P31(He3,pγ)S33andK39(He3,pγ)Ca41Reactions and theγDecay of Analog States inS33andCa41

Abstract

Proton-$\ensuremath{\gamma}$-ray coincidence measurements have been made on the reactions $^{31}\mathrm{P}(^{3}\mathrm{He},p\ensuremath{\gamma})^{33}\mathrm{S}$ and $^{39}\mathrm{K}(^{3}\mathrm{He},p\ensuremath{\gamma})^{41}\mathrm{Ca}$ at incident $^{3}\mathrm{He}$ energies of 12 and 13 MeV. The protons were detected at 0\ifmmode^\circ\else\textdegree\fi{} to the beam direction, thereby preferentially selecting $L=0$ transfers of neutron-proton pairs. A Ge(Li) detector at 90\ifmmode^\circ\else\textdegree\fi{} detected the $\ensuremath{\gamma}$ rays with good energy resolution. Angular distributions for the $^{31}\mathrm{P}(^{3}\mathrm{He},p)^{33}\mathrm{S}$ reaction were measured with a magnetic spectrograph. These, together with previous results on the $^{39}\mathrm{K}(^{3}\mathrm{He},p)^{41}\mathrm{Ca}$ reaction, gave precise information on the intensities of the states excited, as well as their energies. The main purpose of the experiments was to measure the $\ensuremath{\gamma}$ decay of the lowest-lying isobaric analog states in $^{33}\mathrm{S}$ and $^{41}\mathrm{Ca}$. The analog state in $^{33}\mathrm{S}$ was found to decay primarily (85%) to the 842-keV level and secondarily (15%) to the ground state. The analog state in $^{41}\mathrm{Ca}$ decays in a more complex fashion to several states. The strongest transition (55%) is to the 4091-keV state. $\ensuremath{\gamma}$-ray decay schemes for several other low-lying even-parity levels in $^{33}\mathrm{S}$ and $^{41}\mathrm{Ca}$ were also measured. The observation that the 4091-keV level of $^{41}\mathrm{Ca}$ decays (34%) to the ${\frac{7}{2}}^{\ensuremath{-}}$ ground state and (58%) to the ${\frac{3}{2}}^{+}$ level at 2009 keV suggests a ${\frac{5}{2}}^{+}$ assignment. The implications of the measurements are discussed, particularly with reference to the structure of low-lying two-particle-one-hole states in these nuclei.