Shape of the Positron Spectrum ofRb84

The shape of the positron distribution in the decay of ${\mathrm{Rb}}^{84}$ was studied in a magnetic spectrometer. Using both proportional counter and solid-state detectors, it was possible to obtain data which permitted the subtraction of the outer group from the total spectrum without obliterating the subtle details of the inner group. The outer group is fitted very well with the unique once forbidden shape factor. However, an additional empirical factor of $1+\frac{0.2}{W}$ yields an even better fit to the data. The inner group has a distribution which is slightly but measurably different from the statistical shape. This shape cannot be accounted for by an empirical factor of the form $1+\frac{b}{W}$ alone. The end-point energies are found to be 1.657±0.003 MeV and 0.780±0.006 MeV, in good agreement with the weighted average of 0.880 MeV for the gamma ray between the first excited and ground states of ${\mathrm{Kr}}^{84}$.