Precision Alpha-Particle Counting

Abstract

THIS communication gives the results of an experiment designed to compare methods of absolute alpha counting at Harwell and at the Radiation Laboratory of the University of California at Berkeley. The low-geometry methane proportional chamber, used by us, has been described by Hurst and Hall1. The alpha-particles are collimated by three disks, to prevent reflexion from the chamber walls, and a final collimating disk to define the geometry. The alpha-particles then enter the counting chamber where the collecting electrode, a 0.001-in. diameter tungsten wire at earth potential, is held taut by a small brass ball. A concentric brass cylinder forms the cathode. The methane pressure is maintained at about 8 cm. (of mercury) and a flat plateau of ∼ 600 volts is obtained for the dependence of count-rate on the high tension of the electrode. The critical dimensions, the diameter of the collimating disk and the distance of this from the source (∼ 5 in.), can be measured very accurately. The geometry factor of the chamber for a point source is calculated from the difference between the solid angles subtended at the source by the collimating disk and the ball. There is a small correction for the area of the source: 0.14 per cent for a circular source of radius 0.2 in. The calculated geometry factors of our two chambers are: Counter 12 712.5 ± 0.7 Counter 13 717.3 ± 0.8 the quoted error being due to the possible errors in the dimensions used in the calculation. The chambers have been used in conjunction with the following standard British electronic equipment: power pack type 1007, amplifier type 1049A, potentiometer unit type 1007, scalar type 1009A, timing unit 1003B and a special head amplifier which is a modification of a type 1049A (to operate on negative high tension).