The 18-MeV elastic and inelastic alpha-particle cross sections for some states of ${\mathrm{Ne}}^{20}$ (Gnd., 1.632 MeV), ${\mathrm{Na}}^{23}$ (Gnd., 0.439, 2.080 MeV), ${\mathrm{P}}^{31}$ (Gnd., 1.265, 2.232 MeV), ${\mathrm{S}}^{32}$ (Gnd.), and ${\mathrm{Ar}}^{40}$ (Gnd., 1.462 MeV) have been analyzed in terms of the nuclear optical model and the distorted-wave theory. Although good qualitative agreement between the empirical and optical-model cross sections is found, particularly at forward angles, the quantitative agreement is significantly poorer than that found for heavier target nuclei at higher incident energies. Further intercomparison of the results of the present and previous studies strongly suggests that for the prevailing experimental and physical circumstances ($E_{\alpha }\approx 18$ MeV and $A<\mathrm{}40\mathrm{}$) of the present investigation the applicability of the optical model is marginal. The distorted-wave predictions are in fair agreement with the experimental data at the more forward angles ($\theta \lesssim 80°$) in that they generally reproduce the positions and relative amplitudes of two or more of the maxima. For larger angles ($\theta >80\mathrm{}°$) the quality of the fits deteriorates insofar as detail is concerned; however, the calculated curves do follow the general trend of the data. For the inelastic angular distributions the quality of the fits is poorer and the angular range over which the fits are acceptable is smaller than for the corresponding elastic cross sections.

• Received 29 November 1965

DOI:https://doi.org/10.1103/PhysRev.144.972