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

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 $\ensuremath{\alpha}$-disintegration characteristics measured by on-line techniques of $\ensuremath{\alpha}$ spectroscopy. Energy values, excitation functions, and half-lives of $\ensuremath{\alpha}$-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.The new $\ensuremath{\alpha}$ data help greatly in the determination of the trends in $\ensuremath{\alpha}$-decay energies for isotopes of even-$Z$ elements, particularly in the poorly defined region just above the 126 neutron shell. This allows the preparation of a more reliable set of predictions for many unknown nuclei.