Decrease in Efficiency of Grenz-ray Tubes

Abstract

THE so-called Grenz or Bucky rays are X-rays of comparatively long wave length, and are found to be quite readily absorbed, even in materials of very low atomic weight, such as celluloid, air, and the tissues of the body. The construction of cathode-ray tubes from which quantities of these soft rays may be obtained for therapeutic use was successful only when a glass was compounded out of low atomic weight materials. The use of this glass in the tube window permits the escape of a quantity of the radiation sufficient to produce useful biological changes when absorbed in the skin. It is obvious that the deposition of any material of high atomic weight upon the window must reduce the quantity and alter the quality of the radiation escaping. The filament and the anode of the tube are both possible sources of such contamination. The filament operates at a high temperature. At every temperature there is a very definite rate of evaporation. This rate may be extremely small, but where the material evaporating is tungsten, of extremely high atomic weight, even a very minute amount will be of importance if it deposits on the window. The impact of the electrons upon the anode may be sufficient to free a small quantity of the material of which the anode is composed. This may also deposit upon the window, adding to the filtering action of the tungsten. It seems probable that the evaporation of tungsten is the most important source of filtering material. Treatment with the Grenz ray was instituted in this Clinic December 20, 1928. A table of the intensities to be expected at various skin target distances was furnished with the apparatus. These intensities were measured abroad by means of the Küstner apparatus and were given in r-units. After the first tube had been in continuous service for over a year, during which time 260 treatments aggregating 31.38 hours were given with it, the intensity of its radiation was measured. A standard air ionization chamber of the Duane type was used, with a galvanometer to measure the current. The values found were approximately only 60 per cent of what the tube was supposed to deliver when it arrived. It was apparent that either the method of measurement was at fault, or else the window had become contaminated. In order to determine what had happened, a spare tube that had never been used was installed, and the intensity measurement repeated. It was found that the intensity given by this tube checked within experimental error with the values determined abroad by means of the Küstner apparatus. More complete measurements were then made with each tube, and are presented graphically in Figure 1. The tubes were both operated at 10 ma. and 9 kilovolts. Successive filters of celluloid, each 1/100 inch in thickness, were interposed in the beams, and the “absorption curves” determined. These curves show that the old tube is now capable of delivering only 57.5 per cent as intense a beam as the new tube.