Quadrupole collectivity in the neutron-rich sulfur isotopes S38,40,42,44

Shell structure of $^{43}$S and collapse of the $N=28$ shell closure

“…The isobar 43 Cl and the isotone 45 Cl have transitions at 943 and 928 keV which could have contaminated the spectrum. A recent Coulomb excitation experiment confirmed the 977 keV state that is directly excited from the ground state [42]. The 850 keV transition is placed on top of the 625 keV one, since the former was not observed in the proton removal reaction [33].…”

“…The momentum distribution for the 977 and 1856 keV states are asymmetric and very broad, suggesting the population via a non-direct process. This would be expected from a collective rotational band member [27,42].…”

“…The 42 Si nucleus is well deformed, it exhibits a low excitation energy for the first 2 + state [16] and a large R 4/2 ratio [17]. Calculations with the SDPF-MU interaction predict the ground state of 42 Si to be strongly oblate deformed [15].…”

“…Approaching the drip-line [19], the last N = 28 nucleus with excited states known is 40 Mg [20]. The measured two-proton removal cross sections along the N = 28 isotones [17,21] were interpreted as showing a change of the ground state deformation from prolate in 44 S to oblate for 42 Si, and back to prolate at 40 Mg.…”

“…Calculations with the SDPF-MU interaction predict the ground state of 42 Si to be strongly oblate deformed [15]. Detailed spectroscopy of 42 Si, however, questioned the 4 + assignment of Ref. [17] and proposed an excited 0 + 2 state based on the observed population cross section [18].…”