Improvements in the sensitivity of screen-type X-ray films for cascade showers and the charge resolution for heavy ions

Abstract

Nonscreen-type X-ray films were commonly used to detect electromagnetic cascade showers developed in emulsion chambers. The detection threshold energy of the showers with the nonscreen-type X-ray film is about 500 GeV to 1 TeV, depending on exposure and development conditions. In order to improve the detection threshold, we examined various screen-type X-ray films. Thus, we found that combinations of a Gd2O2S: Tb phosphor screen and a green sensitive X-ray film show very high sensitivity, not only to cascade showers, but also to high-energy heavy ions. In order to extract the maximum performance of the screen-type X-ray film for the detection of cascade showers, we widely examined the effect of development conditions for the X-ray film and also analyzed the radiative transfer of photons in a Gd2O2S: Tb phosphor screen. Since we found that the optimum thickness of phosphor is about 500 μm, we prepared a special screen, type-GS, with a phosphor layer of 500 μm and applied it to balloon observations of high-energy galactic electrons. The improvement in the sensitivity for showers is remarkable and a detection threshold of 150 GeV, the predicted value from our analysis, has been attained with this special screen, type-GS. We have also found that a charge resolution of about 1.2 charge units is attainable for relativistic iron nuclei by using the average density of successive dark spots on ten sheets of screen-type X-ray film. The characteristics of the screen-type X-ray film, and its applications to observations of cosmic-ray electrons and heavy primaries and described in this paper.