Absolute calibration of a cobalt-60 gamma-ray beam.

Abstract

This report describes the calorimetric determination of the amount of energy in the collimated gamma-ray beam from a cobalt-60 source, and the amount of ionization produced by this beam under specified conditions. These measurements experimentally relate absorbed dose and quantity of radiation. Comparison of this experimentally determined relationship is made with the calculated relationship predicted by theory, permitting critical evaluation of the theory under the specified conditions. The calorimetric determination of the quantity of radiation will be described in detail. Measurements with two secondary standards were made for correlation with the calorimetric results. The first standard is the roentgen as measured with a 25-r Victoreen thimble chamber calibrated at the National Bureau of Standards with a cobalt-60 gamma ray source. The second standard is a polystyrene wall extrapolation chamber measurement of cavity ionization. X-ray and gamma-ray radiations are usually measured in terms of the roentgen. The roentgen has assumed a dual role in radiology through its use as a measure of dose as well as a measure of output. It is not a direct measure of either energy flux or dose but can be related to both by calculation or experiment. The correlation of the roentgen with our calorimetric determination will be evaluated both theoretically and experimentally. X-ray and gamma-ray radiations are usually measured in terms of the roentgen. The roentgen has assumed a dual role in radiology through its use as a measure of dose as well as a measure of output. It is not a direct measure of either energy flux or dose but can be related to both by calculation or experiment. The correlation of the roentgen with our calorimetric determination will be evaluated both theoretically and experimentally. X-ray and gamma-ray radiations are usually measured in terms of the roentgen. The roentgen has assumed a dual role in radiology through its use as a measure of dose as well as a measure of output. It is not a direct measure of either energy flux or dose but can be related to both by calculation or experiment. The correlation of the roentgen with our calorimetric determination will be evaluated both theoretically and experimentally. X-ray and gamma-ray radiations are usually measured in terms of the roentgen. The roentgen has assumed a dual role in radiology through its use as a measure of dose as well as a measure of output. It is not a direct measure of either energy flux or dose but can be related to both by calculation or experiment. The correlation of the roentgen with our calorimetric determination will be evaluated both theoretically and experimentally. Calorimeter Apparatus A single lead-tungsten absorbing cylinder, illustrated in Figure 1, is suspended in a vacuum chamber for use as the energy absorber for the telecobalt gamma-ray beam. The absorber is suspended with nylon threads through eyelets attached to the roof of the vacuum chamber and accurate alignment is made possible by worm-screw adjustments. The absorber is constructed in three sections. The center section, or socket, consists of a silver-plated cylindrical brass shell filled with lead. The two end sections, or plugs, are machined from “Hevi-met” tungsten alloy. After mounting the thermistor in one plug and the heater element in the other, the separate elements are screwed together and soldered at the joints to insure good thermal conductivity. A Western Electric No. 12A rod-type thermistor is used as the temperature-sensitive element.