Calorimetric Determination of the Power in a 1400 kv X-Ray Beam

The traditional method of measuring the quantity of X-radiation has been to measure the ionization produced in an air cavity by the X-radiation. This procedure is consistent with the definition of the roentgen4 for energies below a few Mev and depends on the establishment, in the wall of the ionization cavity, of an equilibrium between the electrons produced by the incident X-rays and the electrons absorbed. Measurement of the exposure dose in roentgens at higher energies is impractical because it is difficult to determine the ionization produced by long-range secondary electrons originating in 1 cm3 of air. The response of an ionization chamber to X-ray photons has been calculated by Mayneord (1), Johns et al. (2), and others, as a function of the incident photon energy, but has not been adequately checked experimentally. Recent calorimetric measurements by Genna and Laughlin of Co60 7-rays (3) gave a value of 3370 i 130 ergs/cm2 per roentgen. Measurements of the same quantity for 400-kv X-rays by Laughlin et al. (4) gave an average value of 2880 i 90 ergs/cm2 per roentgen.