The Precise Measurement of X-Ray Dosage

Abstract

With the increase in the use of X-radiation in medical therapy it has become more important to be able to express with some exactitude the actual amount of radiation being received by a patient. It is also necessary that this quantity be expressed in a common unit which is readily reproduced and in which there is no ambiguity. I.—Introduction In practice there are three distinct operations necessary in order to describe the radiation at a given location: (1) The measurement of the radiation intensity by means of a dosage meter; (2) the calibration of the dosage meter against a standard which measures the unit of X-radiation exactly; (3) a determination of the wave length distribution of the total radiation. It is in the second part that the Bureau of Standards is particularly interested and more especially with the exact measurement of the unit which was defined at the Second International Congress of Radiology at Stockholm, July, 1928, as follows: . . . “the quantity of radiation which, when the secondary electrons are fully utilized and the wall effect of the chamber is avoided, produces in one cubic centimeter of atmospheric air at 0° C. and 76 cm. mercury pressure such a degree of conductivity that one electrostatic unit of charge is measured at saturation current.” This unit is called the “roentgen” and is designated by small “r.” In a recent paper Behnken2 gives the results of his comparison of the unit between the laboratories of Grebe, Küstner, Holthusen, Duane, Glasser, and the Physikalisch Technischen-Reichsanstalt. According to his measurements there is a discrepancy of about 4 per cent between various laboratories. No explanation for these differences has been given, and it is assumed that they are traceable to experimental error. However, as a result of some preliminary measurements made at the Bureau of Standards with the co-operation of Dr. Behnken, it is evident that improper electrostatic field conditions within the ionization chamber could account for at least some of the variation. To test this, a single ionization chamber was used under the same X-ray conditions but with various combinations of potentials applied to the plates as shown below. II.—X-Ray Generating Equipment During the above-mentioned comparisons of Behnken it was demonstrated very clearly at the Bureau of Standards that, from the point of view of regularity of action, a mechanically rectified system was unsuited to precise X-ray measurements. The wave length energy distribution in the X-ray continuous spectrum excited by constant voltage can be expressed mathernatically3 and can be determined experimentally with fair precision4 as a function of the X-ray tube voltage and other factors, such as target material, etc. The total energy varies approximately as the square of the voltage.