Quantum Efficiency in Photographic X-ray Exposures

The grain yield per absorbed quantum has been evaluated experimentally for four different photographic emulsions over a wavelength range of 1·-0.01 A., i e. from very soft X rays to gamma rays emitted from radium The tests were carried out with both heterogeneous and monochromatic X rays A grain yield of 1.0 per absorbed quantum appears to hold within a range of wavelengths of 1.2-0 37 A. (10 kV -33 kV). From 0.37 A. to short wavelengths (gamma rays) an increase of grain yield linear with quantum energy is obtained At a wavelength of 0.0123 A (1,000 kV.) about 80 grains are rendered developable per quantum absorbed The rate of increase of grain yield with quantum energy is different for different emulsions. For a slow and fine-grain chloro-bromide emulsion the grain yield was found to be less than unity for wavelengths below 0.12 A (i.e 100 kV) The results are consistent with the hypothesis that grains are made developable not only by the direct action of X rays but also by secondary electrons produced by the X rays. These are effective only when they are energetic enough to reach grains in the neighbourhood of that absorbing the original X-ray quantum.