Induced Pseudoscalar Interaction and the Low-Energy Anomalies in Beta Spectra

An unsuccessful attempt is made to interpret in terms of the induced pseudoscalar interaction the ($1+\frac{a}{W}$) anomaly factors which have been found by Langer and co-workers in the shapes of all the allowed Gamow-Teller beta spectra that they have measured. Tadic's recent observation that the Hamiltonian of the induced pseudoscalar interaction in nuclear beta decay is radically different from that of the more familiar "basic" pseudoscalar interaction is taken into account. The shape factors for the transitions in question are derived and found to display an anomaly of the required form. In the case of pure Gamow-Teller transitions arbitrarily large distortions are possible, provided that an appropriate destructive interference takes place between the axial vector and induced pseudoscalar terms. However, for a mixed Fermi-Gamow-Teller transition there is a definite upper limit to the amount of distortion which the induced pseudoscalar interaction can give rise to. This limit is greatly exceeded by the ${\mathrm{Zr}}^{89}$ spectrum and hence the anomaly must have some other origin. It is furthermore concluded that until the reason for these anomalies is understood it would be unsafe to draw any conclusion concerning the strength of the induced pseudoscalar interaction on the basis of low-energy spectrum shapes.