The mechanism by which the rare-earth ions in CaF 2 can be reduced from the trivalent to the divalent state by exposure to x rays or y rays has been systematically studied for all the thirteen available ions in the lanthanide series. Each of the ions was irradiated at 77°K and its temperature subsequently raised above room temperature. During the heating process the intensity of the thermoluminescence and the changes in the optical absorption spectrum were measured as a function of temperature. An accompanying paper describes the changes in the emission spectrum with temperature. In general, the temperatures at which significant changes in absorption occur, and for which there is simultaneous thermoluminescent emission, are independent of the particular rare-earth ion, while the emission spectra is characteristic of the rare earth. The absorption spectra have parts characteristic of the individual ions and parts independent of the ion. On the basis of these observations, it is possible to formulate a consistent model of the irradiationinduced charge-reduction process and the subsequent thermal reoxidation process. From 77 to about 330°K the oxidation process is attributed to thermal activation of a variety of hole traps.

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