An attempt has been made to construct the solid state analogue of a Bragg-Gray cavity ionization chamber. Thin layers of materials sensitive to radiation have been placed between walls of carbon, aluminum, copper, tin, or lead. Two types of sensing agents have been employed: thin layers of anthracene whose ultraviolet-induced luminescence degrades upon irradiation, and 6-μ thick polyethylene terephthalate (Mylar) films whose optical density at 3250 Å increases upon irradiation. Results in a cobalt-60 gamma ray field with the anthracene disagree sharply with theory, while the Mylar film yields excellent agreement (±5 per cent) with theory. A preliminary attempt to discriminate between the fast neutron and gamma ray dose in a nuclear reactor has been unsuccessful.