The internal bremsstrahlung spectra accompanying the nonunique first forbidden beta decay of 170Tm and unique first forbidden beta decay of 89Sr have been measured using an extrapolation procedure with the beta stopper method. In order to avoid errors in the source strength measurement and uncertainty of the normalization procedure, the experimental and the theoretical results are compared in terms of the number of photons of energy k per unit energy interval per unit photon yield. The results for 170Tm show better agreement with the Lewis-Ford theory, particularly in the lower and mid-energy regions, than the Knipp-Uhlenbeck-Bloch (KUB) and Nilsson theories. However the experimental results are higher than those of these theories at the high-energy end. For 89Sr, the experimental results are in agreement with the Lewis-Ford theory in the whole energy region covered by the present measurements and do not favour the KUB and Nilsson theories beyond a photon energy of 400 keV.
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