Production and Decay Properties of Thorium Isotopes of Mass 221-224 Formed in Heavy-Ion Reactions

Abstract

In order to search for new neutron-deficient isotopes of uranium and thorium, targets of ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ were bombarded with beams of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$, ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$, ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ in the Berkeley HILAC. The reaction products were transported from the reaction cell by the helium-jet technique and their $\alpha$-disintegration characteristics measured by on-line techniques of $\alpha$ spectroscopy. Energy values, excitation functions, and half-lives of $\alpha$-particle groups were measured. Only fragmentary evidence for uranium nuclei was found, because of intense losses by nuclear fission, but definite proof was obtained for the previously unknown millisecond isotopes ${}_{}{}^{222}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{221}{}_{}{}^{}\mathrm{Th}$, and for their decay products ${}_{}{}^{218}{}_{}{}^{}\mathrm{Ra}$, ${}_{}{}^{214}{}_{}{}^{}\mathrm{Rn}$, ${}_{}{}^{217}{}_{}{}^{}\mathrm{Ra}$, and ${}_{}{}^{213}{}_{}{}^{}\mathrm{Rn}$. Improvements were made on the literature values for ${}_{}{}^{223}{}_{}{}^{}\mathrm{Th}$ and its decay products.