A method of measuring the wall contribution of an ionisation chamber

A new method of measuring the wall contribution of an ionisation chamber is described. The special equipment required is simple and consists of a hollow shell of approximately water-equivalent material and simple small lead shields. The inner dimensions of the shell are large compared with the ionisation chamber. The shell is thick enough to absorb all the contamination electrons. The principle of the method is described and the results of measurements on an ionisation chamber are given.