Scattering of 40.5-MeV Alpha Particles by C12, C13, N14, N15, O16, and O18

Abstract

Elastic and inelastic scattering of 40.5-MeV α particles from targets of ${\mathrm{C}}^{12}$, ${\mathrm{C}}^{13}$, ${\mathrm{N}}^{14}$, ${\mathrm{N}}^{15}$, ${\mathrm{O}}^{16}$, and ${\mathrm{O}}^{18}$ was studied. Angular distributions were measured for a large number of excited states. It was found that the shape of the angular distribution depends on the nature of the single-particle transition involved. Six examples of quadrupole transitions involving promotion of a $p_{\frac{3}{2}}$ nucleon to the $p_{\frac{1}{2}}$ shell were found. Although the cross sections varied over a tenfold range, the shapes of the angular distributions remained very similar. Six examples of the dipole transition $p_{\frac{1}{2}}\to 2s_{\frac{1}{2}}$ and seven examples of the octupole transition $p_{\frac{1}{2}}\to d_{\frac{5}{2}}$ were also observed. The dipole transitions gave angular distributions of a characteristic and unusual shape. Excitation of the ${\mathrm{N}}^{14}$ levels at 9.41, 9.71, 10.22, and 10.55 MeV suggests that they are all $T=0\mathrm{}$. The levels at 6.05, 6.70, 7.40, and 7.60 MeV were not observed; probably they do not exist. A weak level at 10.85 MeV and two strongly excited levels (or groups of levels) at 11.3 and 12.9 MeV were observed in ${\mathrm{N}}^{14}$. The angular distribution of particles scattered from the 4.45-MeV level of ${\mathrm{O}}^{18}$ suggests strongly that this level is 1-rather than 3+. Several unnatural parity states were observed, but no states known to have isotopic spin different from the ground state. The angular distributions for several scattered particle groups were compared with distorted-wave Born approximation calculations and very approximate reduced transition probabilities for excitation of the levels were obtained. For the quadrupole and octupole excitations the results are in reasonably good agreement with values measured by electromagnetic methods.