Spectral and Dosimetric Characteristics of Soft X Rays for Studies in Radiobioloqy

A DEMOUNTABLE x-ray generator has been developed to investigate the radiobiological effects of soft x rays (1). This generator (Fig. 1) is capable of operating with high D.C. currents at relatively low potentials (e.g., up to 500 MA at 6 kV). I t consists of a Pierce type electron gun (2) and a water-cooled target. The use of the gun permits a large current even at an 11.5-cm target-cathode spacing. This large spacing is required to prevent deposition on the target of a significant amount of tungsten evaporated from the filamentary cathode. Such a deposit would change the target from the selected material to tungsten. The target is constructed from a hollow copper cylinder of square cross section. Each face of the cylinder forms one target and the target material is applied to the copper by vacuum evaporation. The design permits the use of each of the four different target materials without disturbing the vacuum. A thin composite window formed by a beryllium foil and a Mylar film isolates the high vacuum in the generator from a chamber filled with helium at one atmosphere. The x rays emerge from this chamber through a second Mylar film. The total inherent filtration is 0.0005 inch of beryllium and 0.0003 inch of Mylar. In this paper, we report a number of measurements of the characteristics of x-ray beams produced by this generator. A complete description of the generator and its operation will be published elsewhere. Energy Distribution of the X-Ray Photons The x-ray spectrum from various targetsis a mixture of the continuous radiation (bremsstrahlung) and the line radiation characteristic of the target material. It is possible to emphasize the contribution of line radiation by choice of target, choice of accelerating voltage, and choice of filter. A proportional counter (Fig. 2) has been specially designed for measuring the x-ray energy distributions from the generator. The counter is cylindrical, with a 3-inch inside diameter and a O.0025-inch diameter stainless-steel center wire. Field tubes are provided to correct for counter end-effects. A 0.OOOL5-inch Mylar film lying flush with the inner wall of the counter and supported by a stainless-steel grid serves as the entrance window. The counter is operated as a flow device at slightly less than atmospheric pressure with a gas mixture of 90 per cent argon, 10 per cent methane. A krypton-methane mixture would have been superior to this mixture in terms of absorption power but was found to be unsuitable because of the radioactive contamination of atmospheric krypton (3). Even with the argon-methane mixture, however, the large diameter of this counter allows a detection efficiency which is greater than 95 per cent for photons over most of the energy range 0 to 6 keY. Thus, only relatively small corrections arenecessary to relate the measured spectra to the spectral distribution of the total x-ray fluence.