The Design, Construction, and Performance of a Magnetic Focusing, Semi-Circular, Low Energy Beta-Ray Spectrometer

A semi-circular beta-ray spectrometer (Fig. 1) has been constructed for operation between 2 and 120 kev electron energy. This instrument was designed to supplement the ring- focusing beta-ray spectrometer whose lower limit is approximately 25 kev. The 180° spectrometer has an electron trajectory radius of 12 cm. The field is produced by two iron free coils spaced to give a nearly homogeneous field (with some second order focusing) in the region of the electron trajectories . The field current is supplied by a one kw motor- generator set. Stabilization is obtained to 2 parts in 10, 000 with a rotating electro- mechanical unit and an electronic servo system. Source and Geiger counter window dimensions for a standard resolution of 0 .8% are 0.1 cm x 4 cm. With a 10 μ.g/cm^2 formvar window, transmission is assured down to 2 kev electron energy. A recording potentiometer traces the counting rate vs. electron energy. Several examples of chart recordings are reproduced to illustrate the performance of the semi- circular beta-ray spectrometer.