Vapor-deposited CaWO 4 phosphor
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Vapor-deposited CaWO4 phosphor P. F. Carcia, M. Reilly, C. C. Torardi, M. K. Crawford, C. R. Miao, and B. D. Jones DuPont Central Research and Development, Experimental Station, P.O. Box 80356, Wilmington, Delaware 19880-0356 (Received 11 July 1996; accepted 21 January 1997)
In this paper we describe the preparation, microstructure, and x-ray excited luminescence of vapor-deposited CaWO4 films up to about 50 mm thick, comparing them to particulate CaWO4 phosphor screens, used in medical diagnostic imaging. Films that we e-beam evaporated on substrates heated at or above 500 ±C were polycrystalline with the scheelite structure, while on unheated substrates, films were initially amorphous but became crystalline after annealing them in air above about 750 ±C. Crystalline CaWO4 films irradiated with x-rays produced light emission peaked at 430 nm. The emission intensity depended on film thickness and grain size and was comparable to particulate CaWO4 phosphor screens. Because the vapor-deposited films also exhibited superior resolution, they are promising for diagnostic x-ray imaging.
I. INTRODUCTION
While it is generally agreed that a nearly fully dense, thin film phosphor, made by physical or chemical vapor deposition, can have superior resolution1–3 because of inherently smaller grain size and because less lateral scattering occurs in a thinner layer, experience suggests that other phosphor forms, such as powder compactions or particulate/binder systems, have higher luminescent efficiency.4 The common explanation is that a dense thin film phosphor, with its high index of refraction (usually .2), light-pipes or channels the luminescence,5 trapping it within the film. Much of this light may be absorbed, undergoing multiple reflections with the substrate over a long path length. Powder compactions or particulate phosphors, on the other hand, are less dense (often ,50%), and light is forward scattered by multiple reflections out of this aggregate, where it can be viewed or detected by, for example, photographic film. However, this perception of a thin film phosphor is overly simplistic, since a featureless microstructure5 is implicit, whereas the actual microstructure of films6,7 made by physical vapor deposition can be much richer and also controlled by processing. In this paper we show that x-ray stimulated light emission from a dense, vapor-deposited CaWO4 thin film phosphor can be comparable to commercial CaWO4 x-ray screens,8–10 comprised of particulate CaWO4 in a polymeric binder. A phosphor is used in diagnostic imaging8–11 because photographic film is relatively insensitive to x-radiation, requiring unacceptably long exposures for the patient in order to produce a quality image. Because an x-ray photon is much more energetic than a visible photon (,eV), one x-ray photon can produce thousands of visible photons; this amplification that occurs in the phosphor reduces the x-ray dose needed J. Mater. Res., Vol. 12, No. 5, May 1997
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