Recoil Studies of Nuclear Reactions Induced by Heavy Ions

Abstract

The mechanism of nuclear reactions induced by heavy ions was investigated by measuring the recoil ranges of ${\mathrm{Tb}}^{149}$, ${\mathrm{At}}^{211}$ and other alpha-emitting isotopes of At and neighboring elements and by determining the cross sections for the formation of ${\mathrm{Tb}}^{149}$ and ${\mathrm{At}}^{211}$.Recoil ranges were consistent with compound-nucleus formation at all energies studied for the following reactions: ${\mathrm{Pr}}^{141}({\mathrm{C}}^{12}, 4n){\mathrm{Tb}}^{149}$, $\mathrm{Ce}({\mathrm{N}}^{14}, xn){\mathrm{Tb}}^{149}$, ${\mathrm{La}}^{139}({\mathrm{O}}^{16}, 6n){\mathrm{Tb}}^{149}$, ${\mathrm{La}}^{139}({\mathrm{O}}^{18}, 8n){\mathrm{Tb}}^{149}$, and $\mathrm{Ba}({\mathrm{Ne}}^{22}, pxn){\mathrm{Tb}}^{149}$. A similar result was obtained for the reaction ${\mathrm{Pr}}^{141}({\mathrm{O}}^{16}, 2p6n){\mathrm{Tb}}^{149}$ at 138 and at 146 Mev and for the reactions ${\mathrm{Au}}^{197}({\mathrm{O}}^{16}, 2pxn \mathrm{and} 3pxn)\mathrm{At}$, Po at energies below 100 Mev. The excitation functions of the $(\mathrm{HI}, xn){\mathrm{Tb}}^{149}$ reactions seem to be characteristic of an evaporation process but have smaller peak cross sections than do the excitation functions of the reactions $\mathrm{Ba}({\mathrm{Ne}}^{22}, pxn){\mathrm{Tb}}^{149}$ or ${\mathrm{Pr}}^{141}({\mathrm{O}}^{16}, 2p6n){\mathrm{Tb}}^{149}$. We conclude that most reactions probably involve charged-particle emission. The reaction $\mathrm{Ba}({\mathrm{Ne}}^{22}, pxn){\mathrm{Tb}}^{149}$ seems to occur with much greater probability than the reaction $\mathrm{Ba}({\mathrm{Ne}}^{20}, pxn){\mathrm{Tb}}^{149}$.In many cases the compound-nucleus mechanism cannot account for our results. Partial momentum transfer is observed in the reactions ${\mathrm{Au}}^{197}({\mathrm{O}}^{16}, 2pxn \mathrm{and} 3pxn)\mathrm{A}\mathrm{t},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{o}$ at energies above 100 Mev. Partial momentum transfer also occurs when Bi is bombarded at energies 1.3 times the barrier energy or greater. Reactions of Bi with heavy ions (${\mathrm{Ne}}^{20}$ is possible exception) at energies near the Coulomb barrier produce ${\mathrm{At}}^{211}$ with greater recoil energy than expected from a compound-nucleus mechanism. Apparently, particles are emitted in the backward direction. Near the barrier the cross section for the production of ${\mathrm{At}}^{211}$ by ${\mathrm{C}}^{12}$, ${\mathrm{O}}^{16}$, and ${\mathrm{Ne}}^{20}$ bombardment comprises about \textonequarter{} the value calculated for compound-nucleus formation. Therefore, the cross section for all noncompound-nucleus reactions must comprise a large fraction of the total interaction cross section. The experiments with Pb as a target are also consistent with this conclusion.