Determination of the Spectroscopic Quadrupole moment of the first 2+ excited state in 32S

Abstract

In this work we have determined the spectroscopic or static quadrupole moment of the rst excited state (QS (2+1) lying at 2230.6 keV in 32S using the reorientation e ect. The Coulomb-excitation experiment at safe bombarding energies was performed at iThemba LABS's AFRODITE vault, where 32S beams at 120.3 MeV were bombarded onto a 194Pt target of 1 mg/cm2 thickness. The beam energy has been chosen such that the separation between nuclear surfaces is greater than 6.5 fm at all scattering angles, in order to avoid nuclear interactions. A double-sided CD-type S3 silicon detector, with 24 rings and 32 sectors, has been placed upstream (at backward angles) to detect the scattered particles. Gamma rays have been detected with the AFRODITE clover array. This particle-gamma coincidence experiment allows for an angular distribution and Doppler correction of the gamma rays emitted at 9% the speed of light. The cross sections (or gamma-ray integrated yields) measured as a function of scattering angle at backward angles are sensitive to second-order perturbation e ects in Coulomb excitation, i.e., diagonal matrix elements which are directly related to the spectroscopic quadrupole moment. The gamma-ray integrated yields obtained from this experiment are compared with the GOSIA simulations, yielding a new measurement of QS (2+1) = 􀀀0:10 0:7 eb, which corresponds to a prolate shape in the intrinsic frame of the nucleus. The uncertainty of this measurement is limited by statistics. This result agrees with previous measurements and con rms the zig zag of shapes at the end of the sd shell when approaching the doubly-magic nucleus 40Ca. Nonetheless, the mystery continues as a prolate shape for the rst 2+ disagrees with modern theoretical mean- eld calculations and the pairing coupling model.