X-ray intensifying screens

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]. 

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... 

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. 

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