Varian Linatron High-Energy X-ray Applications 2007

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This wedge block is then radiographed with substantially the same film, screens, focal-film distance, and arrangements for the same conditions as future routine inspection work. It should be noted that good collimation and shielding must be used for the test exposures to achieve a low amount of scatter at the film holder. The film is then inspected to determine where the film density is 2.0 on the step or wedge image. The total thickness of the wedge-slab combination corresponding to this spot is measured and the exposure level is recorded. This data provides the first point for the exposure curve. The assembly is then made thicker by adding several slabs, and the process is repeated to obtain the second point. These two points should then be plotted on semilog graph paper and connected by a straight line. The first check on the accuracy of this exposure curve is to determine from the slope of the line if the HVL equals the handbook values or other predicted value for the material. An accuracy check can be obtained by selecting a third exposure that should correspond to another point along the line. The line can be extended down to 1 HVL, and up to 12 HVLs or greater. The curve provides a good approximation of exposure times for the entire range of thickness the Linatron can inspect. Since the same wedge exposure will have a region where the film density is about 3.0, it could provide points for an exposure curve for the density 3.0. These data also provide important information about contrast and latitude. Latitude. A common task in high-energy Linatron installations is to radiograph objects that have varying shapes and thicknesses. A single film exposure can cover a range of useful film densities where sensitivity and interpretability are accurate and valid. The thickness range that corresponds to the range of useful densities, usually 1.5 to 3, is called the latitude of the exposure. Latitude depends on the film gradient, or contrast, and the attenuation of the material, or HVL. By noting the points on the wedge image that correspond to densities 1.5 and 3, the wedge exposure used to generate the exposure curves can also be used to provide data for determining latitude. Figure 4-9 shows a plot of latitude for each energy. page 26 FIGURE 4-9. Latitude ranges for Linatron exposures of steel. Hint: While a single film has a limited latitude, two films of different speed can be combined in one exposure to increase the total latitude of the exposure. Exposure Times and Material Densities. When all other variables remain constant the correct radiographic exposure time and level for an object depend on its thickness and density. Two objects with different densities (d 1 and d 2 ) can have approximately the same exposure time, when their thickness (X 1 and X 2 ) are related by the expression Example d 1 X 1 = d 2 X 2 A 1.00-inch-thick (25.4 mm) slab of copper alloy with a density of 8.8 gm/cc will have approximately the same exposure time and rads as a 1.13-inch (28.7 mm) thick slab of steel with a density of 7.87 gm/cc. Likewise, an object with unknown radiographic exposure characteristics may be compared with one whose exposure values are known. To accomplish this, first approximate the objects linear attenuation coefficient (μ 2 ) from its density (d2) by Varian Linatron applications μ 1 /d 1 = μ 2 /d 2
and then determining the approximate exposure level from the known value of exposure for object number 1 (Rl ), by R2e -μ2X2= R1e -μ1X1 Example The exposure of a 4-inch-thick (101.6 mm) bronze alloy (density = 9) may be estimated from the steel exposure curve at 2 MV by: μbronze = 0.693/0.8 X 9/7.85 = 1 inch-1 For 4 inches (101.6 mm) of steel, the exposure rads: R l = 150 rads. Therefore for bronze: R2 = 150 e-(.866)(4) e = 256 rads -(1)(4) Thus, an exposure curve for the bronze alloy can be constructed by calculating the exposure rads at two thicknesses, plotting these points, and connecting them with a straight line. Table 4-4 lists the physical density ratio conversion factors for converting steel or solid propellant exposure curve into curves for a wide variety of materials. • Physical density may be used for a first approximation, but a more accurate value results from t using linear attenuation coefficients. Multiply by factors belowto convert steel exposures to approximate exposures for these materials: Zirconium 0.83 Copper 1.13 Molybdenum 1.3 Silver 1.33 Lead 1.44 Uranium or Tungsten 2.38 Gold 2.45 page 27 Using Film Response Curves to Generate New Exposure Curves. When an exposure curve is obtained and verified for one film and at one value of film density, new exposure curves for other films and for other film densities can be generated without remaking the entire series of exposures. To determine response curves for new films, refer to the film response curves that plot film density versus the exposure level for X-rays, in the Linatron energy ranges. Example Table 4-4 Physical Density Ratios for Common Materials When Using High Energy X-Rays If an exposure curve for steel is made with AGFA-Gevaert Type D-2 x-ray film for a density of 2.0, and it is desired to have a similar curve for Type D-4 film at density 2.0, the film response data of Table 4-3 (presented earlier) and the characteristic curve plots for x-ray film in the AGFA- Gevaert Handbook supply the data necessary for the new plot. Table 4-3 shows that at a film density of 2.0, D-2 has a film speed index of 7 and D-4 has a film speed index of 3. These indices are in proportion to the exposure rads. Therefore, at each thickness, the factor of the D-4 film speed index relative to the D-2 film index (or 3/7 when multiplied by the exposure rads from the D-2 exposure curve) will determine the correct rad exposure at each thickness for the D-4 film. the Agfa-Gevaert Handbook indicates the same relative speed indices for these films, but with another series of index numbers. Multiply by factors below to convert propellant exposures to approximate exposures for these materials: Sodium 0.57 Lucite 0.69 Magnesium 1.02 Carbon 1.3 Concrete 1.38 Aluminum 1.6 Titanium 2.67 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 26 and 27: FIGURE 4-5. Plot of density versus
Page 28 and 29: The total unsharpness is reduced to
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

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