Volume 17 - Nondestructive Evaluation and Quality Control Part 9 pptx

areas outside the facility. An unrestricted area is usually defined as an area where employees are not required to wear personal radiation-monitoring devices and where there is unrestricted access by either employees or the public. Second, main work areas are shielded in accordance with applicable requirements. Some facilities must be inspected by an expert in radiation safety in order to qualify for licensing; it is usually recommended that the owner consult an expert when planning a new facility or changes in an older facility to ensure that all local requirements are met. Appropriate shielding usually consists of lead or thick concrete on all sides of the main work area. The room in which the actual exposures are made is the most heavily shielded and may be lined on all sides with steel or lead. Particular attention is given to potential paths of leakage such as access doors and passthroughs and to seemingly unimportant paths of leakage such as the nails and screws that attach lead sheets to walls and doors. Third, portable radiation sources are used in strict accordance with all regulations. In most cases, safe operation is ensured by a combination of: • Movable shielding (usually lead) • Restrictions on the intensity and direction of radiation emitted from the source during exposure • Exclusion of all personnel from the immediate area during exposure The best protection is afforded by distance because radiation intensity decreases in proportion to the square of the distance from the source. As long as personnel stay far enough away from the source while an exposure is being made, portable sources can be used with adequate safety. Finally, access to radiographic work areas, including field sites and radioactive-source storage areas, must at all times be under the control of competent and properly trained radiographers. Radiographers must be responsible for admitting only approved personnel into restricted work areas and must ensure that each individual admitted to a restricted work area carries appropriate devices for monitoring absorbed radiation doses. In addition to keeping records of absorbed dose for all monitored personnel, radiographers must maintain accurate and complete records of radiation levels in both restricted and adjacent unrestricted areas. Radiation Monitoring. Every safety program must be controlled to ensure that both the facility itself and all personnel subject to radiation exposure are monitored. Facility monitoring is generally accomplished by periodically taking readings of radiation leakage during the operation of each source under maximum-exposure conditions. Calibrated instruments can be used to measure radiation dose rates at various points within the restricted area and at various points around the perimeter of the restricted area. This information, in conjunction with knowledge of normal occupancy, is used to evaluate the effectiveness of shielding and to determine maximum duty cycles for x-ray equipment. To guard against the inadvertent leakage of large amounts of radiation from a shielded work area, interlocks and alarms are often required. Basically, an interlock disconnects power to the x-ray source if an access door is opened. Alarms are connected to a separate power source, and they activate visible and/or audible signals whenever the radiation intensity exceeds a preset value, usually 0.02 mSv/h (2 mrem/h). All personnel within the restricted area must be monitored to ensure that no one absorbs excessive amounts of radiation. Devices such as pocket dosimeters and film badges are the usual means of monitoring, and often both are worn. Pocket dosimeters should be direct reading. One disadvantage of the remote-reading type is that it must be brought to a charger unit to be read. Both types are sensitive to mechanical shock, which could reduce the internal charge, thus implying that excessive radiation has been absorbed. If this should happen, the film badge should be developed immediately and the absorbed dose evaluated. Under normal conditions, pocket dosimeters are read daily, and the readings are noted in a permanent record. At less frequent intervals, but at least monthly, film badges are developed and are evaluated by comparison with a set of reference films. Because of various factors, the values of absorbed radiation indicated by dosimeters and film badges may differ. This is particularly true at low dose rates and with high radiation energies (1 MeV or more). However, the accuracy of both dosimeters and film badges increases with increasing dose, and at dose rates near the maximum allowable dose of 0.0125 Sv (1 rem) per calendar quarter, the readings usually check within ±20%. Access Control. Permanent facilities are usually separated from unrestricted areas by shielded walls. Sometimes, particularly in on-site radiographic inspection, access barriers may consist only of ropes. In such cases, the entire perimeter around the work area should be under continual visual surveillance by radiographic personnel. Signs that carry the international symbol for radiation must be posted around any high-radiation area. This helps to inform bystanders of the potential hazard, but should never be assumed to prevent unauthorized entry into the danger zone. In fact, no interlock, no radiation alarm, and no other safety device should be considered a substitute for constant vigilance on the part of radiographic personnel. References cited in this section 1. Radiography & Radiation Testing, Vol 3, 2nd ed., Nondestructive Testing Handbook, American Society for Nondestructive Testing, 1985 2. R. Halmshaw, Nondestructive Testing, Edward Arnold Publishers, 1987 Radiographic Inspection Revised by the ASM Committee on Radiographic Inspection * X-Ray Tubes X-ray tubes are electronic devices that convert electrical energy into x-rays. Typically, an x-ray tube consists of a cathode structure containing a filament and an anode structure containing a target all within an evacuated chamber or envelope (Fig. 7). A low-voltage power supply, usually controlled by a rheostat, generates the electric current that heats the filament to incandescence. This incandescence of the filament produces an electron cloud, which is directed to the anode by a focusing system and accelerated to the anode by the high voltage applied between the cathode and the anode. Depending on the size of the focal spot achieved, x-ray tubes are sometimes classified into three groups: • Conventional x-ray tubes with focal- spot sizes between 2 by 2 mm (0.08 by 0.08 in.) and 5 by 5 mm (0.2 by 0.2 in.) • Minifocus tubes with focal-spot sizes in the range of 0.2 mm (0.008 in.) and 0.8 mm (0.03 in.) • Microfocus tubes with focal-spot sizes in the range of 0.005 mm (0.0002 in.) and 0.05 mm (0.002 in.) The design of conventional and microfocus x-ray tubes is discussed in the following section. Fig. 7 Schematic of the principal components of an x-ray unit When the accelerated electrons impinge on the target immediately beneath the focal spot, the electrons are slowed and absorbed, and both bremsstrahlung and characteristic x-rays are produced. Most of the energy in the impinging electron beam is transformed into heat, which must be dissipated. Severe restrictions are imposed on the design and selection of materials for the anode and target to ensure that structural damage from overheating does not prematurely destroy the target. Anode heating also limits the size of the focal spot. Because smaller focal spots produce sharper radiographic images, the design of the anode and target represents a compromise between maximum radiographic definition and maximum target life. In many x-ray tubes, a long, narrow, actual focal spot is projected as a roughly square effective focal spot by inclining the anode face at a small angle (usually about 20°) to the centerline of the x-ray beam, as shown in Fig. 8. Fig. 8 Schematic of the actual and effective focal spots of an anode that is inclined at 20° to the centerline of the x-ray beam Tube Design and Materials The cathode structure in a conventional x-ray tube incorporates a filament and a focusing cup, which surrounds the filament. The focusing cup, usually made of pure iron or pure nickel, functions as an electrostatic lens whose purpose is to direct the electron beam toward the anode. The filament, usually a coil of tungsten wire, is heated to incandescence by an electric current produced by a relatively low voltage, similar to the operation of an ordinary incandescent light bulb. At incandescence, the filament emits electrons, which are accelerated across the evacuated space between the cathode and the anode. The driving force for acceleration is a high electrical potential (voltage) between anode and cathode, which is applied during exposure. The anode usually consists of a button of the target material embedded in a mass of copper that absorbs much of the heat generated by electron collisions with the target. Tungsten is the preferred material for traditional x-ray tubes used in radiography because its high atomic number makes it an efficient emitter of x-rays and because its high melting point enables it to withstand the high temperatures of operation. Gold and platinum are also used in x-ray tubes for radiography, but targets made of these metals must be more effectively cooled than targets made of tungsten. Other materials are used, particularly at low energies, to take advantage of their characteristic radiation. Most high-intensity x-ray tubes have forced liquid cooling to dissipate the large amounts of anode heat generated during operation. Tube envelopes are constructed of glass, ceramic materials or metals, or combinations of these materials. Tube envelopes must have good structural strength at high temperatures to withstand the combined effect of forces imposed by atmospheric pressure on the evacuated chamber and radiated heat from the anode. The shape of the envelope varies with the cathode-anode arrangement and with the maximum rated voltage of the tube. Electrical connections for the anode and cathode are fused into the walls of the envelope. Generally, these are made of metals or alloys having thermal-expansion properties that match those of the envelope material. X-ray tubes are inserted into metallic housings that contain an insulating medium such as transformer oil or an insulating gas. The main purpose of the insulated housing is to provide protection from high-voltage electrical shock. Housings usually contain quick disconnects for electrical cables from the high-voltage power supply or transformer. On self- contained units, most of which are portable, both the x-ray tube and the high-voltage transformer are contained in a single housing, and no high-voltage cables are used. Microfocus X-Ray Tubes. Developments in vacuum technology and manufacturing processes have led to the design and manufacture of microfocus x-ray systems. Some of these systems incorporate designs that allow the opening of the tube head and the replacement of component parts, such as targets and filaments (Fig. 9). A vacuum is generated by the use of a roughing pump and turbomolecular pumps that rapidly evacuate and maintain the system to levels as low as 0.1 × 10 -3 Pa (10 -6 torr). Electrostatic or magnetic focusing and x-y deflection are used to provide very small focal spots and to guide the beam to various locations on the target. Focal-spot sizes are adjustable from 0.005 to 0.2 mm (0.0002 to 0.008 in.). If the target becomes pitted in one area, the beam can be deflected to a new area, extending the target life. When the target, filament, or other interior component is no longer useful in producing the desired focal-spot size or x-ray output, the tube can be opened and the component replaced at minimal cost. The tube can then be evacuated to operating levels in a few minutes or hours, depending on the length of time the tube is open to the atmosphere and the amount of contamination present. Fig. 9 Schematic of a microfocus x-ray tube These systems are available with voltages varying from 10 to 360 kV at beam currents from 0.01 to 2 mA. To avoid excessive pitting of the target, the beam current is varied according to the desired focal-spot size and/or kilovolt level (Fig. 10). Fig. 10 Maximum ratings for no burn-in with 200-kV cathode/120-kV anode for electron beam widths of 10 m (0.4 mil) Microfocus x-ray systems having focal spots that approach a point source are useful in obtaining very high resolution images. A radiographic definition of 20 line pairs per millimeter (or a spatial resolution of 0.002 in.), using real-time radiography has been achieved with microfocus x-ray sources. This high degree of radiographic definition is accomplished by image enlargement, which allows the imaging of small details (see the section "Enlargement" in this article). Microfocus x-ray systems have found considerable use in the inspection of integrated circuits and other miniature electronic components. Microfocus x-ray systems with specially designed anodes as small as 13 mm (0.5 in.) in diameter and several inches long also enable an x-ray source to be placed inside otherwise inaccessible areas, such as aircraft structures and piping. The imaging medium is placed on the exterior, and this allows for the single-wall inspection of otherwise uninspectable critical components. Because of the small focal spot, the source can be close to the test area with minimal geometric unsharpness (see the section "Principles of Shadow Formation" in this article for the factors that influence geometric unsharpness). X-Ray Tube Operating Characteristics There are three important electrical characteristics of x-ray tubes: • The filament current, which controls the filament temperature and in turn the quantity of electrons that are emitted • The tube voltage, or anode-to-cathode potential, which controls the energy of impinging electrons and therefore the energy or penetrating power, of the x-ray beam • The tube current, which is directly related to filament temperature and is usually referred to as the milliamperage of the tube The strength, or radiation output, of the beam is approximately proportional to milliamperage, which is used as one of the variables in exposure calculations. This radiation output, or R-output, is usually expressed in roentgens per minute (or hour) at 1 m (as mentioned in the section "Radiation Sources" in this article). X-Ray Spectrum. The output of a radiographic x-ray tube is not a single-wavelength beam, but rather a spectrum of wavelengths somewhat analogous to white light. The lower limit of wavelengths, min , in manometers, at which there is an abrupt ending of the spectrum, is inversely proportional to tube voltage, V. This corresponds to an upper limit on photon energy, E max , which is proportional to the tube voltage, V: E max = aV (Eq 4) where a = 11 eV/volt. Figure 11 illustrates the effect of variations in tube voltage and tube current on photon energy and the intensity (number of photons). As shown in Fig. 11(a), increasing the tube voltage increases the intensity of radiation and adds higher- energy photons to the spectrum (crosshatched area, Fig. 11a). On the other hand, as shown in Fig. 11(b), increasing the tube current increases the intensity of radiation but does not affect the energy distribution. Fig. 11 Effect of (a) tube voltage and (b) tube current on the variation of intensity with wavelength for the bremsstrahlung spectrum of an x-ray tube. See text for discussion. The energy of the x-rays is important because higher-energy radiation has greater penetrating capability; that is, it can penetrate through greater thickness of a given material or can penetrate denser materials than can lower-energy radiation. This effect is shown in Fig. 12, which relates the penetrating capability to tube voltage. An applied voltage of about 200 kV can penetrate steel up to 25 mm (1 in.) thick, while almost 2000 kV is needed when the thickness to be penetrated is 100 mm (4 in.). Fig. 12 Effect of tube voltage on the penetrating capability of the resulting x-ray beam As previously stated, most of the energy in the electron stream is converted into heat rather than into x-rays. The efficiency of conversion, expressed in terms of the percentage of electron energy that is converted into x-rays, varies with electron energy (or tube voltage), as shown in Fig. 13. At low electron energies (tube voltages of about 100 to 200 kV), conversion is only about 1%; about 99% of the energy must be dissipated from the anode as heat. However, at high electron energies (above 1 MeV), the process is much more efficient, varying from about 7% conversion efficiency at 1 MeV to 37% at 5 MeV. Therefore, in addition to producing radiation that is more penetrating, high-energy sources produce greater intensity of radiation for a given amount of electrical energy consumed. Fig. 13 Effect of tube voltage or electron energy on the efficiency of energy conversion in the target of an x- ray source The R-output of an x-ray tube varies with tube voltage (accelerating potential), tube current (number of electrons impinging on the target per unit time), and physical features of the individual equipment. Because of the last factor, the R- output of an individual source also varies with position in the radiation beam, position usually being expressed as the angle relative to the central axis of the beam. Effect of Tube Voltage. Both the mean photon energy and the R-output of an x-ray source are altered by changes in tube voltage. The effect of tube voltage on the variation of intensity (R-output) is shown in Fig. 11(a). The overall R- output varies approximately as the square root of tube voltage. The combined effect of greater photon energy and increased R-output produces, for film radiography, a decrease in exposure time of about 50% for a 10% increase in tube voltage. The effect is similar with other permanent-image recording media, as in paper radiography and xeroradiography. Effect of Tube Current. The spectral distribution of wavelengths is not altered by changes in tube current; only the R- output (intensity) varies (Fig. 11b). Because tube current is a direct measure of the number of electrons impinging on the target per unit of time, and therefore the number of photons emitted per unit of time at each value of photon energy, R- output varies directly with tube current. This leads to the so-called reciprocity law for radiographic exposure, expressed as: it = constant (Eq 5) where i is the tube current (usually expressed in milliamperes) and t is the exposure time. The reciprocity law (Eq 5) is valid for any recording medium whose response depends solely on the amount of radiation impinging on the testpiece, regardless of the rate of impingement (radiation intensity). For example, the reciprocity law is valid for most film and paper radiography and most xeroradiography but not for fluoroscopic screens or radiometric detectors, both of which respond to radiation intensity rather than to total amount of radiation. However, radiographic films, when used with fluorescent screens, exhibit reciprocity law failure because the response of film emulsions varies with the rate of impingement of photons of visible light (screen brightness) and with total exposure. If tube current is decreased and exposure time is increased according to the reciprocity law, a fluorescent screen will emit the same amount of light (same number of photons) but at a lower level of brightness over a longer period of time. The lower brightness level will result in lower film density compared to an equivalent exposure made with a higher tube current. Deviations from reciprocity law are usually small for minor changes in tube current, causing little difficulty in resolving testpiece features or interpreting radiographs made on screen-type film. However, when the tube current is changed by a factor of four or more, there may be a 20% or more deviation from reciprocity law, and it will be necessary to compensate for the effect of screen brightness on film density. In most cases, when tube voltage is maintained constant, exposures made in accordance with the reciprocity law should produce identical film densities. This assumes that tube voltage does not vary with tube current. Heavy-duty equipment designed for stationary installations contains electrical circuitry (current stabilizers and voltage compensators) that tend to maintain R-output in accordance with the reciprocity law. However, equipment intended for on-site use is often designed to minimize weight, size, and cost and does not contain such complex circuitry. Consequently, the R-output and x-ray spectrum of many portable or transportable machines vary with tube current. These types of machines may exhibit apparent reciprocity law failure at both ends of the useful range of tube currents. For example, at currents approaching the maximum rated current, the tube voltage tends to be depressed because of the heavy electrical load on the transformer. This produces lower values of both R-output and mean photon energy than would normally be expected. Conversely, at very low tube currents, the tube voltage may actually exceed the calibrated value because the transformer is more efficient. This produces higher values of both R-output and mean photon energy than are normally expected. These deviations from expected values are not always indicated on the monitoring instruments attached to x-ray machines. Because of deviations from reciprocity law caused by equipment characteristics, it is often desirable to prepare exposure charts (usually graphs) for each x-ray unit. Such charts or graphs should be based on film exposure data at several values of tube voltage within the useful kilovoltage range of the unit. Use of these charts for determining exposure times, especially at abnormally high or low tube currents, will help to avoid unsatisfactory image quality. Effect of Electrical Waveform. In all x-ray tubes that are powered by transformer circuits, the waveform of the tube voltage affects R-output. X-ray tubes are direct current (dc) devices, yet almost all power supplies are driven by alternating current (ac). If ac power is supplied directly to the x-ray tube, the tube itself will provide half-wave rectification. Many low-power x-ray machines use self-rectifying x-ray tubes. As illustrated in Fig. 14(a), half-wave rectification results in a sinusoidal variation of instantaneous tube voltage with time, except that no tube current flows (and there is no effective tube voltage) during the portion of each cycle when the anode is electrically negative, and consequently there is no emission of x-rays during that portion of the cycle. Fig. 14 Waveforms for accelerating potential (tube voltage) and tube current for four generally used types of x- ray-tube circuits. (a) Half-wave rectified circuit and (b) full-wave rectified circuit, in w hich tube voltage is equal to the peak voltage of the electrical transformer. (c) Villard-type circuit and (d) constant- potential circuit, in which tube voltage is twice the peak voltage of the transformer In a full-wave-rectified circuit (Fig. 14b), the instantaneous tube voltage varies sinusoidally from zero to peak voltage (kVp) and back to zero, then repeats the sinusoidal pattern during the second half of each cycle. This effectively doubles the time during which x-rays are emitted on each cycle, thus doubling the R-output compared to half-wave-rectified equipment. In Villard-circuit equipment, the electrical input to the x-ray tube varies sinusoidally, but as Fig. 14(c) shows, the zero potential occurs at the minimum instantaneous tube voltage rather than at midrange. Consequently, the accelerating potential across the tube has a peak value (kVp) that is twice the peak voltage of the transformer; for example, 2-MeV electrons can be produced with transformer rated at 1 MVp. X-rays produced by Villard-circuit equipment are harder (that is, of higher mean photon energy) than x-rays produced by rectified equipment having a transformer that operates at the same peak voltage. Villard circuits exhibit a variation in instantaneous tube current that parallels the variation in instantaneous tube voltage. A modified Villard circuit, also known as a Greinacher circuit or a constant-potential circuit, produces an accelerating potential and instantaneous tube current that are nearly constant, varying only slightly with time, as shown in Fig. 14(d). The chief value of constant-potential equipment lies in the fact that its R-output is about 15% higher than for a full-wave- rectified unit of equivalent tube voltage. This means that a nominal 15% reduction in exposure times can be achieved by using constant-potential instead of rectified equipment. Heel Effect. X-ray tubes exhibit a detrimental feature known as the heel effect. When the direction in which x-rays are emitted from the target approaches the anode heel plane, the intensity of radiation at a given distance from the focal spot is less than the intensity of the central beam because of self-absorption by the target. Figure 15(a) shows the heel effect schematically, and Fig. 15(b) is a graph of the variation of intensity as a function of angle of emergence relative to the anode heel plane. The direction of the central beam is at 20° for this particular tube design, an angle equal to the anode heel angle. [...]... in steel mm in X-ray tubes 150 kV Up to 15 Up to 250 kV Up to 40 Up to 1 400 kV Up to 65 Up to 2 1000 kV (1 MV) 5 -9 0 -3 High-energy sources 2.0 MeV 5-2 50 -1 0 4.5 MeV 2 5-3 00 7.5 MeV 6 0-4 60 1-1 2 2 20.0 MeV 7 5-6 10 -1 8 3-2 4 R-Output of High-Energy Sources The R-output of a pulsed x-ray generator such as a linear accelerator or betatron depends on several factors, including pulse length and frequency of... occurring and artificially produced unstable isotopes In all respects other than their origin, γ-rays and x-rays are identical Unlike the broad-spectrum radiation produced by an x-ray tube, γ-ray sources emit one or more discrete wavelengths of radiation, each having its own characteristic photon energy The two most common radioactive isotopes used in radiography are iridium- 192 and cobalt-60 Ytterbium-1 69. .. High-Energy X-Ray Sources Above about 400 kV, the conventional design of an x-ray tube and its high-voltage iron-core transformer becomes cumber-some and large Although x-ray machines with iron-core transformers have been built for 600 kV (maximum), there are no commercial versions operating above 500 kV For higher-energy x-rays, other designs are used Some of the machine designs for the production of high-energy... the material and design of its encapsulation, and the degree of concentration at the time the source was originally produced Table 4 Characteristics of -ray sources used in industrial radiography γ-ray source Half-life Photon energy, MeV Radiation output, RHM/Ci(a) Thulium -1 70 128 days 0.054 and 0.084(b) 0.003 Penetrating power, mm (in.) of steel 13 ( ) Iridium- 192 74 days 12 rays from 0.2 1-0 .61 0.48... from the relationship: (Eq 9) where M is the degree of enlargement (magnification), Si is the size of the image, So is the size of the object, Li is the source-to-film or source-to-detector distance, and Lo is the source-to-object distance Variations in the position of a given object relative to the source and recording surface affect image size For example, when the source-to-image distance, Li, is decreased... filter material and thickness varies with different combinations of testpiece material and thickness, film or detector type, and bremsstrahlung spectrum as determined by the type of x-ray tube and tube voltage used Also, the optimum combinations of filter material and thickness, and tube current and exposure time, must be evaluated on the basis of a compromise between image quality and costs Generally,... one of the lightest of metals and is more transparent to x-rays than any other metal The beryllium-window tube has a minimum of inherent filtration and allows most of the very low energy x-rays to escape from the tube, as shown by the comparison with an ordinary oil-insulated glass tube in Fig 19 The results are quite noticeable with both film and real-time radiography, particularly in contrast improvement... low-density materials Nevertheless, real-time microradiography has found application in the inspection of high -quality castings and high-integrity weldments Fig 24 Geometric magnification of radiographic images with a microfocus x-ray tube (a) Greater geometric magnification and unsharpness occur when the testpiece is moved away from the detector and toward the radiation source With a microfocus x-ray... diameter; this restricts the size of the x-ray beam and the corresponding field diameter Fig 19 Comparison of the R-output of a beryllium-window tube with that of an ordinary oil-insulated glass xray tube In general, the higher the tube voltage, the larger the tube must be, and consequently the thicker the glass walls must be to support the internal elements and to withstand external atmospheric pressure Furthermore,... radiation Tube Rating X-ray tubes produce a great amount of heat At 100-kV tube voltage, only about 1% of the electrical energy is converted to x-rays; the remaining 99 % is converted to heat Heat removal constitutes the most serious limitation on x-ray tube design The size of the focal spot and the design of the anode are the main factors that determine the rating of a particular x-ray tube A tube rating . (1 MV) 5 -9 0 -3 High-energy sources 2.0 MeV 5-2 50 -1 0 4.5 MeV 2 5-3 00 1-1 2 7.5 MeV 6 0-4 60 2 -1 8 20.0 MeV 7 5-6 10 3-2 4 R-Output of High-Energy Sources. The R-output of a pulsed x-ray generator. Thulium -1 70 128 days 0.054 and 0.084 (b) 0.003 13 ( ) Iridium- 192 74 days 12 rays from 0.2 1-0 .61 0.48 75 (3) Cesium-137 33 years 0.66 0.32 75 (3) Cobalt-60 5.3 years 1 .17 and. iridium- 192 and cobalt-60. Ytterbium-1 69 has also gained a measure of acceptability in the radiography of thin materials and small tubes, especially boiler tubes in power plants. Cesium-137,