Determining the Spectroscopic Quadrupole Moment Qs of the 2+1 state in 40Ar

Abstract

The rst reorientation-e ect Coulomb-excitation experiment has been performed at iThemba LABS by bombarding 40Ar beams on a heavy 208Pb target at a \safe" energy of Elab =143.2 MeV. The goal was to determine, the spectroscopic quadrupole moment, QS , of the rst excita- tion 2+ 1 at 1.461 MeV in 40Ar at \safe" energies. The scattered particles were detected at backward angles using a double-sided CD-type S3 silicon detector, composed of 24 rings and 32 sectors for angular distribution and Doppler correction, respectively, in coincidence with de-excited rays collected by 8 clover detectors in the AFRODITE array. The collected coincidence data were analysed using a state-of-the-art sorting code spe- cially developed for this kind of measurements, which allowed, by setting up di erent energy and time conditions, a clean -ray spectrum for further analysis with the Coulomb-excitation coupled-channels code, GOSIA. A diagonal matrix element of h2+ 1 j jE^2 j j2+ 1 i = +0:047(22) eb has been determined, which yields QS (2+ 1 ) = +0:036(17) eb, after detailed GOSIA analysis and minimisation. This value is 8.5 times more precise than the only previous measurement of QS (2+ 1 ) = +0:01(4) eb by Nakai and col- laborators in 1970, and further supports the zig-zag of quadrupole shapes observed at the end of the sd shell. A possible explanation regards the in uence of proton holes driving the nuclear shape towards an oblate con- guration.