Excitation Functions for the (α, αn) and (α, 2pn) Reactions onCe142

Cross sections have been measured radiochemically for the reactions ${\mathrm{Ce}}^{142}(\alpha , \alpha n){\mathrm{Ce}}^{141}$ and ${\mathrm{Ce}}^{142}(\alpha , 2pn){\mathrm{Ce}}^{143}$ in the helium-ion energy range 16.8-40.1 Mev. The cross section for the ($\alpha , \alpha n$) reaction begins to rise sharply at about 25 Mev and reaches a value of 69±5 mb at 40.1 Mev. The cross section for the ($\alpha , 2pn$) reaction begins to rise at about 32 Mev and reaches a value of 2.5±0.4 mb at 40.1 Mev. An upper limit of ∼0.1 mb for the cross section for the reaction ${\mathrm{Ce}}^{142}(\alpha , \alpha p){\mathrm{La}}^{141}$ in the energy range covered by this study has also been obtained. The results for the ($\alpha , \alpha n$) and ($\alpha , \alpha p$) reactions are discussed in terms of these possible mechanisms: compound nucleus formation and decay, knock-on, and direct inelastic scattering followed by neutron evaporation. The results seem to be most consistent with the last mechanism. The existence of a measurable cross section for the ($\alpha , 2pn$) reaction in this energy region suggests that the reaction proceeds mainly by ${\mathrm{He}}^3$ emission, probably by a stripping mechanism. The data reported are consistent with the hypothesis that in this energy range at most one particle is emitted as a result of direct interaction.