18 Figure 1 A schematic figure of the xeroradiographic process by Rawls and Owen19: (a) The charged photoconductor acting as an insulator; (b) Exposure ensures charge to be conducted away from the surface; Tubacin alpha-tubulin (c) particles collect in charged areas, giving positive image. … IMAGE DEVELOPMENT The generated latent image is developed through an electrophoretic development process using liquid toner. The process involves the migration to and subsequent deposition of toner particles suspended in a liquid onto an image reception under the influence of electrostatic field forces.18 That is, by applying negatively charged powder (toner) which is attracted to the residual positive charge pattern on the photoconductor, the latent image is made visible and the image can be transferred to a transparent plastic sheet or to paper.
The toner is thereafter fixed to a receiver sheet onto which a permanent record is made. The plate is then cleaned of toner for reuse.19 THE XERORADIOGRAPHIC PLATE This plate is made up of a 9 ? by 14 inch sheet of aluminum, a thin layer of vitreous or amorphous selenium photoconductor, an interface layer, and an overcoating on the thin selenium layer (Figure 2).9,19 Figure 2 A schematic figure of a xeroradiographic plate.19 THE ALUMINUM SUBSTRATE The substrate for the selenium photoconductor should present a clean and smooth surface. Surface defects affect the xeroradiographic plate��s sensitivity by giving rise to changes in the electrostatic charge in the photoconductor.9 THE INTERFACE LAYER This is a thin layer of aluminum oxide between the selenium photoconductor and aluminum substrate.
The oxide is produced by heat treating the aluminum substrate. As a nonconductor, the interface layer prevents charge exchange between the substrate and the photoconductor surface.9 THE SELENIUM COATING The thickness of this layer varies from 150 ��m for powder toner development plate to 320 ��m for liquid toner development. Amorphous or vitreous selenium coating, melting point 216��C, is formed by depositing a vapor form of liquefied selenium in high vacuum. Because of its ease of use, fabrication and durability, inherent property of electrically conducting when exposed to x-rays and ability to insulate well when shielded from all sources of light, make selenium a xeroradiographic material of choice. On the other hand, any form of impurity adversely affects its performance.
Amorphous form is used in xeroradiography because crystalline selenium��s electrical conductivity is very high which makes it unsuitable in xeroradiography. However, amorphous selenium Cilengitide undergoes a dark decay of about 5% per minute. A new system of xeroradiography which uses plates with thicker selenium layer (320��m) gives about 50% x-ray absorption.9 SELENIUM PROTECTIVE COATING The protective coating is a 0.1 ��m cellulose acetate overcoat. The coat bonds intimately with selenium photoconductor.