Varian Linatron High-Energy X-ray Applications 2007

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FIGURE 4-5. Plot of density versus steel steps for lead and leadphosphor at 9 MeV. * Figure 4-5 is a plot of density versus steel steps for lead and metalphosphor screens. It shows the increased contrast achieved with phosphor screens. Multifilm Techniques. It has been shown that a single film will exhibit a range for film densities that bears a relationship to the range of thicknesses in the object. If there are thicknesses in the object being radiographed that exceed the corresponding limits of a single film, the thicknesses may still be covered in a single exposure by placing two or more films in the film holder. This is true with the additional film having the same or a different speed. Example It may be desired to maintain a minimum density of 1.5 to achieve the required radiographic quality for an exposure where the film density ranges from 1.0 to 1.7. If a second film of the same type is used and the two films are exposed simultaneously, the resultant exposure is about the same. The density of each film will again range from about 1.0 to 1.7. The two densities will sum and the density range will be 2.0 to 3.4 when they are viewed together. page 22 For this example, there wasn’t an increase in exposure time to get a very desirable density range. Using two films of the same type also permits the interpreter to distinguish between an artifact (blemish or flaw) in the film and a defect in the object. This procedure can also be used with two films of different speeds. The first is exposed to a density ranging from 1.0 to 1.7, as before. If the second is a faster film, its density may range from 2.0 to 3.5. The interpreter may then view the darker portion of the lower density film and the low density portion of the higher density film, and thus obtain coverage of the entire thickness range in one exposure. Double film loads can be used between a single pair of screens. There is also mutual intensification of the touching film using this technique. Also, many electrons from metallic screens are sufficiently energetic to pass through multiple layers of film and intensify the succeeding layers. Using interleaved lead screens will ensure better resolution in each film. Interleaving is also employed to adjust the speed relationships. This makes the second layer darker. The interleaved lead foil screens must be without backing material and be between 0.005 and 0.010 inch thick. Definition and Unsharpness It is often desirable to obtain the very best image available in radiography, i.e., one with high contrast, minimal distortion, low graininess, and optimum definition. To accomplish this, an intermediate exposure time is required, since it allows improved density variations to occur on the film. Therefore, compromises are required where there is a need for the shortest possible exposure times. Some approaches to this include: • Sharpness is increased by using thin screens (and sometimes eliminating the back screen), by using single emulsion film and fine-grained film, and by using effective filters between the object and the screen. HOWEVER, Exposure times are reduced by using thick screens, double-emulsion films, coarser- grained films, and by deleting a separate filter between the object and screen. Varian Linatron applications
• Moving the film farther from the object (increasing “TI’) reduces scatter, which improves image quality. HOWEVER, Exposure time and geometric unsharpness are increased. • Increasing the source-to-object distance provides greater area coverage and reduces geometric unsharpness. HOWEVER, Exposure time increases by the inverse square law. • Using composite metal-phosphor screens shortens exposure times and provides higher contrast in the image. HOWEVER, Composite metal-phosphor screens result in increased graininess and a loss of sharpness and detail resolution. Unsharpness in a radiograph can be defined as blurring of image edges. This causes a loss of fine crack definition, image detail, and penetrameter (Image Quality Indicator) detail. Unsharpness and lack of contrast are not synonymous. Film contrast is the difference in film density between two areas of a radiograph. A film may have high contrast but lack sharpness because of image edge blurring, and vice versa. Good image quality needs sharpness and high contrast for good radiographic sensitivity. Three sources of unsharpness have been identified in general radiography. These are geometric, inherent, and scatter (see Figure 4-6). U F (inherent unsharpness of the film and screens), and U G (geometric unsharpness due to focal spot size and object thickness and arrangement) contribute mostly to unsharpness. U F is usually the major element of unsharpness in high-energy X-ray radiography. It increases with increasing radiation energy and film grain size. It is a function of screen material and thickness, and is affected by the film processing technique. We can expect the following values of U F with Linatron radiography using lead screens, fine-grained film (i.e., speed index = 4, at density = 2.0), and automatic processing: page 23 FIGURE 4-6. Sources of unsharpness. Energy, MeV UF, mm 1 0.15 2 0.3 4 0.4 6 0.5 9 0.6 15 1 .0 U G is a linear unsharpness, described by the adjacent diagram and the expression: U G = S/(D/T) Where S = source spot size, D = distance from the source spot to the front surface of the object and T = distance from the front surface of the object to the film, or usually the thickness of the object. The total unsharpness (UTOT ) results from the combined effect of the inherent, scatter and geometric unsharpnesses, and can be expressed as the square root of the sum of the square of each. UTOT = (U 2 F + UG2 + US2 ) 1/2 Varian Linatron applications
Page 1: Varian Linatron High-Energy X-ray A
Page 5: Preface This manual presents theory
Page 8 and 9: FIGURE 1-3. Linatron M System Diagr
Page 10 and 11: Linatron targets are specially desi
Page 12 and 13: FIGURE 2-3. Half-value layer for pr
Page 14 and 15: Beaming and Field Flatness. Electro
Page 16 and 17: FIGURE 3-2. Linatron x-ray intensit
Page 18 and 19: It is most convenient to have expos
Page 20 and 21: Vacuum cassettes are recommended wh
Page 22 and 23: Table 4-3 lists some relative speed
Page 24 and 25: Manufacturers publish density versu
Page 28 and 29: The total unsharpness is reduced to
Page 30 and 31: This wedge block is then radiograph
Page 32 and 33: Increasing Latitude With Multiple F
Page 34 and 35: FIGURE 4-12. Linatron 2 MeV typical
Page 38 and 39: FIGURE 4-16. Linatron typical expos
Page 44 and 45: Radiographic Procedures General Con
Page 46 and 47: When radiographing curved sections,
Page 48 and 49: Radiography of Welds Radiography is
Page 50 and 51: The width of separation required fo
Page 52 and 53: The propellant in large motors is o
Page 54 and 55: Grain Radiography Coverage Requirem
Page 56 and 57: Bibliography Books Practical Radiog
Page 58 and 59: Normally, the light generated in th
Page 60 and 61: FIGURE 6-3. Real-time automated ins
Page 62 and 63: Real-time image acquisition display
Page 64 and 65: D/T RATIO Ratio of the distance bet
Page 66 and 67: LINEAR ATTENUATION COEFFICIENT Cons
Page 68: Security & Inspection Products 6883

Varian Linatron High-Energy X-ray Applications 2007

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