Nuclear-Structure Studies of the Low-Lying Levels ofCa39

Abstract

The low-lying levels of ${\mathrm{Ca}}^{39}$ have been studied by means of the ${\mathrm{Ca}}^{40}({\mathrm{He}}^{3}, \ensuremath{\alpha}\ensuremath{\gamma}){\mathrm{Ca}}^{39}$ reaction. Angular-correlation measurements were carried out in a collinear geometry for the ground-state decays of the first three excited states of ${\mathrm{Ca}}^{39}$. The results of these correlation studies, in conjunction with the results of previous reaction mechanism studies involving the first three ${\mathrm{Ca}}^{39}$ levels, indicate $J={\frac{1}{2}}^{+}, {\frac{7}{2}}^{\ensuremath{-}}, \mathrm{and} {\frac{3}{2}}^{\ensuremath{-}}$ for the 2.47-, 2.80-, and 3.03-MeV levels, respectively. E3 and E1 multipolarities appear to dominate the 2.80- and 3.03-MeV transitions, respectively. $\ensuremath{\gamma}$-ray decay schemes from the first eight ${\mathrm{Ca}}^{39}$ levels were also studied. Assuming at least partial dipole character for the $\ensuremath{\gamma}$ rays of energy less than 2 MeV, a $\frac{7}{2}$ or $\frac{9}{2}$ assignment was made for the 3.66- and 3.88-MeV levels, and a \textonehalf{} or $\frac{3}{2}$ assignment was made for the 4.02-MeV level. The predominant ground-state transition for the 3.84-MeV level is consistent with a ${\frac{5}{2}}^{\ensuremath{-}}$ assignment for the level, and with E1 character for the transition. A comparison with the measurements which have been made on the mirror nucleus ${\mathrm{K}}^{39}$ is made. A hole+${\mathrm{Ca}}^{40}$ core-excitation picture for the ${\mathrm{Ca}}^{39}$-${\mathrm{K}}^{39}$ pair is viewed as a simplified model for the low-lying levels, and the parameters for the model were found to be similar to those previously used for the ${\mathrm{Si}}^{27}$-${\mathrm{Al}}^{27}$ pair.