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Europium-activated barium halides, especially BaFBr:Eu are used. as photostimulable x-ray phosphors in computerized radiography. The kinetics of the photostimulated luminescence (PSL) has been investigated. The measurements support the mechanism of PSL involving F + ~ F and Eu 2+ ~ Eu 3+ centers as proposed by Takahashi et al. (1,2). Values for the cross section of optical excitation of trapped electrons and for the calculated oscillator strength have been determined. Conventional x-ray radiographs usually are produced by combination of a photographic film and an intensifying screen. The screen converts the x-ray energy into visible light and an image is formed on the exposed film. This technique is hampered by the limited latitude of the film.Superior images have been reported by using computerized radiography (3). This system utilizes an imaging plate which consists of the photostimulable phosphor BaFBr:Eu (4). During x-ray exposure part of the electrons and holes created in the phosphor by the x-ray is trapped. This charge is subsequently read out by scanning the imaging plate with a focused He/Ne laser beam. During readout the intensity of the resulting blue Eu 2 § emission ( 5) is proportional to the trapped charge.A mechanism for the photostimulated luminescence (PSL) in europium-activated barium halides has been proposed by Takahashi et al. (1,2). Based on optical,

We report donor-acceptor-pair line luminescence from Na-doped ZnSe. The luminescence is "type II," and analysis gives an energy of 20 ±2 meV for the donor. From the spectrum type and the energy value we conclude that the donor must be an alkali-metal (Na or Li) interstitial ion. Prior evidence for such species in II-VI compounds has been circumstantial, and no reliable energy values have been available. Moreover, the magnitude of the energy value raises serious questions on the theory of shallow levels in compound semiconductors.

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