The Photographic Action of X Rays

This paper describes the results of a research into the photographic action of X rays, with special reference to the application of this method to the measurement of X-ray dosage-rates. It is shown that the photographic action of X rays is similar in many respects to that of ordinary light, and that exposures can conveniently be specified in terms of the röntgen, which is analogous to the candle metre-second unit used for visible light. The characteristic curves for various emulsions tested were found to have long toes, and the photographic density only becomes proportional to the logarithm of the exposure at comparatively high densities, of the order of 3 for double-coated films. At densities less than about unity the density is nearly proportional to the exposure for most ordinary developers. The curve relating density (D) to the exposure (Q) was found to be well represented by the Busé equation:— B = C log (A Q + 1) where A and C are constants. It was also found that the form of the characteristic curve is unaffected either by variations in the quality or the intensity of the incident radiation, and that the sensitivity of the emulsion to radiations of different qualities may be related to their half-value layers in copper or aluminium. The emulsions tested showed a maximum sensitivity for a half-value layer of 0·25 mm. of copper. The validity of the Bunsen-Roscoe reciprocity law was investigated, and the law found to hold over an intensity-range of 100 to 1, and was confirmed with a less accuracy over a range of 10,000 to 1. No intermittency effect was found when blackenings produced by equal quantities of X rays excited by constant and by pulsating voltages were compared.