Gamma ray studies in 40Ar, 33S and 19O

Abstract

The particle-gamma coincidence method was used to investigate the properties of states in 40Ar, 33S and 19O. In 40Ar, Ge(Li) and NaI(Tl) γ-ray spectrometry was used to determine mean lifetimes, spins and decay modes, while in 33S and 19O, spin and decay mode data were determined using NaI (Tl) spectrometry alone. The lifetimes of levels in 40Ar excited by the 40Ar (p, p') reaction were investigated using the Doppler shift attenuation method. The p-γ coincidence technique allowed data from many levels to be accumulated simultaneously. The results include the following data for states between 3.51 and 4.48MeV for which lifetimes have not been reported previously: τm(3.51)=0.12±0.045ps; τm(3.68)=0.14+0.09 -0.07ps; τm(3.92)=0.44±0.06ps; τm(4.48)<0.1ps. Particle-gamma angular correlations were also measured to determine level spins and branching and mixing ratios in 40Ar. Using these data and the lifetime results, the spins of the 3.21 and 4.48MeV levels were determined to be 2 and 1 respectively and the ranges of possible spins for many other levels were restricted. The results are compared with data from previous investigations and with the predictions of a recent shell model study and are discussed in terms of a simple model that considers mixing between shell model and collective states. The 32S(d,p) reaction was used to excite states in 33S in order to carry out p-γ correlation experiments on this nucleus. A unique spin assignment of 3/2 was made for the 3.94MeV level. The spin and mixing ratio data obtained agreed with previous results, but new decay modes were found for some of the levels of higher excitation energy. The experimental level scheme is compared with shell model and intermediate coupling model predictions. Correlation measurements were also performed to determine the spins of levels in 19O excited by the 18O(d,p) reaction. A thin 18O-enriched ice target was used. Previous spin assignments of 3/2 and 9/2 for the 2.37 and 2.78MeV levels respectively were found to be incorrect. Some features of the structure of this nucleus are well reproduced by an asymmetric rotor model: this may indicate that 19O is non-spherical.