THE USE OF THE WILSON CLOUD CHAMBER FOR MEASURING THE RANGE OF ALPHA-PARTICLES FROM WEAK SOURCES

Abstract

A large Wilson cloud chamber operated by a synchronizing mechanism similar to that used by Blackett is described. It is shown how, by the use of mirrors, it is possible to take two photographs of the whole volume of the cloud chamber with a single lens, which are equivalent to photographs taken by two cameras at right angles. The design of such a camera is discussed in detail. The photographs are examined by replacing the developed film in the camera and using it as a projection apparatus. The two images can be combined on a suitably oriented plate as one image the same size as the original track. The method is thoroughly tested by photographing glass fibers and by range determinations. The reduction of the track lengths in the chamber to α-particle ranges under standard conditions is discussed fully. It is shown that the interval between expansions must be made much longer than is customary if precision is to be attained in this reduction. The temperature in the chamber is carefully examined throughout the single cycle and repeated cycles to determine this interval and other thermal conditions of operation. The whole technique has been developed with the aim of utilizing as many tracks per expansion as possible for the purposes of range measurement.