Energy Levels inSi28Excited by Alpha-Particles onMg24

Alpha-particles produced by the Wisconsin electrostatic generator were used to bombard thin ${\mathrm{Mg}}^{24}$ ${\mathrm{F}}_2$ targets evaporated onto pure graphite backings. The reaction products separated by a 90° magnetic analyzer were observed at a laboratory angle of 164±5°. Excitation curves for the reactions ${\mathrm{Mg}}^{24}(\alpha ,\alpha ){\mathrm{Mg}}^{24}$ and ${\mathrm{Mg}}^{24}(\alpha ,p){\mathrm{Al}}^{27}$ were obtained and compared to the excitation curves of the ${\mathrm{Al}}^{27}\mathrm{}(p,p)\mathrm{}{\mathrm{Al}}^{27}$ and the ${\mathrm{Al}}^{27}(p,\alpha ){\mathrm{Mg}}^{24}$ reactions that had been obtained earlier. Cross sections for the reaction ${\mathrm{Mg}}^{24}(\alpha ,p){\mathrm{Al}}^{27}$ were found to bear a simple qualitative relationship to those of the inverse reaction ${\mathrm{Al}}^{27}(p,\alpha ){\mathrm{Mg}}^{24}$, as suggested by the principle of detailed balance. These two excitation curves could be brought into good correspondence by assuming a $Q$-value of -1.613 Mev for the reaction ${\mathrm{Mg}}^{24}(\alpha ,p){\mathrm{Al}}^{27}$. Some $J$-values have been assigned to the levels formed in the compound nucleus ${\mathrm{Si}}^{28}$.