Super-allowed Fermiβdecay: Half-lives ofO14andKm38

Abstract

The half-lives of the Fermi super-allowed $\beta$-ray transitions from ${}_{}{}^{14}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{38}{}_{}{}^{}\mathrm{K}_{}^m{}_{}{}^{}$ have been measured by multiscaling $\gamma$ rays (from ${}_{}{}^{14}{}_{}{}^{}\mathrm{O}$) or $\beta$ rays (from ${}_{}{}^{38}{}_{}{}^{}\mathrm{K}_{}^m{}_{}{}^{}$) detected in a plastic scintillator. The activities were made via the reactions ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^3{}_{}{}^{}\mathrm{He}, n){}_{}{}^{14}{}_{}{}^{}\mathrm{O}$ at $E_{{}_{}{}^3{}_{}{}^{}\mathrm{He}}=2.9\mathrm{}$ MeV and ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}(\alpha , n)$ ${}_{}{}^{38}{}_{}{}^{}\mathrm{K}_{}^m{}_{}{}^{}$ at $E_{\alpha }=8.0\mathrm{}$ MeV respectively. Results for the half-lives were as follows: ${}_{}{}^{14}{}_{}{}^{}0$-70.613 ± 0.025 sec, and ${}_{}{}^{38}{}_{}{}^{}\mathrm{K}_{}^m{}_{}{}^{}$-921.71±0.65 msec. Recommendations as to the "best" half-life values for the set of eight most accurately measured Fermi super-allowed transitions are presented and comments are made on the present status of knowledge of the nucleon's mean quark charge as extracted from such transitions following acceptance of Cabibbo universality and the gauge theories as exploited by Sirlin. The value recommended for the vector coupling coefficient effective for nucleon $\beta$ decay (including the "inner" radiative correction) is $g_{\beta \mathrm{V}}^R=(1.412\mathrm{}48\pm 0.00044)\times {10}^{-49\mathrm{}}$ erg ${\mathrm{cm}}^3$.