Decay ofSi26

Abstract

A (2.1±0.3)-sec activity was observed when vacuum distilled magnesium was bombarded with 8-Mev ${\mathrm{He}}^3$ ions. Half-life studies using NaI(Tl) scintillation counters yielded evidence that this activity was due to the decay of a positron emitting isotope with a maximum kinetic energy greater than 3.5 Mev. The features of gamma-ray spectrum with the exception of a weak line at 824±15 kev could be understood in terms the decay characteristics of known radioisotopes. An internally consistent argument based on the known decay characteristics of reaction products that may be expected from energy considerations, the results of half-life studies, experimental gamma spectra, and nuclear systematics can be made to support the conclusion that the (2.1±0.3)-sec half-life is that of ${\mathrm{Si}}^{26}$ produced in the reaction ${\mathrm{Mg}}^{24}({\mathrm{He}}^3, n){\mathrm{Si}}^{26}$, and a consistent decay scheme can be proposed. The ground state of ${\mathrm{Si}}^{26}$(0+) decays by the emission of two positron groups to excited states of ${\mathrm{Al}}^{26}$. The most intense transition, $E_0=3.76\mathrm{}$ Mev, is to the 0.228-Mev state (0+) of ${\mathrm{Al}}^{26}$. The second transition, $E_0=2.94\mathrm{}$ Mev, is to the 1.05-Mev state (1+) of ${\mathrm{Al}}^{26}$. The 1.05-Mev and 0.228-Mev states are then connected by a (824±15)-kev gamma transition. The energies of the positron transitions are derived from the known levels of ${\mathrm{Al}}^{26}$ and the ${\mathrm{Si}}^{26}$-${\mathrm{Al}}^{26}$ mass difference.