A series of empirical arguments has developed which leave two alternatives for the law of $\beta$-interaction: an $\mathrm{STP}$ (scalar+tensor pseudoscalar) or a $\mathrm{VTP}$ ($V\equiv \mathrm{vector}$) combination. We offer a new argument, based on an interpretation of the reported "allowed" shapes of once-forbidden spectra, which favors the $\mathrm{STP}$ over the $\mathrm{VTP}$ form. This new development may be important because it is a possible last step in arriving at a unique law of $\beta$-decay (i.e., all three components of the $\mathrm{STP}$ form would become both admissible and necessary).

A critical stage in the new argument is the explanation of the "allowed" shapes. The theory may yield such shapes if the Coulomb potential energy at the nucleus is sufficiently larger than the kinetic energy with which electron and neutrino are emitted. That condition happens to be best fulfilled just for the cases in which the "allowed" shapes are the most accurately observed. However, the dominance of the Coulomb effect cannot be a sufficient condition except with the $\mathrm{STP}$ or $\mathrm{VA}$ forms of $\beta$-interaction. Other forms make deviations from the allowed shape possible even when the Coulomb energy is supposed indefinitely large.

The last type of deviation has the same origin as the well-known Fierz interference in allowed spectra. The absence of the latter effect has been widely quoted but apparently never heretofore examined in detail. It is important in one of the series of arguments mentioned above. Hence, we investigate the empirical limits on Fierz-type deviations not only in once-forbidden but also in allowed spectra.

• Received 20 August 1952

DOI:https://doi.org/10.1103/PhysRev.88.1266