Patent Number: 050251635
Section: description

DETAILED DESCRIPTION In FIG. 1 the basis of the invention is illustrated by a particle of high-Z metal 10, which may be spherically shaped, being completely surrounded by a coating of luminescent material 12. When a photon of incident radiation 14 is absorbed by the high-Z metal particle, secondary electrons 18 are released in random directions as shown. Because the coating of luminescent material 12 completely surrounds the particle 10, all secondary electrons which escape from the particle encounter the luminescent coating thereby enabling the production of photons of visible light. Proper sizing of the particle will assure that substantially all of the secondary electrons will escape from the particle. For example, it can be shown that with incident X-rays of 60-80 keV, particles of about 5 microns in diameter will allow substantially all of secondary electrons to escape. Coated particles as shown in FIG. 1 can be produced in a variety of known ways. For example, the selected high-Z material can be ball milled to the proper size and subsequently slurried with a liquid mixture of the luminescent material. Alternatively, the coated particles can be produced directly by the process of spark machining schematically illustrated in FIG. 2. In this process, larger particles 20 of the selected high-Z metal suspended in a dielectric liquid 22 containing thiourea are subjected to a high potential alternating electric field applied by electrodes 23a, 23b from voltage source 24 which causes the particles to break up and form into microscopic spheres 26. The existence of thiourea and appropriate dopants, such as copper chloride, in the liquid can result in a cadmium sulfide (CdS) luminescent coating being formed on the spheres 26. Examples of suitable high-Z metals useful for particle 10 are bismuth (Bi), tungsten (W), osmium (Os) or tantalum (Ta). Examples of suitable luminescent materials useful for the coating 12 are cadmium sulfide (CdS), zinc oxide (ZnO), zinc sulfide (Zns) or CdSe. In FIG. 3, a segment of a radiographic imaging screen is shown in which a layer 30 of coated particles as described in FIG. 1 is deposited on a radiation transparent substrate 32 and held on the substrate by means of a suitable binder 34, such as urethane or polymethylmethacrylate. Preferably the binder should be transparent to both the incident radiation as well as to the visible light photons generated by the luminescent coating on the particles. It can be shown that a substantial percentage, e.g. two thirds, of incident X-rays at energy levels of 60 to 80 keV can be stopped by a layer of high-Z material 100 microns thick. Consequently, a suitable image screen can be made by depositing the coated particles on the substrate, more or less closely packed, in a layer about 100 microns in depth. Thinner layers may be employed to increase image resolution, as little as about 50 microns, however, the thinner the layer the lower the absorption efficiency of the screen. Although absorption efficiency can be further enhanced by increasing the thickness beyond 100 microns, the thickness is limited by decreasing resolution and increasing light loss as the layer is made thicker. In FIG. 4, an alternative embodiment of the radiation imaging screen is shown that does not require the precoating of the high-Z particles. The same reference numerals are used for like components from previous figures. In this embodiment, the particles 10 are slurried in a liquid mixture of the luminescent material 12 and then coated to the desired depth and dried directly onto substrate 32. The particles 10, being uniformly dispersed in the luminescent material, are completely surrounded by the luminescent material thus assuring that secondary electrons emitted from the particles in any direction encounter the luminescent material to enable production of visible light photons. In preparing the slurry, the volume packing fraction of the luminescent material and the high-Z particles should suitably adjusted to balance the absorption efficiency of the resulting layer with denser particle packing versus the light emission from the layer since the particles themselves will tend to block the light to some extent. The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.