High energy fluoroscopic screen
BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     The present invention relates to high energy fluoroscopic inspection systems, and more particularly to a high energy fluoroscopic screen for inspecting large objects. 
     2. Description of Prior Art. 
     The current practice in radiography of large objects is to use lead intensifying screens to obtain images on an X-ray film. The sensitivity is limited to approximately 2 percent, and the resolution for crack detection is limited by the geometry, i.e., the X-ray beam must be aligned within 5° to detect a crack. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a high energy fluoroscopic screen which is essentially a three-stage photon-to-light converter. High energy photons absorbed by a specimen provide a varying flux across the face of the screen having three layers. A high-Z foil stops a portion of the photons and emits secondary radiation which causes a phosphor layer to luminesce. A Polycellular Image Converter (PIC) gathers the light as well as converting additional photons to light. The light is piped to the open face of the PIC and visually sensed by a low light level TV chain. 
     Therefore, it is an object of the present invention to provide a filmless radiographic inspection technique. 
     Another object of the present invention is to provide a radiographic inspection technique of greater sensitivity and greater certainty of crack detection. 
     Still another object of the present invention is to provide a dynamic radiographic inspection technique. 
     Yet another object of the present invention is to provide an efficient photon-to-light high energy fluoroscopic screen. 
     Other objects, advantages and novel features wll be apparent from the following detailed description when read in view of the appended claims and attached drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a filmless radiographic inspection system according to the present invention. 
     FIG. 2 is a cross-sectional view of the high energy fluoroscopic screen according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 a high energy X-ray source 10, such as a betatron, linear accelerator, cobalt 60 or the equivalent, emits a high energy photon beam 12 toward a specimen 14, such as a rocket motor, a large casting, a thick weld, a patient or the like. The high energy photons are absorbed by the specimen 14, providing a varying flux across the face of a fluoroscopic screen 16. The screen 16 converts the photons to light which is sensed by a low light level TV camera 18. The signal from TV camera 18 is processed by a signal processing and recording circuit 20. The output of the signal processing and recording circuit 20 is displayed on a high resolution TV monitor 22. The specimen 14 may be continuously moved with respect to the high energy photon beam 12 to permit 100 percent inspection of the specimen. 
     The fluoroscopic screen 16 is shown in greater detail in FIG. 2. A high-Z foil 24, selected from tungsten, tantalum, uranium, platinum, rhenium or the like, is coated with a luminescent phosphor 26, such as the oxysulfides of lanthanum, gadolinium or lutetium activated with trivalent terbium disclosed in U.S. Pat. Nos. 3,725,704 and 3,829,700. A polycellular image converter (PIC) 28, such as is described in U.S. Pat. No. 3,225,193 by Hilton et al, is attached to the phosphor 26. 
     The high-Z foil 24 stops X percent of the photons and emits secondary radiation of low energy characteristic X-rays, electrons and photofission fragments. The secondary radiation causes the phosphor 26 to luminesce. The PIC 28 gathers the light emitted from the phosphor 26. The PIC 28 also collects ε(100-X) percent of the remaining photons, where ε is the efficiency of the PIC crystals, and converts these photons to light. The light is piped to the open face 30 of the PIC 28 where it is visually sensed by the low light level TV camera 18. 
     Thus, the present invention provides a filmless radiographic inspection system with a high energy fluoroscopic screen which achieves sensitivities to 0.01 percent with crack detection approaching a probability of certainty.