THICK-TARGET EXTERNAL-BREMSSTRAHLUNG SPECTRA OF PM-147 AND S-35 BETA-RAYS

External-bremsstrahlung spectra excited by soft beta particles of Pm-1 47 (E(beta)max = 225 keV) and S-35 (E(beta)max = 167 keV) in targets o f Al, Cu, Sn, and Pb have been studied. The experimental and theoretic al results are compared in terms of the number of photons of energy k per m0c2 per unit photon yield to exclude the uncertainty in the sourc e strength measurement and overcome the inherent inadequacy of the nor malization procedure used by earlier workers. The results of present m easurements for medium- and high-Z elements show better agreement with the theory of Tseng and Pratt [Phys. Rev. A 3, 1714 (1976)] than with Elwert's corrections [Ann. Phys. (N.Y.) 34, 78 (1939)] to the Bethe-H eitler theory [Proc. R. Soc. London Ser. A 14, 83 (1934)], particularl y at the higher-energy ends. However, for low-Z elements, both theorie s are found to be adequate.