Ar40 (p,p′ γ)Reaction Mechanism at Low Energies

The angular distributions of the protons inelastically scattered on ${\mathrm{Ar}}^{40}$ at the bombardment energies of 4.1 and 7.3 MeV were measured with semiconductor detectors. The angular distribution at $E_p=4.1\mathrm{}$ MeV, corresponding to the first excited state $Q=-1.46\mathrm{}$ MeV of ${\mathrm{Ar}}^{40}$, and the angular distributions at $E_p=7.3\mathrm{}$ MeV on the higher excited states of the target are well explained by the statistical model of the compound nucleus with the inclusion of the ($p,n$) outgoing channels in the calculation. The angular distribution at $E_p=7.3\mathrm{}$ MeV on the first excited state of ${\mathrm{Ar}}^{40}$ shows a forward peak characteristic of the direct-interaction mechanism. The part taken by the direct interaction in the excitation of the first level of ${\mathrm{Ar}}^{40}$ is confirmed by the ($p,p_1\gamma$) angular correlations measured at $E_p=5.8\mathrm{}$ MeV. An attempt is made to separate the contributions of the two reaction mechanisms to the excitation of the first excited state of ${\mathrm{Ar}}^{40}$, in the 3-17-MeV bombardment-energy range. The spins for some levels of ${\mathrm{Ar}}^{40}$ are also suggested.