The Use of Photographic Films for Monitoring Stray X-Rays and Gamma Rays

IN RECENT YEARS, particularly in the atomic-energy project, photographic films have been used extensively to monitor exposures to stray radiation received by personnel. Frequently, films are employed in conjunction with pocket ionization meters, each serving as a check upon the other. The most significant difference between the two methods is that a properly calibrated ionization meter records roentgens directly over a wide range of x-ray qualities, while the film response, in terms of roentgens, varies greatly over certain quality ranges. Other differences between the two methods are that an ionization recording can be evaluated immediately after the exposure, whereas the films must first be processed; the films will integrate the exposure satisfactorily over a period of several weeks, while the ionization measurement is best limited to an exposure of one day. Contamination of the insulation in the pocket meter, or mechanical shock, may result in an electrical discharge of the device independently of any exposure to radiation, thereby producing an excessive reading. A simultaneous film recording will act as a check against this possibility. The film also provides a long-lasting direct record of the exposure. By themselves, photographic films cannot give a measurement of the quantity of radiation. They must be suitably calibrated so that the densities observed on the monitoring films can be evaluated in terms of roentgens. Ordinarily, some films of the same emulsion number as the films worn by personnel are given a known exposure, or a series of exposures, expressed in roentgens, to primary radiation of suitable quality. These calibration films are processed with the monitoring films, and the exposures on the latter are evaluated by comparing their densities with those of the calibration films. If calibration films are developed with each batch of monitoring films, a high degree of constancy of the development conditions is not essential, although it is desirable to maintain uniform recommended temperatures and other properly controlled developing conditions. The value of the calibration exposure may be made equal to the maximum tolerance dose for the period during which the monitoring films were carried by the personnel. By comparison of the resulting densities, it can be determined whether the monitoring films received more or less than the tolerance exposure. Another possibility is to give the calibration films a series of known exposures. By comparison of the film densities, the values of the actual exposures of the test films can be determined. A simple visual comparison is sufficient for qualitative determination, but if greater precision is desired, and if the actual exposures are to be recorded and permanent exposure records maintained, the film densities should be read on a densitometer.