Thick-target inverse-kinematics proton scattering fromAr46and theN=28shell belowCa48

Abstract

Low-lying excited states of $^{46}\mathrm{Ar}$ have been studied via inverse-kinematics proton scattering with a thick target. Coupled-channels calculations have been used to extract the deformation length of the ${2}_{1}^{+}$ state. This result, combined with existing Coulomb excitation data, yields a ratio of the neutron-to-proton transition matrix elements of ${M}_{n}/{M}_{p}=1.19(25)N/Z$, showing a departure from the proton dominance observed in the $N=28$ isotones above $^{48}\mathrm{Ca}$. The status of the $N=28$ shell below $^{48}\mathrm{Ca}$ is discussed.