Decay of a New Isotope,S30

Abstract

Scintillation techniques were used to study the beta and gamma radiation from high-purity natural silicon targets after irradiation with 8-Mev ${\mathrm{He}}^3$ ions. In addition to activities associated with well-known radioisotopes, an activity with a (1.35±0.10)-sec half-life was observed. A (677±10)-kev gamma ray was associated with the 1.35-sec half-life. Decomposition of decay curves constructed from data obtained by observing annihilation radiation revealed a component with the same half-life. Half-life measurements using positrons with energies in excess of 3.15 Mev also indicated the presence of a 1.35-sec activity. The beta spectrum in coincidence with two annihilation quanta extended to ≈5.0 Mev, a higher energy than can be accounted for by positrons from the known reaction products. The beta spectrum in coincidence with the (677±10)-kev gamma ray had an end-point energy of (4.30±0.15) Mev. The assignment of the (1.35±0.10)-sec activity to the decay of ${\mathrm{S}}^{30}$ produced in the reaction ${\mathrm{Si}}^{28}({\mathrm{He}}^3, n){\mathrm{S}}^{30}$, and the proposed decay scheme are supported by arguments formulated from the known characteristics of reaction products, half-life studies using both beta and gamma radiation, the features of the experimental beta and gamma spectra, beta-gamma coincidence spectra, nuclear systematics, and nuclear theory. The decay of the ground state of ${\mathrm{S}}^{30}$ takes place by at least two positron transitions: ${\beta }_1$, a (4.98±0.15)-Mev superallowed transition to the 1+, $T=0\mathrm{}$ ground state of ${\mathrm{P}}^{30}$; ${\beta }_2$, a (4.30±0.15)-Mev superallowed transition to the 0+, $T=1\mathrm{}$ (0.677±0.010)-Mev first excited state of ${\mathrm{P}}^{30}$. No evidence was found for ${\beta }_3$, presumably an allowed transition to the 1+, (0.704±0.005)-Mev second excited state of ${\mathrm{P}}^{30}$, but an experimental upper limit of 25% is placed on its branching percentage. Branching percentages of (19±2)%, (73±7)%, and (8±10)% for ${\beta }_1$, ${\beta }_2$, and ${\beta }_3$ were calculated using the measured ${\mathrm{S}}^{30}$ half-life, a ${\mathrm{S}}^{30}$-${\mathrm{P}}^{30}$ mass difference of (6.01±0.15) Mev, assumed charge independence of nuclear forces, and the fact that $log\mathrm{ft}$ for $0^{+}$ to $0^{+}$ positron transitions within $T=1\mathrm{}$ charge multiplets is almost constant.