On Ce, Clambda, and the effective wall material of a Baldwin-Farmer chamber

An expression relating the constant A to the characteristics of an ionisation chamber is developed. This constant, used to obtain the mass ionisation from the instrument reading, appears in expressions for Ce and Clambda, and is a function of the chamber wall material. Measurements with a Baldwin-Farmer chamber are described, and indicate that the effective wall material is more nearly equivalent to Lucite than to air in 60Co gamma radiation. The resulting value of A is nearly 1.5% lower than the figure commonly used.