Ga66ground stateβspectrum

Abstract

The ground state branch of the $\ensuremath{\beta}$ decay of $^{66}\mathrm{Ga}$ is an allowed Fermi (${0}^{+}\ensuremath{\rightarrow}{0}^{+}$) transition with a relatively high $ft$ value. The large $ft$ and the isospin-forbidden nature of the transition indicates that the shape of the $\ensuremath{\beta}$ spectrum of this branch may be sensitive to higher order contributions to the decay. Two previous measurements of the shape have revealed deviations from an allowed spectrum but disagree about whether the shape factor has a positive or negative slope. As a test of a new iron-free superconducting $\ensuremath{\beta}$ spectrometer, we have measured the shape of the ground state branch of the $^{66}\mathrm{Ga}$ $\ensuremath{\beta}$ spectrum above a positron energy of 1.9 MeV. The spectrum is consistent with an allowed shape, with the slope of the shape factor being zero to within $\ifmmode\pm\else\textpm\fi{}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ per MeV. We have also determined the endpoint energy for the ground state branch to be $4.1535\ifmmode\pm\else\textpm\fi{}0.0003$ (stat.) $\ifmmode\pm\else\textpm\fi{}0.0007$ (syst.) MeV, in good agreement with and a factor of 3 more precise than the presently accepted value.