Investigations using a Permanent-magnet Double  -Ray Spectrograph with Coincidence Counting - I. Experiments with Thorium Active Deposit

A detailed description is given of a semicircular focusing β-particle spectrograph in which a thin source is centrally placed and sheaves of oppositely moving electrons are investigated using two thin-windowed Geiger counters. These counters can be used in a coincidence arrangement, and each is capable of movement so as to accept electrons which have been focused with any radius of curvature between 2.5 cm. and 6 cm. The arrangement was tested using sources of thorium active deposit. During this work the (coincidence) β-particle spectrum for the ThB.C transition leading to the 238 keV. state of ThC was obtained separately for the first time and its end-point determined as 355 keV. A feeble higher-energy β-component due to ThB was also detected and the end-point estimated at 589 keV. Redeterminations of the end-points of the β-particle spectra of ThC and ThC'' (by β-γ coincidences) gave values of 2.256 MeV. and 1.805 MeV., respectively. The intensity of the F-line of ThB was found to be 0.288 photoelectrons per disintegration.