A sandwich arrangement of indium and calcium tungstate on both sides of an X-ray film, all inside a standard medical aluminium film cassette, was used in making ,shadowgraphs with fast neutrons. When exposed to a neutron flux, the indium plates will store a latent shadowgraph of an object, placed between the neutron source and the sandwich, through an (n,y)--reaction in the indium. The information conveyed by a fast neutron shadowgraph is partially complementary to that of an X-ray picture because fast neutrons are absorbed about equally by both light and heavy elements whereas X-rays are absorbed preferably by the heavy elements. With a sufficiently strong neutron flash source this technique should be applicable in recording fast events. Owing to the relatively long half-life of the activated indium, any other radiation accompanying the neutrons can be eliminated simply by inserting the X-ray film into the cassette only after the exposure of the indium sandwich. The optical density curve of this arrangement was recorded with a continuous source of 2.5-MeV neutrons. It shows that about 1012 neutrons/m2 should produce a useable image. However, this is a quite severe requirement for a flash source of fast neutrons.
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