d5/2 single particle transition.
Approaching the Island of Inversion: 34P
Yrast states in {sup 34}P were investigated using the {sup 18}O({sup 18}O,pn) reaction at energies of 20, 24, 25, 30, and 44 MeV at Florida State University and at Lawrence Berkeley National Laboratory. The level scheme was expanded, {gamma}-ray angular distributions were measured, and lifetimes were inferred with the Doppler-shift attenuation method by detecting decay protons in coincidence with one or more {gamma} rays. The results provide a clearer picture of the evolution of structure approaching the ''island of inversion,'' particularly how the one- and two-particle-hole (ph) states fall in energy with increasing neutron number approaching inversion. However, the agreement of the lowest few states with pure sd shell model predictions shows that the level scheme of {sup 34}P is not itself inverted. Rather, the accumulated evidence indicates that the 1-ph states start at 2.3 MeV. A good candidate for the lowest 2-ph state lies at 6236 keV, just below the neutron separation energy of 6291 keV. Shell model calculations made using a small modification of the WBP interaction reproduce the negative-parity, 1-ph states rather well.
Crossing of single-particle energy levels resulting from neutron excess in the sd shell
It has been shown that the large discrepancies between calculation and experiment in the binding energies of neutron-rich isotopes of sodium and magnesium can be removed by extending the model space to include the f7/2 shell. It is shown that the physical explanation of this result lies in the severe distortion of the underlying single-particle spectrum by the large neutron excess.