Nuclear deformation and super-allowed Fermi beta-decay

Abstract

The Q EC values for the eight bodies from whose super-allowed Fermi beta-decay we derive our most accurate knowledge of the vector coupling constant are now determined with such precision that the associated uncertainties in the phase-space factor amount, on that account, to an average of only 0.030%; we must therefore consider all possible influences at the 0.01% level. Although the bodies in question are of J = 0 they possess, or may possess, large intrinsic deformations that: (i) affect the radial distribution of the charge whose field operates upon the emitted positron; (ii) affect the leptonic-nucleonic convolution of the shape factor. These effects are evaluated in an approximation that exaggerates their influence on the phase-space factor; using for the nuclear deformation values deriving from Q 2+-values measured by the re-orientation effect in Coulomb excitation the resultant magnitude of the change in the phase-space factor due to the deformation is found to be everywhere less than 0.01% although approaching that figure in some cases.