The $3s$ strength-function resonance has its maximum value in nuclei for which the major 28-neutron shell is near closure or just beyond. In view of this fact, it seemed desirable to investigate this mass region more extensively, and to do so we have studied the even-even compound nuclei which lie in the region of the $3s$ strength-function resonance. We find that the resonances in ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ and, to a certain extent, in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ show an uncommon tendency to cluster. The result is that for ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ the strength function below 10 keV has a relatively large value of about 12×${10}^{-4\mathrm{}}$, whereas in the energy interval 3-60 keV the value is about 5×${10}^{-4\mathrm{}}$, in accord with the optical-model prediction. This situation suggests that the relatively large peak values of about 10×${10}^{-4\mathrm{}}$ which certain experiments show for the $3s$ strength-function resonance may result from a tendency for several nuclides in the vicinity of ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ to have rather large "local" strengths in a manner similar to that of ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$. This means that the maximum value of the $3s$ strength function is actually about 4.5×${10}^{-4\mathrm{}}$. A quasiparticle calculation is made to try to understand the spacings observed. Excepting ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$, the calculations showed good agreement with experiment, in that the observed levels are predominantly of the four-quasiparticle type, since two-quasiparticle types are relatively so few in number that they are not likely to be seen.

• Received 30 March 1966

DOI:https://doi.org/10.1103/PhysRev.151.912