The ft value of the superallowed fermi beta decay 26mAl( β+) 26Mg

Abstract

The threshold for the reaction 26Mg(p, n) 26mAl has been measured in terms of the accurately known energies of the α-particle groups from a ThC source. The result E th = 5208.5 ± 1.7 keV corresponds to a Q-value of −5013.5 ± 1.6 keV. The weighted mean of this and an earlier independent Q-value measurement gives for the end-point in the beta decay 26mAl( β +) 26Mg the result 3208.6±1.3 keV. The half-life for the beta decay has been measured as 6.346±0.005 sec. The ft value for the transition is thus found to be 3081±7 sec, including radiative corrections. This result is lower than those for other superallowed Fermi decays. It corresponds to a value for the vector coupling constant G β v = (1.4129±0.0016) x 10 −49 erg · cm 3. The value inferred for the Cabibbo angle is cos θ β v = 0.985±0.001, excluding uncertainties in the radiative corrections. This result appears to differ appreciably from the value inferred from the rate of K + → π 0e + v decay.