## Abstract

The $\beta $-spectra of Th (C+C$^{\prime \prime}$) and Th (B+C+C$^{\prime \prime}$) have been measured using semicircular focusing. The sources were mounted between two thin films to prevent the escape of Th C$^{\prime \prime}$ by $\alpha $-recoil. The intensities of seven of the $\beta $-ray lines were measured and the continuous spectra of Th B. C and Th C. C$^{\prime}$ were found by subtraction using the known spectrum of Th C$^{\prime \prime}$.D. The $\beta $-ray end-point of Th C. C$^{\prime}$ is at 2$\cdot $250 MeV, and the shape of the $\beta $-ray spectrum differs but little from that of an allowed transition for energies above 0$\cdot $9 MeV. A detailed comparison is made with the shape of the compound spectrum required to account for the known $\gamma $-rays of Th C$^{\prime}$, using the theory of forbidden transitions. A spin change of $\Delta $I = $\pm $ 2 is ruled out, but $\Delta $I = 0 or $\pm $ 1 is possible. The $\beta $-spectrum of Th B seems to be composite with its principal end-points at 0$\cdot $331 and 0$\cdot $569 MeV. The intensity of the high-energy transition between the ground states is about 0$\cdot $12 $\pm $ 0$\cdot $02 electrons per disintegration. The K internal conversion coefficient of the F $\gamma $-ray was found to be about 0$\cdot $377, 16% higher than the theoretical value for magnetic dipole radiation.