Journal Notes

The sensitivity of photographic emulsions to x‐rays can be increased by an estimated factor of about one hundred through the use of an ingenious method devised by K. S. Lion of MIT. As reported in the March issue of the Journal of Applied Physics, this technique involves fastening a sheet of photosensitive material to one of the plates of a parallel plate spark counter. Such a counter operates like an ordinary Geiger‐Muller counter, with an electrical discharge occurring between its electrode plates, across which an appropriate potential is placed, upon the passage of ionizing radiation through it. In a parallel plate counter this discharge is localized at the point where the radiation is incident, and does not spread laterally. Hence an x‐ray beam striking a counter with an emulsion on one of its plates not only produces an image by its direct photographic action but the spark discharge that accompanies it also contributes to the image. According to Lion, control experiments indicated that the effect was not caused by secondary radiation from the electrodes or by the effects of electric currents in the emulsion.