Physical Investigation of the Contribution of the Photo-Electrons from Sulphur to X-ray Ionisation

The fractional increase in the energy increment per unit volume of tissue exposed to a given beam of X radiation which would result from completely surrounding the small volume element of tissue by sulphur of thickness equal to the maximum range of the photo-electrons was investigated by means of the chamber shown in Fig. 1. The only materials used in its construction were graphite, bamboo, amber, wax, and “Aerion.” The graphite top and bottom of the chamber could be removed and replaced by others nearly identical in dimensions but having inner surfaces of sulphur or other material. The collecting electrode consisted of a circle and cross made from thin bamboo fibre, coated with “Aquadag.” This electrode thus made a negligible contribution to the secondary electron emission. Collecting fields of 60 and 120 volt/cm. were always used.* The corresponding ionisation currents, in general, differed by ∼ 2 per cent. All measurements were made at approximately the same gas pressure, and ionisations which were to be compared were measured with the same collecting voltage. With such small separations between walls and electrode, however, it was quite possible that the field was not always exactly the same; this may account for the fact that, while ionisations measured with a given chamber assembly were consistent to ∼ 0·5 per cent., the same quantity measured on different occasions varied by a few per cent. (see Table 1).