Energetics of theAl27(B11, p3n)Cl34mReaction from Recoil Studies

Abstract

We have studied, in detail, the ${\mathrm{Al}}^{27}({\mathrm{B}}^{11}, p3n){\mathrm{Cl}}^{34m}$ reaction at bombarding energies from 30 to 115 MeV. This paper presents measurements of cross sections, average recoil ranges, and angular distributions of the reaction product ${\mathrm{Cl}}^{34m}$, the high-spin member of an isomeric pair. The excitation function behavior is unusual, exhibiting a relatively slow decrease in cross section at higher energies. The peak cross section is about 16 mb, and at 65 MeV above the excitation-function peak, the cross section is approximately 2% of its maximum value. Our recoil range results provide strong evidence that the reaction proceeds by compound-nucleus formation at all energies investigated. At the highest bombarding energy, approximately 66 MeV must be dissipated as particle kinetic energies and photons in the compound nucleus de-excitation. The analysis of our angular-distribution experiments indicates that most of this energy appears as kinetic energy of emitted particles. The total particle kinetic energy increases approximately linearly with available energy. Relatively little de-excitation energy is released in the form of gamma radiation, of the order of 7 MeV or less. The trend of total gamma-ray emission energy with increasing available energy is roughly constant, but seems to show a slight decrease. There is no direct evidence that the formation of ${\mathrm{Cl}}^{34m}$ involves preferential selection of compound nuclei with greater than average angular momentum. On the contrary, if the presence of gamma-ray competition with particle evaporation is taken as a criterion of high angular momentum, then our results may imply a strong discrimination against high-angular-momentum events. Our range-energy data for ${\mathrm{Cl}}^{34m}$ in Al are compared with the predictions of stopping theory. The high velocities encountered in our experiments provide a test of the theoretical formulation relating to electronic stopping. Although general agreement is good, there are systematic deviations which may indicate the approximate character of the theoretical assumptions.