Further Experiments with X-ray Tubes for High Voltages up to One Million Volts

Abstract

An improved, sealed-off, one million-volt “Metalix” X-ray tube is described. With a wall thickness of 2·5 mm. Cu, the output of this tube at 800 kV constant potential, in a direction perpendicular to the beam of electrons, has been found to be approximately 14 r/min. for 1 metre focal-skin distance and 1 mA tube current. Half-value layers up to 10·3 mm. Cu and 4·2 mm. Pb were measured. The amount of back scatter, the percentage depth dose at 10 cm. below the surface and the exit dose at 20 cm., have been measured under various conditions with a waterphantom. At one million volts and 1 mA approximately 90 mm. lead is required in order to reduce the primary radiation, at a focus skin distance of 1 metre, to the so-called tolerance dose (10−5 r/sec.). The scattered radiation, however, emerging from a paraffin block struck by the unfiltered primary beam, can readily be absorbed by a few mm. lead. Finally, the output, in the direction of the beam of primary electrons, was measured. At 800 kV pulsating tension,...