The “Quality” of High-Voltage Radiations. Part II

The “effective wavelength” of X radiation within a scattering medium has been measured by two methods, one depending essentially on the differential increase in photoelectric emission from the walls of metal and carbon ionisation chambers with increasing wavelength, and the other on the variation of absorption with wavelength in a cylindrical copper sheath. For reasons given, it is thought that the former method is less open to objection. It is shown that the effective wavelength depends on the size of the field of irradiation, being large for large fields on the surface, and still greater at depths of 10–15 cm. A primary radiation produced at 350 kV may be so changed that its effective wavelength at a depth is little less than that of primary radiation produced at 250 kV. Measurements have also been made of the amount and quality of primary radiation reaching a depth in water when scatter is excluded. The fraction of scattered radiation at varying depths under therapeutic conditions has also been investigated. It is shown that the scattered radiation may amount to some 80 per cent, of the total dose at a depth of 14 cm. in large fields. The bearing of these results on the question of relative biological effects of different X-ray wavelengths is briefly emphasised.