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

Makabata Mokgolobotho
2017 unpublished
The first reorientation-effect Coulomb-excitation experiment has been performed at iThemba LABS by bombarding 40 Ar beams on a heavy 208 Pb target at a "safe" energy of E lab =143.2 MeV. The goal was to determine, the spectroscopic quadrupole moment, Q S , of the first excita-tion 2 + 1 at 1.461 MeV in 40 Ar 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
more » ... ular distribution and Doppler correction, respectively, in coincidence with de-excited γ rays collected by 8 clover detectors in the AFRODITE array. The collected coincidence data were analysed using a state-of-the-art sorting code specially developed for this kind of measurements, which allowed, by setting up different energy and time conditions, a clean γ-ray spectrum for further analysis with the Coulomb-excitation coupled-channels code, GOSIA. A diagonal matrix element of 2 + 1 | | ˆ E2 | |2 + 1 = +0.047(22) eb has been determined, which yields Q S (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 Q S (2 + 1) = +0.01(4) eb by Nakai and collaborators in 1970, and further supports the zigzag of quadrupole shapes observed at the end of the sd shell. A possible explanation regards the influence of proton holes driving the nuclear shape towards an oblate configuration .