Some Operating Characteristics of the Wilson Cloud Chamber

Abstract

The following characteristics of cloud-chamber behavior were studied: (a) the dependence of minimum pressure on expansion time, (b) the ion threshold expansion ratio as a function of expansion time, (c) the sensitive time and its dependence on the percent expansion, (d) the rate of pressure rise after completion of a fast expansion, and (e) the rate of drop growth in N2, H2, and He with vapor from 95 percent C2H5OH or a liquid mixture of one part H2O to three parts C2H5OH. The experiments were performed with large cloud chambers (30-cm diameter and 10- to 30-cm depth) filled, in most cases, with N2 as the permanent gas and with the vapor from a 1 : 3 liquid mixture of H2O and C2H5OH. (a) The minimum pressure is found to be proportional to the square root of the expansion time. (b) The ion threshold expansion ratio increases linearly with expansion time. (c) The sensitive time increases with expansion ratio, reaches a maximum when the general background becomes appreciable, and approaches zero for higher expansion ratios. (d) The initial pressure rise after completion of an expansion is proportional to the square root of the time. (e) The surface area of a drop increases linearly with time in all cases. The temperature of a drop calculated from the rate of growth and from thermal constants is much lower for H2 and He than for N2. The results in (a), (b), (c), and (d) agree with results predicted by Williams' equations or equations obtained from similar considerations. The results for the rate of growth in (e) are in reasonable accord with values calculated from the diffusion equation.