Precise Shape Measurements of Beta Spectra ofY90andY91

Precise measurements of the detailed shapes of the beta spectra of ${\mathrm{Y}}^{91}$ and ${\mathrm{Y}}^{90}$ were made in order to determine better to what extent the theory of beta decay in its present approximation is adequate to describe the distributions which are observed for these unique, $\Delta I=2\mathrm{}$, party changing transitions. The use of an integrally biased semiconductor as the detector in a high resolution magnetic spectrometer provided an extremely low background which facilitated the accurate determination of the end points with a minimum amount of extrapolation. A proper end-point determination is necessary for a meaningful shape-factor analysis. In both cases, it was found that the experimental shape factor shows a small but definite deviation from the theoretical expectation. For a theoretical shape factor, $L_0{q}^2+9\mathrm{}{L}_1$, with $L_0$ and $L_1$ based on exact electron radial wave functions including the effects of a finite nuclear radius, the necessary correction can be supplied by an empirical factor of the form $1+\frac{b}{W}$ with $0.3\le b\le 0.4$. The end points of the ${\mathrm{Y}}^{91}$ and ${\mathrm{Y}}^{90}$ spectra were determined to be 1.545±0.005 MeV and 2.273±0.0005 MeV, respectively.