Summary. --The luminescence of barium fluoride excited by X-rays has been recorded vs. wavelength. The spectrum shows two components due to recombination of excitons bound to Frenkel defects and to luminescent centres also active in the thermoluminescent process. The results are interpreted by means of a model based on the Prener and Williams recombination scheme.PACS 78.60 -Luminescence and radiative recombination. PACS 71.35 -Excitations and related phenomena.Samples of polycrystalline barium fluoride, BaF2, excited at room temperature (295 K) with soft X-rays (Cu Ka), showed a radioluminescent (RL) emission in the form of a continuous spectrum of intensity decreasing from 380 nm to 600 nm, followed by a broad band at 659 nm. A second very faint band at about 420 nm is barely visible (see fig. 1). These samples, when heated from room temperature to about 700 K at a rate of one K/s, originated a thermoluminescent (TL) emission in which no continuous spectrum was detectable, but a clear glow peak was present with a maximum at about 400 K and 660 nm (see fig. 2). This peak corresponds to the 659 nm band of the RL spectrum. The emissions observed imply the existence in BaF2 of deep trapping levels active in the TL process, accompanied by levels unable to store carriers when the ionizing X-ray flux is interrupted.To explain these results, we have to consider the actual structure of BaF2. In its crystal cell, like in the fluorite CaF2 cell, eight equivalent positions exist in which Ba + § ions can be held. Since the cell contains only four Ba + + ions, room is available for interstitials. On this ground, the most probable origin of the traps responsible for the TL emission is merely the displacement of Ba + § ions in interstitial positions lying in cells placed at a distance from the initial ion positions. In this way, separated couples of vacancies and interstitials bearing two opposite-sign electron charges are introduced in the lattice. Such kinds of crystal defects have already been considered in connection with the TL emission of CaF2 [1]. As to the continuous spectrum, it can be ascribed to the recombination of excitons bound to Frenkel defects originated by the displacement in interstitial positions of fluorine F-ions [2]. In other words, an electron and a hole are captured by a fluorine ion vacancy and an interstitial fluorine producing F-and H-centres, respectively. Recombinations of the nearest F-H pairs give rise to 1107
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