Recoil Studies of Nuclear Reactions of Bi209 and Pb208 with O16

Abstract

Projected range distributions in stacked ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ foils were determined for the $\alpha$-active products from the reactions ${\mathrm{Bi}}^{209}({\mathrm{O}}^{16},X)Y$ at various bombarding energies from 76 to 163 MeV and ${\mathrm{Pb}}^{208}({\mathrm{O}}^{16},X)Y$ at 123 and 163 MeV. The products $Y$ identified were ${\mathrm{Po}}^{206,208,210}$, ${\mathrm{At}}^{209,211}$, ${\mathrm{Rn}}^{210,211,212}$, and ${\mathrm{Fr}}^{212}$. Stacked Al leaf foils were used for further study of the particularly interesting region of bombarding energies near the Coulomb barrier. Here, projectile residues are ejected in a preferentially backward direction, giving the recoiling products, $Y$, energies and ranges up to about three times the values possible from a compound-nucleus reaction. For bombarding energies above about 1.3 times the Coulomb barrier, the range distributions for the products typically peak at ranges from ¼-½ the compound-nucleus range, but most have a long-range tail. With the help of angular-distribution experiments, it was possible to clarify the mechanisms in a qualitative way. No evidence was obtained which indicates that $\alpha$ transfer might be a preferred mode. Improved half-lives were obtained for ${\mathrm{Rn}}^{210,211,212}$. These are 2.42±0.05 h, 14.6±0.6 h, and 25.5±1.0 min, respectively, where the uncertainties are standard deviations.