A new isotope of berkelium has been identified through bombardments of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ with ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$, and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ with both ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$. The new activity decays by electron capture with a half-life of 7.0±1.3 minutes. The $\alpha$-branching ratio for this isotope is less than 1% and that for spontaneous fission is less than 0.03%. Analysis of the excitation functions, the chemical behavior of the isotope, and the milking of the ${}_{}{}^{242}{}_{}{}^{}\mathrm{Cm}$ daughter leads to the positive identification of this isotope as ${}_{}{}^{242}{}_{}{}^{}\mathrm{Bk}$. A preliminary search was made for ${}_{}{}^{241}{}_{}{}^{}\mathrm{Bk}$ but was unsuccessful. Examination of the (heavy ion, $\mathrm{pxn}$) (Cm) and (heavy ion, $\alpha \mathrm{xn}$) (Am) transfer products in comparison with the (heavy ion, $\mathrm{xn}$) (Bk) compound nucleus products produced in the above reactions revealed transfer reaction cross sections to be equal to or greater than the compound nucleus cross sections. The data suggest that in some cases the yield of an isotope produced via a $\mathrm{pxn}$ or $\alpha \mathrm{xn}$ transfer reaction might be higher than its production via an $\mathrm{xn}$ compound nucleus reaction.

NUCLEAR REACTIONS Compound nucleus, proton and alpha transfer reactions: ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}({}_{}{}^{10,11}{}_{}{}^{}\mathrm{B},X)I$; ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}({}_{}{}^{10}{}_{}{}^{}\mathrm{B},Q)R$; ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}({}_{}{}^{14,15}{}_{}{}^{}\mathrm{N},M)T$; measured excitation functions for ${}_{}{}^{242}{}_{}{}^{}\mathrm{Bk}$; measured $\sigma$ for ${}_{}{}^{238}{}_{}{}^{}\mathrm{Am}$, ${}_{}{}^{239}{}_{}{}^{}\mathrm{Am}$, ${}_{}{}^{240}{}_{}{}^{}\mathrm{Am}$, ${}_{}{}^{240}{}_{}{}^{}\mathrm{Cm}$, ${}_{}{}^{242}{}_{}{}^{}\mathrm{Cm}$, ${}_{}{}^{243}{}_{}{}^{}\mathrm{Bk}$.

• Received 11 December 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1794