The sensitivity of photographic films to X radiation at very low temperatures

Experiments have been carried out to determine the sensitivity of photographic films to x radiation when exposures are made with the films maintained at temperatures obtainable with liquid nitrogen, hydrogen, and helium. The sensitivity at any temperature T is defined as the ratio of the exposure times required to produce images of equal density (0.25) by exposures made at room temperature and at the temperature T , the intensity of the radiation being the same for each exposure. It is found that the sensitivity appears to decrease uniformly down to about 100° K., but below that temperature the decrease becomes less rapid, until, at temperatures below 20° K., the sensitivity remains practically constant at a value considerably higher than that found for visible light by Berg and Mendelssohn. The results obtained for X rays are compared with those found by Berg and Mendelssohn for visible light, and it is suggested that the relatively high value found for the limiting sensitivity in the case of X rays indicates that a considerable number of free electrons must be formed by the absorption of the X radiation while the film is at low temperature.