Dose build-up curves for cobalt-60 irradiation: a systematic error occurring with pancake chamber measurements

Abstract

This note presents: (a) measurements of build-up curves J(x) with ionisation chambers of various geometries; (b) observations of the scattering of electrons from the side walls of a cavity; (c) a method of calculating D(x) in a homogeneous material; and (d) a discussion of the differences between J(x) and D(x). The authors believe that to obtain the true build-up function D(x) in a solid substance (such as skin), pancake chambers have to be selected according to construction conditions which sufficiently reduce the systematic error due to multiple scattering of electrons from the side wall.