Differential spectra of the internal bremsstrahlung from the first-forbidden beta decay of 143Pr and 204Tl

Abstract

In the case of both the non-unique first-forbidden 143Pr decay and the unique first-forbidden 204Tl decay, the integral measurements performed on the internal bremsstrahlung (IB) spectrum contradict each other and the pertaining theories. On the basis of our differential measurement at 90° carried out with special care to eliminate any possible instrumental distortion effects, it was found that in the case of the IB spectrum from the non-unique first-forbidden decay of 143Pr, no real deviation from the Nilsson corrected 26) KUB 24,25) theory seems probable. In the case of the unique first-forbidden decay of 204Tl, this study confirms the presence of a positive deviation of the experimental IB spectrum from both the allowed and the forbidden theoretical ones in a wide region of the spectrum.