Positron Decays ofCa38andK38

Abstract

The positron decay of ${}_{}{}^{38}{}_{}{}^{}\mathrm{Ca}$ has been investigated using the ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}({}_{}{}^3{}_{}{}^{}\mathrm{He},n)$ reaction with an enriched ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ gas target. A delayed 1568-keV $\gamma$ ray with a half-life of 0.47±0.02 sec is attributed to a positron branch to the 1695-keV level of ${}_{}{}^{38}{}_{}{}^{}\mathrm{K}$. Upper limits of 3% or better were found for transitions to other low-lying levels. Using the measured half-life and the decay energies, the branching ratio to the 1695-keV level is calculated from $\beta$-decay systematics to be (21±4)%. The $log\mathrm{ft}$ for this transition (3.41±0.09) establishes $J^{\pi }=1^{+}$ for the 1695-keV level. An additional result was the observation of an allowed transition from ${}_{}{}^{38}{}_{}{}^{}\mathrm{K}$ to the 3936-keV level of ${}_{}{}^{38}{}_{}{}^{}\mathrm{Ar}$.