New Isotopes of Thorium Studied with an Improved Helium-Jet Recoil Transport Apparatus

Five neutron-deficient isotopes of thorium lighter than mass 218 were studied at the Berkeley Heavy Ion Linear Accelerator by bombardment of enriched ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ with ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. Silicon (Au) surface-barrier detectors were used in on-line measurements performed to determine $\alpha$-decay characteristics. Mass-number assignments of ${}_{}{}^{213}{}_{}{}^{}\mathrm{Th}$ through ${}_{}{}^{217}{}_{}{}^{}\mathrm{Th}$ were made on the basis of excitation functions and systematic trends in $\alpha$-decay properties. Half-lives and $\alpha$-particle energies were determined. New information was also obtained about the $\alpha$-decay characteristics of ${}_{}{}^{216}{}_{}{}^{}\mathrm{Ac}$, ${}_{}{}^{215}{}_{}{}^{}\mathrm{Ra}$, ${}_{}{}^{214}{}_{}{}^{}\mathrm{Fr}$, and ${}_{}{}^{212}{}_{}{}^{}\mathrm{At}$. Systematic occurrence of complex structure in the $\alpha$ decay of even-$Z$ 125-neutron nuclei is discussed, and spins and parities are suggested for three lowest-energy levels of ${}_{}{}^{211}{}_{}{}^{}\mathrm{Ra}$, ${}_{}{}^{209}{}_{}{}^{}\mathrm{Rn}$, and ${}_{}{}^{207}{}_{}{}^{}\mathrm{Po}$. Systematic occurrence of isomerism in odd-$Z$ 127-neutron nuclei is also discussed, and spins and parities are suggested for several levels. On the basis of the extreme regularity of the experimental $\alpha$ energies, predictions are made for the energies of several unknown neutron-deficient isotopes of thorium, protactinium, uranium, and neptunium. In the experimental work, the helium-jet transport method was used to remove the $\alpha$-emitting products from the target region. Technical details concerning the application of the method to millisecond activities are discussed.