A Comparison of Photographic and Ionization Measures of Radiation Quality

Abstract

I. The Photographic Determination of the Half Value Layer AMONG the many methods for obtaining a description of the quality, or absorption characteristics of a roentgen-ray beam, one that in recent years has gained more and more attention is the photographic determination of the half value layer. First suggested by the work of Christen (1), the Swiss mathematician and physician, and preceded by such devices as the Benoist penetrometer (2), the idea of a photographic method has been developed up to its present frequent use in roentgenology. The half value layer describes roentgen radiation quality in terms of that thickness of an absorbing material, such as copper or aluminum, which reduces the incident intensity of a beam by 50 per cent. Photographic determinations of the half value layer of a given roentgen-ray beam are ideally accomplished by the simple procedure of matching a photographic darkness, produced upon a film by a beam of half intensity, with the darkness under a metal filter stairs exposed to the full intensity of radiation. There are several apparent recommendations for such a method of measurement: a group of intensity measurements can be obtained at one time; the film acts as a recording medium, yielding a permanent record of intensitites which may be expressed relatively by the film darkenings; the method is simple, and certainly it is inexpensive. Meyer (3) has developed this process of half value layer measurements; his work also mentions the use of the stair filter with a 1.0 mm. copper strip to derive copper-aluminum equivalents photographically. In the procedure generally followed, a suitable film or plate in a light-tight container is covered by a strip of lead, and by stair filters either of aluminum or copper, or both. This device is exposed to the roentgen-ray beam. At one-half the necessary exposure time, the beam is cut off, and the lead strip is removed without disturbing the stairs. The film is then exposed again for another interval of one-half time, so that the region under the stairs has received one full exposure, while that area under the lead has received one-half of the exposure. After proper development, those regions under the steps in which the photographic opacity is the same as that of the half-exposure-time strip are determined, and the step thicknesses corresponding to those areas are designated as half value layers. It seemed worth while to make a study comparing the half value layers obtained from photographic observation with those derived from the more generally accepted ionization absorption measurement of radiations produced by the same conditions. II. Response of the Photographic Emulsion to Roentgen Rays For measurements of any radiation by photographic methods, it is necessary to know the characteristics of response of the emulsion to the radiation.