Evidence for Small Deviations from the Allowed Shape in the Comparison of the Beta Spectra ofNa24andP32

Two low-$Z$, similar-in-energy, presumably allowed beta transitions, ${\mathrm{Na}}^{24}(E_0=1394\mathrm{} \mathrm{kev}, 4+\to 4+, log{f}_{0}t=6.1)$ and ${\mathrm{P}}^{32}(E_0=1711\mathrm{} \mathrm{kev}, 1+\to 0+, log{f}_{0}t=7.9)$ have been compared by means of the Argonne double-lens spectrometer at 2% resolution. Vacuum-distilled sources on 200 μg/${\mathrm{cm}}^2$ Al had thicknesses of 50 to 75 μg/${\mathrm{cm}}^2$ for ${\mathrm{Na}}^{24}$ and ≪50 μg/${\mathrm{cm}}^2$ for ${\mathrm{P}}^{32}$. Considerable attention has been given the question of detector efficiency and instrumental scattering. The results show that ${\mathrm{Na}}^{24}$ has very nearly the allowed shape; its experimental shape factor, ($\frac{N}{\left[f{(E_0-E)}^2\right]}$, is within 0.5% that given by $L_0$ between 110 and 1300 kev. In contrast, the experimental shape factor for ${\mathrm{P}}^{32}$ decreases about 3% from 270 to 1630 kev with a slope ∼4 times that of $L_0$. We conclude that there is definite evidence for small deviations from the allowed shape in one of these transitions, and further, with a lower confidence level, that the slower ${\mathrm{P}}^{32}$ transition shows the deviation. While the deviation in ${\mathrm{P}}^{32}$ is not inconsistent with previous limits set on $T-A$ interference, the large $f_{0}t$ of ${\mathrm{P}}^{32}$ suggests that second-order effects may hold the explanation.