A Scintillation Spectrometer Study of the Decay ofTl204

By means of a special technique of scintillation spectroscopy, whereby radioactivity is introduced into a NaI(Tl) crystal, the $\beta$-spectrum of ${\mathrm{Tl}}^{204}$ is investigated and the assignment of the transition as "first forbidden" is confirmed. A reported weak intensity photon radiation of 70-kev energy is uni$\stackrel{\mathrm{´}}{\mathrm{q}}$uely identified as being due to $K$-electron capture, and the branching ratio of the $K$ capture process to the beta-emission is determined to be ∼0.015. Utilizing the formulas for forbidden $\beta$-transitions developed by Marshak, the transition energy for the orbital capture is calculated to be ∼400 kev. The inner bremsstrahlung associated with the $K$-capture is detected by coincidence measurements, and its end-point value is found to be in agreement with the transition energy calculated from the branching ratios. An upper limit of 0.2 percent is set for any possible $K$ capture in ${\mathrm{Au}}^{198}$.