This article discusses the possibility of screen-type X-ray film for the observation of heavy cosmic-ray primaries. We investigate in detail the optical characteristics of the intensifying screen Gd2O2S:Tb, and perform a simulation calculation for three-dimensional diffusion of scintillation light emitted from numerous δ-rays (knockon electrons produced by a heavy primary) in the screen. On the basis of these theoretical speculations, we report the experimental results obtained recently by a balloon-borne emulsion chamber with full use of the screen type X-ray film, focusing to the charge resolution of heavy primaries and their flux with energy larger than the rigidity cutoff at Sanriku latitude, Ec∼ 4 GeV/n. All results show that the emulsion chamber with multilayered screen-type X-ray film is quite valid and powerful for the observation of heavy cosmic-ray primaries.
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