X-ray intensifiers

Screen Preparations, 100 micron thick x-ray intensifying screens were prepared using standard doctor blade coating techniques. The final phosphor volume was 50% when the coatings were dried. In most instances, the phosphor suspensions were prepared using polyvinyl butyral binders with viscosities adjusted to 2000 centipoise for the doctor blade operation and care was taken to avoid convection cell formation (9). A cross section of the screen construction is shown in Figure I. The completed screens consist of polyester (Mylar) base about 10 mil. thick, a 50 micron thick (TI02 (rutile) reflector layer, a 100 micron thick phosphor layer, a 10 micron thick clear cellulose acetate butyrate top protective layer. 

To be used in x-ray intensifying screens, rare earth phosphors must also have high conversion efficiencies (nc Figure 1) in addition to high intrinsic absorptions of x-rays and suitable emission characteristics. The several rare earth phosphors listed in Table I have conversion efficiencies from about 10% for La202S Tb to about 20% for LaOBr Tb (5,, ) as compared to about 6% for CaWOi. ... 

Yttrium tantalate and yttrium niobate-tantalate have good X-ray absorption and are used in X-ray intensifying screens because of their high conversion factor (Section 5.5.4.2.). Substitution of tantalum by small quantities of niobium considerably increases the blue fluorescence when excited by X rays. 

The advantage of the X-ray excitable tantalates lies in the fact that the emission spectrum extends into the near UV. The films used with X-ray intensifying screens are particularly sensitive in this region. 

Oxyhalides. The oxyhalides of yttrium, lanthanum, and gadolinium are good host lattices for activation with other rare-earth ions such as terbium, cerium, and thulium. The use of LaOCl Tb3+ as the green component in projection-television tubes has been discussed [5.419]. LaOBr Tb3+ and LaOBr Tm3+ exhibit high X-ray absorption, and they are used in X-ray intensifying screens [5.420]. 

Both primary and secondary electron models (Atoyan Voelk 2000, Brunetti et al. 2001, Blasi Colafrancesco 1999, Miniati et al. 2001) have been analyzed to reproduce the spectral and spatial features of the EUV excess in Coma without a definite solution. Additional experimental information has been recently added to the complexity of the problem in particular, the EUV intensity distribution seems to be highly correlated with the thermal X-ray intensity and produce a constant ratio between the azimuthally averaged EUV and X-ray intensifies (Bowyer et al. 2004). Specific secondary models seem, at present, one of the few viable possibilities to reproduce the EUV emission features of Coma. 

Rare earth phosphors used in medical radiography The following are the needs for more efficient X-ray intensifying screens ... 

X-ray phosphors are also used in X-ray intensifying screens. The X-rays are first converted into visible photons, which subsequently irradiate the film. In most cases, the photographic film is sandwiched between two phosphor sheets. Light moving into the direction away from the photographic film can nevertheless be used by application of Ti02 reflecting layers. The typical thickness of such a phosphor layer is of the order of a few hundred micrometers. 

X-ray intensifying screens are used in radiological diagnosis and in industrial non-destructive testing. X-ray fluoroscopic screens are used mainly in health examinations and in radiological examination of luggage... 

Totally reflection process effectively adds incident and reflected beam x-ray field amplitudes. This yields up to 4x x-ray intensify at surface. 

The search for new X-ray phosfdiors for conventional X-ray intensifying screens is reaching its end. A number of sati.sfactory materials are available, and efficiencies cannot be expected to become much higher. For specialized applications there may be some need for improvement. 

Molycorp, a wholly owned subsidiary of Unocal Corp., was the only company to mine rare earth minerals in the United States in 2002. The rare-earth separation plant operations stopped in 2003. Molycorp mined bastnasite, a rare earth fluorocarbonate mineral, as a primary product at Mountain Pass, California. The value of domestic ore production was estimated at 31 million in 2002 the estimated value of refined rare earth minerals was more than 1 billion. The end uses for rare earth products in 2000 were as follows automotive catalytic, 22 percent glass polishing and ceramics, 39 percent permanent magnets, 16 percent petroleum refining catalysts, 12 percent metallurgical additives and alloys, 9 percent rare earth phosphors for lighting, televisions, computer monitors, radar, and x-ray intensifying film, 1 percent, and miscellaneous,... 

Pieexposed Kodak X-OMAT R film with a calcium tungstate x-ray intensifying film. 

The activated rare earth phosphate phosphors can be used in all applications developed for phosphors in fluorescent lamps, cathode-ray tubes. X-ray intensify-... 

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