A Radio Frequency High Voltage Apparatus for X-Ray Therapy

Part II IN THE previous issue of this Journal a radio frequency high voltage apparatus for x-ray therapy was described. The construction of the apparatus, as can be determined from the previous description, is such that the high voltages are made and kept within a vacuum tank. Therefore, it is not possible to measure the peak voltages by the usual sphere gap method or by a voltmeter. It is necessary, therefore, to state the quality of the radiation. The commonly adopted methods of specifying the quality of an x-ray beam are the following: (1) the complete absorption curve; (2) the “equivalent voltage” of Lauritsen; (3) the constant potential equivalent of Taylor; (4) the half value layer; (5) the effective wave length of Duane, and (6) the average wave length. The effective wave length and the average wave length methods are not used in the range of voltages here described. Method of Obtaining Percentages of Absorption.—A lead diaphragm 2.5 cm. thick, with a circular aperture 2 cm. in diameter in its center, was placed just outside the wall of the tube at a distance of 42 cm. from the target. A second similar diaphragm was placed at the entrance to a lead cylinder 12 cm. in diameter and 60 cm. long. Thus a beam of rays could be directed down the axis of the lead cylinder so as not to touch its walls. An aluminum ionization chamber, 14 cm. long and 7 cm. in diameter, with walls 0.3 mm. thick, was placed at the far end of the lead cylinder. It was connected to a string electrometer (Fricke-Glasser dosimeter). The quantity of radiation was measured by the. time taken for the indicator to pass over the complete scale. The influence of scattered radiation from the room was largely eliminated by taking initial readings with the entrance to the lead tube blocked by 10 cm. of lead. The wall of the tube consists of 0.5 mm. of copper plus 3 mm. of steel, both of which constitute a permanent initial filter. Complete Absorption Curve.—Lead is the absorber usually used for obtaining curves up to nearly complete absorption in the high voltage range because the total thickness required need not be more than 2 centimeters. The physical data concerning the absorption in this metal are complicated by the photo-electric effect, whereas those for the lighter elements are not. Therefore, it is less convenient for the estimate of the equivalent voltages. Because the nearly complete curve is obtained so easily, however, such curves are valuable for comparing the performance of different x-ray tubes, and the operation of a given tube under different conditions. In general, the slope of the curve of the percentage of transmission becomes steeper for radiation produced by lower equivalent voltages. The curves obtained are shown in Figure 7 along with those obtained by Lauritsen (2) and Coolidge (3). Their voltages were measured by direct methods.