Understanding Neutron Radiography NDTHbook 4-CHP16 Reading 2016-3–NRT My ASNT Level III, Pre-Exam Preparatory Self Study Notes 4th August 2016
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Trinity, 1945
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Trinity, 1945
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The Magical Book of Neutron Radiography
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数字签名者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang @qq.com, c=CN 日期：2016.08.04 06:01:42 +08'00' Charlie Chong/ Fion Zhang
ASNT Certification Guide NDT Level III / PdM Level III NR - Neutron Radiographic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of penetrating radiation • Interaction between penetrating radiation and matter • Neutron radiography imaging • Radiometry 2. Equipment/Materials • Sources of neutrons • Radiation detectors • Non-imaging devices
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3. Techniques/Calibrations
• Electron emission radiography
• Blocking and filtering
• Micro-radiography
• Multifilm technique
• Laminography (tomography)
• Enlargement and projection
• Control of diffraction effects
• Stereoradiography
• Panoramic exposures
• Triangulation methods
• Gaging
• Autoradiography
• Real time imaging
• Flash Radiography
• Image analysis techniques
• In-motion radiography • Fluoroscopy
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4. Interpretation/Evaluation • Image-object relationships • Material considerations • Codes, standards, and specifications 5. Procedures • Imaging considerations • Film processing • Viewing of radiographs • Judging radiographic quality 6. Safety and Health • Exposure hazards • Methods of controlling radiation exposure • Operation and emergency procedures Reference Catalog Number NDT Handbook, Third Edition: Volume 4, Radiographic Testing 144 ASM Handbook Vol. 17, NDE and QC 105 Charlie Chong/ Fion Zhang
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Chapter 16 Neutron Radiography
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PART 1. Applications of Neutron Radiography Neutron radiation is similar to X-radiation. The radiation can originate from an effective point source or can bec ollimated to shine through an object in a coherent beam. The pattern of penetrating radiation can then be studied to reveal clues about the internals of the object. The information conveyed can be very different from that obtainable with X-rays. Whereas X-rays are attenuated by dense metals more than by hydrocarbons,neutrons are attenuated more by hydrocarbons than by most metals. The difference can mean much more than the reversal of a positive image to a negative image. Neutrons, for example, can reveal details within high density surroundings that cannot be revealed by other
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A typical application for neutron radiography is shown in the images of a pyrotechnic device (Fig. 1), where the small explosive charge is encased in metal. Other applications include inspection of explosive cords used in pilot ejector mechanisms; inspection of gaskets, seals and O-rings inside metallic valves; confirmation that coolant channels in jet engine turbine blades are free of blockage; studies of coking in jet engine fuel nozzles; and screening of aircraft panels to detect low level moisture or early stage corrosion in aluminum honeycomb (Fig. 2).
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FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit.
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FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit.
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User’s Guide Unlike many other forms of nondestructive testing, neutron radiography is not a do-it-yourself technique. There have been neutron radiography service centers in the United States since 1968. To try out neutron radiography on an object of interest, it is simply necessary to locate the services currently available and, if agreed, mail your item to them. Typically, the neutron radiograph and your item will be mailed back within a day or two. The cost could be less than 1 or 2 h of an engineer’s time. If assistance is required to interpret the findings, this too may be requested ona service basis, as may referrals to more specialized neutron radiographic techniques. The providers of neutron radiography services use equipment and expertise that is highly specialized. Even though one or more neutron radiography service centers have been operating successfully for over 30 years, there has been no inhouse neutron radiography available at any general service, commercial nondestructive testing center.
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FIGURE 2. Comparison of neutron radiographs of moisture globules in aluminum honeycomb panel, later dried: (a) before processing; (b) after processing. (a) before processing
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(b) after processing.
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The interested user is therefore advised to seek a supplier of neutron radiographic services using leads such as society directories or the published literature. Because neutrons are fundamentally different from X-rays, any object that is a candidate for inspection by X-radiography could also be a candidate for neutron radiography. If X-rays cannot give sufficient information, then trials with neutron techniques may be prudent. The most frequently successful complement to X-radiography is static radiography with thermal neutrons. This approach is reviewed next. Then more specialized neutron radiology techniques are reviewed, such as neutron computed tomography, dynamic neutron imaging, high frame rate neutron imaging, neutron induced autoradiography and neutron gaging. For each of the neutron radiology techniques different neutron energies may be selected. The user should be aware that many of the specialized services are only available at one or two centers worldwide. It is therefore important to shop in the global market and to take advantage of the excellent communications existing between neutron radiography centers in various countries.
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PART 2. Static Radiography with Thermal Neutrons
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Neutron Energy Thermal energy neutrons are those that have collided repeatedly with a moderator material, typically graphite or water, such that they reach an equilibrium energy with the thermal energy of the moderator nuclei. The attenuation coefficients for thermal neutrons differ from material to material in a way that is different from X-rays as shown in Table 1. As a consequence, a high degree of contrast between the elements in an object is possible. In addition, thermal neutrons are relatively easy to obtain and easy to detect
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TABLE 1. Comparison of X-ray and thermal neutron attenuation.
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Neutron Collimation Because the source of thermal neutrons is a dispersed moderator volume, rather than a point source, it is necessary to use a collimator between the source and the object. In preference to a single tube parallel sided collimator or a multiple slit collimator, the most frequently used design uses divergent beam geometry.16 The collimator may be used to extract a beam in any one of a variety of different geometries including horizontal or vertical, radial or tangential to the source. A collimator that is tangential to the source can provide a thermal neutron beam relatively free of fast neutron and gamma ray contamination. An incidental consequence of the divergent collimator principal is that even very large objects can be radiographed using an array of side-by-side films (Fig. 3).
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FIGURE 3. Radiographs of full size motorcycle: (a) neutron radiograph; (b) x- radiograph.
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FIGURE 3. Radiographs of full size motorcycle: (a) neutron radiograph; (b) x-radiograph.
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Neutron Imaging Collimation Ratio The collimation ratio is the ratio L¡D–1 of the collimator length L to aperture diameter D. This ratio helps to predict image sharpness. Imaging Processes For static thermal neutron radiography of nonradioactive objects, two important imaging processes are 1. the gadolinium converter with single emulsion X-ray film and 2. the neutron sensitive storage phosphor (neutron imaging plate). For static neutron radiography of radioactive objects, additional imaging processes are (transfer methods) 1. dysprosium foil activation transfer to film, 2. Indium foil activation transfer to film and 3. Track etch imaging using a boron converter and cellulose nitrate film.
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The established direct imaging technique uses thin gadolinium layer vapor deposited on a solid converter screen, which is held flat against a single emulsion film inside a vacuum cassette of thin aluminum construction. An exposure of 109 neutrons per square centimeter can give a high resolution, high contrast radiograph if careful dust free film darkroom procedures are used. Neutron sensitive imaging plates consist of a thin phosphor layer containing a mixture of storage phosphor, neutron converter and organic binder. Following the neutron exposure stage is the information readout phase, in which the plate is scanned by a thin laser beam stimulating the emission of a pattern of light. Merits of this neutron imaging technique include five decades of linearity, wide dynamic range, direct availability of digital data for processing converter efficiencies of 30 to 40 percent, and spatial resolution acceptable for some applications
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For neutron radiography of highly radioactive objects, dysprosium and indium foil activation transfer to film and track etch imaging each offer complete discrimination against gamma ray fogging. Examples of nuclear fuel neutron radiography are shown in Fig. 4. Dysprosium transfer can be combinedwith a cadmium indium foil sandwich for dual energy radiography. Alternative track etch techniques have been developed to yield more precise dimensional measurements.
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FIGURE 4. Neutron radiographs of nuclear fuel: (a) longitudinal cracks in pellets; (b) missing chips in compacted fuels; (c) inclusions of plutonium in pellets; (d) accumulation of plutonium in central void; (e) deformed cladding; (f) hydrides in cladding. (a) longitudinal cracks in pellets;
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(b) missing chips in compacted fuels;
(c) inclusions of plutonium in pellets;
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(d) accumulation of plutonium in central void;
(e) deformed cladding;
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(f) hydrides in cladding.
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Image Quality Indicators For any nondestructive system, the best measure of quality is to compare the image of the test object with an image of a similar object that contains a known artificial discontinuity, a defect standard, or reference standard. However, neutron radiography has the same problems as other nondestructive testing methods: the quantity of reference standards required is too large to obtain and maintain. In lieu of a reference standard, neutron radiographers have chosen to fabricate a resolution indicator that emulates the worst case scenario with gaps placed between and holes placed beneath different plastic thicknesses. For defining the neutron beam characteristics a beam purity indicator has been devised to accompany the sensitivity indicator. The image quality indicator system of ASTM International has become the primary or alternate system for most manufacturing specifications on an international basis. The no umbra device, a device to measure resolution, is described in ASTM E 803-91 and can be used to determine the collimation ratio L¡D–1 of the neutron radiography facility.
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Nuclear Reactor Systems A nuclear reactor system operated for over 30 years solely to provide a commercial neutron radiographic service is illustrated in Fig. 5. The reactor core, positioned underground in a tank of water, is only about 0.38 m (15 in.) in diameter and operates at 250 kW power. The tangential beam tube is orientated vertically with air displaced by helium. Parts for neutron radiography can therefore be supported on horizontal trays. Usually the neutron imaging uses a gadolinium converter with fine grain radiographic film and the exposure time at a selected collimation is typically about 2 min.
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FIGURE 5. Representative neutron radiographic service center for nonnuclear applications.
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Another reactor that has provided neutron radiography services since 1968 is illustrated in Fig. 6. It is above ground and the fuel of the 100 kW core is arranged in an annulus with a moderator region in the center. Two horizontal beams are extracted from the central moderator, one for direct film neutron radiography of nonradioactive objects, the other for dysprosium activation transfer neutron radiography of radioactive nuclear fuel.
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FIGURE 6. Representative neutron radiographic service center for nuclear and nonnuclear applications.
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Another service for static neutron radiography of radioactive nuclear fuel has been provided by a 250 kW nuclear reactor installed in a hot cell complex (Fig. 7). Also several university reactors in the United States have been equipped for neutron radiography. Worldwide, over fifty nuclear reactors have contributed to development of this field.
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FIGURE 7. Hot cell fuel inspection system.
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Accelerator Based Systems An initial user of neutron radiography need not, in general, be concerned with accelerator source options unless there is an established need either for an in-house system or for a transportable system. Almost all neutron radiography service providers use a nuclear reactor source. One exception has been the powerful spallation type accelerator in Switzerland; the accelerator is a multipurpose facility comparable in complexity and cost to a research reactor. An in-house system that was operated successfully for over 15 years at the United States Department of Energy’s Pantex Plant used a van de graaff accelerator. The operation of this machine, which accelerates over 200 ΟA of deuterons at 3 MeV into a beryllium target, is illustrated in Fig. 8.
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The system provided a peak thermal neutron flux of about 109 neutrons per square centimeter second, two orders of magnitude less than the reactor systems described above but sufficient for low throughput work using 2 h exposure times and a relatively low beam collimation ratio. Cyclotrons and radio frequency quadrupole accelerators are other candidates for a potential custom designed in-house neutron radiographic system. Neutron radiographic performance data have been reported for designs with a variety of sizes, neutron yields and costs. For transportable systems much of the development work has used sealed tube acceleration of deuterium tritium mixtures. This can consist of a source head that is maneuverable with long high tension cable linking it to the high voltage power supply and control unit as illustrated (Fig. 9). The particular type shown yields a peak thermal neutron flux of about 108 neutrons per square centimeter second with a tube operation half life of about 200 h.
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FIGURE 8. Cross section showing van de graaff principle.
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FIGURE 9. Components of mobile deuterium tritium neutron radiographic system: (a) deuterium tritium source head, typically on 6 m (20 ft) cables; (b) cooling unit (left) and power supply; (c) control unit.
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High Intensity Californium-252 Systems Of the many radioactive neutron sources, such as polonium-210 beryllium and americium-244 beryllium, one has dominated interest for neutron radiography: californium-252. This transplutonic isotope is produced as a byproduct of basic research programs. In the United States, some government centers have been able to obtain the source on a low cost loan basis from the Department of Energy. The isotope yields neutrons by spontaneous fission at a rateof 2 Ă— 109 neutrons per second per milligramand has a half life of 2.5 years.
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smaller than a tube of lipstick (Fig. 10). An in-house stationary system has operated at the United States Department of Energy’s installation at Pantex with a total source strength of 150 mg californium-252. It provided sets of nine films, each 350 × 425 mm (14 × 17 in.), approaching reactor quality by using gadolinium with a very fine grain X-ray film; a collimator ratio of 65; And exposure time of under 24 h. A maneuverable source system has operated at McClellan Air Force Base with a total source strength of 50 mg californium252. It provided single neutron radiographs using a fast scintillator screen; High speed, light sensitive film; a collimator ratio of 30; and an exposure time of 12 min. This system was designed for the specific application of scanning intact aircraft to detect hidden problems at an early stage, such as moisture or corrosion in aluminum honeycomb. Another example of a high yield californium-252 system design uses a subcritical multiplier to amplify the central neutron flux. This design (Fig. 11) produces a peak central flux of 7 × 108 neutrons per square centimeter second when loaded with 40 mg californium-252.
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Low Cost In-House System There is evidence that an extremely low intensity californium-252 neutron source could provide a convenient, low cost in-house system. A source size of only 100 Îźg can provide useful quality neutron radiographs by using highly efficient imaging systems that need only 105 neutrons per square centimeter exposure. This is 10 000 times less than the exposure used typically with gadolinium and single emulsion film. The small source size would mean an inexpensive source and also inexpensive shielding, handling and interlock requirements.26 Therefore, a nondestructive testing center with a variety of X-ray, ultrasonic and other inspection capabilities could easily incorporate a small californium-252 based neutron radiographic capability using an underground storage geometry in an existing radiographic bay. Because neutron radiography yields unique information, such an inexpensive in-house capability could be an important complement to an otherwise full service nondestructive testing center.
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FIGURE 10. Californium-252 sources compared in size to postage stamp.
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FIGURE 11. Elevation of subcritical multiplier system.
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PART 3. Special Techniques of Neutron Radiography
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Dynamic Neutron Radioscopy Services that provide different types of dynamic neutron radioscopy have been developed at numerous nuclear reactor centers worldwide. They cover frame rates that range from 30 frames per second (real time motion display similar to television) to 1000 frames per second range (a high frame rate) or to 10 000 frames per second (a very high frame rate). An example of a real time dynamic neutron radioscopic application is illustrated in Figure 12. A beam from a 28 MW reactor was used to study the flow characteristics of lubricant inside an operating jet engine. Other applications have included studies of absorption and compression refrigerator designs, studies of automotive parts in motion and a large range of two-phase flow studies. For high throughput dynamic neutron imaging one reactor center has been equipped with three separate beams, each with its neutron imaging system and digital image interpretation system.
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Other reactor centers have developed techniques for simultaneous neutron and gamma ray dynamic imaging using a pair of scintillator screens in conjunction with a low light leveltelevision camera and video processing.30 The development of dynamic neutron radioscopic services with a high frame rate of 1000 frames per second has capitalized on the availability of very high intensity steady state neutron beams (with a flux of 108 neutrons per square centimeter second) and very high frame rate video cameras used with rapid response neutron sensitive scintillator screens. A very high frame rate capability, up to 10 000 frames per second, uses the ability of certain reactors to be pulsed, giving a high neutron yield for a time duration of a few milliseconds. The event to be studied, such as the burn cycle of a pyrotechnic event, is synchronized to the neutron pulse time.
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FIGURE 12. Frames from real time studies of operating aircraft engine: (a) first view; (b) second view.
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Subthermal Neutron Radiology The neutron attenuation coefficient of a particular material can change significantly as the neutron energy is changed. The pattern of this variation also changes abruptly from one element to another. Therefore, selection of different energy neutrons provides possibilities for quite different neutron radiology penetration and contrast. Neutron radiology service reactors have developed neutron beams of selected subthermal or cold neutrons using three techniques:
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1. beam filtration by polycrystal beryllium, which passes only long wavelength, low energy neutrons below 0.005 eV, 2. a refrigerated moderator volume and 3. selection of longer wavelength, low energy neutrons by multiple internal reflection in a gently curved guide tube. The effect of this energy selection is typically to increase the transparency of certain materials while simultaneously increasing the contrast or detectability of hydrogenous materials (see Table 2 and Fig. 13). Just as thermal neutron radiography gives different information to X-radiography, so subthermal or cold neutron radiography gives information different from that of regular thermal neutron techniques. An example is given in Fig. 14. It is possible, using a guide tube, to select only very cold neutrons (that is, energies below 0.001 eV) and this can provide high sensitivity for very thin hydrogenous specimens.
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FIGURE 13. Attenuation of materials for thermal and cold neutrons.
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TABLE 2. Relative neutron attenuation coefficients.
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FIGURE 14. Neutron radiographs of explosive bridge wire igniter: (a) thermal neutron image; (b) cold neutron image.
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Epithermal and Fast Neutron Radiology A reactor beam, although consisting primarily of thermal neutrons, will contain a proportion of both subthermal and epithermal (high energy) neutrons. With a filter such as cadmium, the thermal and subthermal neutrons can be removed and only the epithermal part of the neutron energy spectrum will be transmitted. For the inspection of enriched nuclearfuel the higher penetration of epithermal neutrons provides a valuable difference from thermal or subthermal neutronradiography. Indium has a high resonance capture cross section at about 1.4 eV epithermal energy. Cadmium wrapped indium foil activation transfer imaging techniques have been used for this application. Another epithermal neutron technique uses an indium foil filter in the incident beam to remove neutrons close to the specific resonance energy.
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This beam is passed through the object and an indium detector is used on the far side. The technique can provide high sensitivity to small quantities of hydrogen in the object because hydrogen can change the energy of an incident neutron more than heavier elements. The term fast neutron radiography refers normally to those neutron energies yielded by an unmoderated accelerator source or radioactive source. Fast neutron radiography provides high penetration but little contrast between elements. The accelerator can provide a point source. Tantalum is one of several detector materials for direct exposure and scintillator screens can be used. Alternatively, foil activation transfer with holmium has been demonstrated
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Neutron Computed Tomography Computed axial tomography has been developed for neutron radiography and can provide detailed cross sectional slices of the object to be analyzed. Although the principle is similar to that of X-ray computed tomography, the information conveyed by neutrons can be unique. In a typical facility the object is rotated in the neutron beam and data are stored for upward of 200 angles. Detectors used have included a scintillator screen 6LiF-ZnS (Ag), viewed by a cooled charge coupled device camera and alternatively a storage phosphor image plate loaded with Gd2O3 combined with an automatic laser beam scanner. Using a high intensity neutron radiography beam of over 108 neutrons per square centimeter second, computed tomography of two-phase flow volumes has been processed as a time averaged three-dimensional analysis.
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Neutron Gaging and Neutron Probe Techniques Neutron gaging is the measurement of attenuation of a collimated small diameter beam of radiation as it is transmitted by a specimen. A neutron radiology service center equipped with a nuclear reactor has demonstrated that the imaging techniques can be complemented by the more quantitative techniques of gaging. The gaging technique can inspect items of greater thickness than can be inspected with neutron radiography. It has been used for static gaging of discrete assemblies and for continuous scanning of long objects for acceptable uniformity. There are also a variety of neutron probe techniques in which radiation, typically gamma, is observed as a result of neutron radiation incident on the object. For example the associated particle sealed tube neutron generator enables the flight time of the incident neutron to be used in conjunction with gamma ray spectroscopy to indicate the chemical composition within an object. This technique has been developed for identification of hidden explosives, drugs or nuclear materials. Another example of a neutron probe is neutron interferometry to detect phase shifts of the neutron wave properties. This neutron phase topography has been proposed for very high sensitivity material testing.
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Neutron Induced Autoradiography By exposing a painting to thermal or cold neutrons and later imaging the radioactivity induced in the various paint components, a technique has been developed sensitive to many elements including manganese, potassium, copper, sodium, arsenic, phosphorus, gold, iron, mercury, antimony and cobalt. The neutron exposures were originally performed in a moderator block (thermal column), close to a reactor core. However, beams similar to those used for transmission neutron radiography have been used for this neutron induced autoradiography of paintings. Typically, a series of autoradiographs is taken using a range of neutron exposure times and different decay times before imaging. This, combined with a range of scintillator screen and film sensitivities, can provide extensive information about successive layers of each painting.
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Closing Industry standards have been published on neutron radiographic testing
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Peach – 我爱桃子
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Good Luck
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Good Luck
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