Evidence for Small Deviations in the Allowed Positron Spectrum ofZr89

Abstract

The decay of ${\mathrm{Zr}}^{89}$ has been carefully studied with magnetic and scintillation spectrometers with special emphasis on the detailed shape of the positron spectrum. The decay scheme has been verified. The positron decay is by a single, allowed group followed by a single 915-kev gamma ray. The ${\mathrm{Zr}}^{89}$ spectrum has a nonstatistical shape corresponding to an excess of low-energy beta particles. Theoretical refinements for screening and finite deBroglie wavelength were applied but were found to be much too small to explain the observed deviation from a statistical spectrum. The same shape factor that was found to fit the ${\mathrm{In}}^{114}$, ${\mathrm{Y}}^{90}$, ${\mathrm{P}}^{32}$, and ${\mathrm{Na}}^{22}$ data (in addition to the once forbidden, unique shape factor for ${\mathrm{Y}}^{90}$) also fits the ${\mathrm{Zr}}^{89}$ data, i.e., ($1+\frac{b}{W}$) with $0.2\le b\le 0.4$. It is significant that the deviation has the same direction and approximate magnitude as was found for the electron spectra.