Angular distributions of 42-MeV $\alpha$ particles, elastically scattered from ${}_{}{}^{40,42,44,48}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{46,48,50}{}_{}{}^{}\mathrm{Ti}$, ${}_{}{}^{52}{}_{}{}^{}\mathrm{Cr}$, ${}_{}{}^{54,56}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{58,60,62}{}_{}{}^{}\mathrm{Ni}$ have been measured in the forward region with an absolute accuracy of ±0.1°; in particular, the locations of the minima near 35° (c.m. system) have been determined to within ±0.1°. The theoretical analysis has been done in three ways, using (a) the Fraunhofer model, (b) a direct parametrization of the scattering amplitude in terms of three parameters, and (c) a standard four-parameter optical model. Strong-absorption radii have been deduced from the critical angular momenta extracted from the scattering amplitudes. In addition, attempts have been made to determine, for the optical-model analysis, other sorts of strong-absorption radii that are more characteristic of the scattering than the usual midpoint radius of the potential. Our analyses indicate that the rate of increase of almost all such strong-absorption radii is smaller within the Ca and Ti isotopes than within the Fe and Ni isotopes, the radius of ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ being anomalously small for all types of analysis.

• Received 13 October 1969

DOI:https://doi.org/10.1103/PhysRevC.1.523