Search

The neutron-rich $N=28$ nucleus $^{44}\mathrm{S}$ was studied using the two-proton knockout reaction from $^{46}\mathrm{Ar}$ at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4${}^{+}$ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the ``intruder'' configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in $^{44}\mathrm{S}$, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.

The recent assertion that shape coexistence occurs in the neutron-rich isotope $^{43}\mathrm{S}$ implies that a state observed at 940 keV in a previous study is a rotational excitation of the deformed ground state. Here we use results from two intermediate-energy reactions to demonstrate that this state---assigned an energy of 971 keV in the present work---is indeed a rotational state. This result strengthens the case for shape coexistence in $^{43}\mathrm{S}$.

Neutron hole states in the exotic $N=27$ isotope $^{44}\mathrm{Cl}$ have been populated using the intermediate-energy single-neutron knockout reaction $^{9}\mathrm{Be}(^{45}\mathrm{Cl},^{44}\mathrm{Cl})\mathrm{X}$. The momentum distribution of the residual $^{44}\mathrm{Cl}$ nuclei after direct population of the ground state is consistent with removal of an $l=1$ neutron. This observation and comparison with a shell-model calculation imply that ${p}_{3/2}$ neutrons from above the $N=28$ major shell closure play important roles in the ground states of both $^{44}\mathrm{Cl}$ and $^{45}\mathrm{Cl}$. The present result is significant because $^{44}\mathrm{Cl}$ is even closer to the valley of stability than $^{43}\mathrm{S}$, where a similar result was recently obtained using a $g$-factor measurement.