Phosphor-intensifying screen

Fig. 7.1. Three types of autoradiography can be distinguished (i) direct exposure (direct contact of specimen with film, usually separated just by a Saran wrap for wet specimens) (ii) fluorography in which fluors are excited by the radiation resulting in multiple photons per -particle (iii) phosphor-intensifying screens which can absorb radiation under the emission of photons (most stable in the cold). The proper choice of method depends on the penetration level of the radioisotope and the intensity of radiation.

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. ... 

Screen-Film Systems. According to Ludwig (5), crucial to the application of x-ray phosphors for intensifying screens is the efficiency with which the incident x-ray energy is converted to useful light energy as given by the expression ... 

Intensifying screens consist of cardboard or plastic sheets (e.g., polyester) as substrates. A reflective layer (e.g., TiOz) is first applied to this and then the phosphor-binder layer. 

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. 

A medical radiography system based on the use of an intensifying screen is represented in Fig. 8.2. The X-ray radiation transmitted by the patient is detected by the X-ray phosphor which is applied as a screen. The emitted luminescence is detected by photographic film. The spectral film sensitivity should coincide optimally with the spectral energy distribution of the emitted luminescence. Although the medical application of this pinciple is best known, other applications are also in use. An example is nondestructive materials control. 

Enhancement of detection sensitivity in the range of 1-16 fold has been obtained by using an inorganic phosphor-coated intensifying screen placed behind the film. The phosphor emits light when beta-particles pass through the film, increasing film exposure. 

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