GS Lecture -1 - 01_07_2013

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CONVENTIONAL AND HIGH ENERGY INDUSTRIAL RADIOGRAPHY

Dr. GURSHARAN SINGHAssociate Director

Radiochemistry and Isotope GroupBhabha Atomic Research Centre

Mumbai - 400 085

INTRODUCTION

Ever since the discovery of Radium by Madam Curie, there have been constantefforts all over the world to utilize the radiations emitted by radioisotopes in various fields.However, a great boost to these applications was triggered when it became possible toproduce a variety of radioisotopes artificially in nuclear reactors. The applications ofradioisotopes depend upon their half lives, energy and type of radiation emitted by them.The overall purpose of these applications is to develop non-destructive and non -invasive,simple and safe techniques to obtain important and reliable information about varioussystems under investigation.

Radiography testing is an important member of the NDT family. Introduction of thistechnology on industrial scale in many countries was initiated when engineering industriestook up fabrication of nuclear and space components having stringent specifications. Sincethen, its use has grown very rapidly and is now extensively used as mandatoryrequirement in the manufacture of pressure vessels, turbines, space vehicles, aircrafts,ships, bridges, offshore rigs and platforms, transport pipe lines, a host of other weldedspecimen, castings, and assemblies etc.

Radiography testing is the process of detecting discontinuities in objects by passingpenetrating ionizing radiation through them and recording the transmitted radiation patternon X-ray films. The radiation sources used for industrial radiography are gamma ray

sources, X-ray machines and in a few cases Neutrons.

A radiography set-up consists of:

Radiation source Test object X-ray film kept between a pair of lead screens and enclosed in a light proof cassette.

Formation of radiographic image is based on the principle of differential absorption ofradiation while passing through the specimen. Variations in density, composition, thicknessor presence of materials of different absorption characteristics can be easily detected. The

image on X-ray film, after processing, is converted into black and white pattern. The typeof pattern obtained on film depends on structure of test object.

OBJECT IMAGE RELATIONSHIP

The image of discontinuities in the object is formed due to differential attenuation ofradiation, the intensity of transmitted radiation through the object having a discontinuity ofthickness t in a thickness of X is represented by the following equation;

I = Ioe-m(X - t)
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RADIATION SOURCE

Selection of a radiation source mainly depends upon;

Material of specimen Thickness of specimen Required image quality.Following table gives characteristics of the commonly used gamma ray sources.

CHARACTERISTICS OF GAMMA RAY SOURCES

Source Half Life Energy(Mev)

RHM Approx. Usefulsteel thickness

range (mm)

Tm-170 127 d 0.08 0.009 2.5 - 12.5

Ir-192 74 d 0.4

(average)

0.5 12 - 65

Cs-137 30 y 0.66 0.32 20 - 90

Co-60 5.26 Y 1.17, 1.33 1.3 50 - 150

Gamma ray sources decay with time, whether in use or not. The present activity (curies)of a source can be obtained from its decay chart.

Conventional penetrating radiation based non-destructive testing methods use X andgamma ray sources as radiation sources with industrial X-ray film as detector. By choosinga variety of source - film combinations, varying degrees of flaw detection, in differentmaterials can be achieved. However, the existing techniques, though more convenient fromthe point of view of portability of the equipment, particularly for gamma ray equipment,have their inherent limitations in flaw detection sensitivity for examination of thicker &thinner sections of materials. Present techniques are suitable for examination of steelequivalent thickness between 15 - 175 mm. Thickness outside this range result in reducedflaw detection sensitivity.

While radioisotopes offer important and unique properties for many applications,they have fixed energies and low intensities, especially when required for examination of

thick structures of steel and high density materials. Such applications include testing ofthick concrete and composite materials made up of concrete, lead and steel. For theseapplications, high energy X- ray emitting sources like linear accelerators, betatrons etc. areneeded. These machines provide high energy X-ray beams with very high radiationintensities, and hence are very useful to inspect high thicknesses (upto 500 mm steelequivalents) in short exposure times. Presently, in India, 9 such accelerators are beingused for various applications.

HIGH-ENERGY RADIGRAPHY
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Basic Principles

Radiography using X-ray energies of 1MeV or greater is commonly considered to be in thehigh energy range. The basic principles of high-energy radiography are the same asthose of conventional low and medium energy X- radiography. Standard types ofcommercial X-ray film, with lead or other intensifying screens, are used to producethe radiographic image of the object being examined. The arrangement of the source,

object and film, the shielding, masking, and other scatter reduction techniques andthe use of penetrameters and identification numbers are all similar to methods usedin Radiography with other energies. The differences between high and conventional low-energy radiography arise from several distinctive characteristics of a high-energy X-raysource, some of which prove to be advantageous.

Advantages

The major advantages of high-energy radiography are:

1. The higher energy photons are more penetrating. Greater penetration means thatRadiography of thick sections is practical and economically feasible.2. Large distance-over-thickness ratios (D/t) can be used with correspondingly lowdistortion.

3. Short exposure times and high production rates are possible.4. The wide thickness latitude, good contrast and reduced amounts of high angle

scatter reaching the film results in high quality radiographs, with excellentpentrarrneter sensitivity and good detail resolution.

5. Some machines have high output intensity, making possible the use of largefocal-film distances, large areas of coverage and greater use of the low speed,fine grained and high contrast films.

Latitude

A common task in high-energy installations is the radiography of objects with varyingshapes and thicknesses. A single film exposure can cover a rang in determining exposuretechniques. Typical broad beam half-value layers are shown in table below.

Material (Density) Typical Half- value layer1MeV 2 MeV 4 MeV 6 MeV 8 MeV 10 MeV 16 MeV

Tungsten(18 g/cm3)HVL (CM)HVL (in)

0.550.21

0.900.36

1.150.45

1.200.48

1.200.48

1.200.48

1.150.45

Lead(11.3g/cm3)

HVL (cm)HVL (in.) 0.750.30 1.250.49 1.600.63 1.700.67 1.700.67 1.700.67 1.650.65

Steel (7.85 g/cm3)HVL (cm3)HVL (in.)

1.600.63

2.000.79

2.501.00

2.801.10

3.001.20

3.201.25

3.301.30

Aluminium (2.70 g/cm3)HVL (cm)HVL (in.)

3.901.50

5.402.10

7.502.90

8.903.50

9.603.80

10.003.90

11.004.30

Concrete (2.35 g/cm3)HVL (cm)HVL (in.)

4.501.80

6.202.40

8.603.40

10.204.00

11.004.30

11.504.50

12.705.00

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Solid Propellant (1.7 g/cm3)HVL (cm)HVL (in.)

6.102.40

8.403.30

11.604.60

13.805.40

14.905.90

16.506.50

20.408.00

Lucite (1.2 g/cm3)HVL (cm)HVL (in.)

10.504.10

12.104.80

16.806.60

19.907.80

21.508.50

23.809.40

29.5011.60

Energy Quality

Since the linear attenuation coefficient and the HVL have definite values for eachmaterial and for each photon energy, these quantities are also used to express thequality of energy, or energy makeup, of the beam from an X-ray generator. Practicalradiography setups use broad beam radiation; that is, scatter is present in theexposure. In that arrangement, the demonstrated or measured HVL thickness at agiven generator energy setting may vary with each setup, depending on the amount ofscatter that the film or detector receives. The slope of the exposure curve and thecontrast and latitude achieved in a step block exposure are indicators of the HVLand the effect of scatter. In high-energy X-radiography, the types and thicknesses ofthe test materials determine to a large extent the generator energy that should be

used. The broad beam HVL is a useful material index for the radiographer to use inthe energy selection, since it is related directly to exposure time. Following figuresillustrate HVL as a function of incident election energy for steel, rocket propellant, leadand concrete. These values represent equilibrium half-value layers.
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HVT vs Energy for various materials

Scatter Radiation

Scatter will be present in every high-energy radiographic application. Because this scattercan be as of useful film densities where sensitivity and inter-pretability are accurateand valid; the thickness range that corresponds to the range of useful densities iscalled the latitude of exposure. Latitude depends on the film gradient, or contrast, andon the attenuation of the material. Naturally, when two films of different speeds areused to image the same object in one exposure, the latitudes of the films aresummed, to expand the total latitude for the exposure.

when a wedge-shaped object is used to generate the exposure curve, the points onthe wedge image, corresponding to the minimum and maximum film densities allowedby the appropriate specifications, will provide data for determining latitude. Followingfigure shows a plot of latitude in steel for several energies.
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Latitude in steel for several energies.

BEAMING AND FIELD FLATNESS

Beaming

In high-energy X-ray machines, electrons reach speeds approaching that of light. Most ofthe electrons continue to travel in the forward direction after their initial interactionswith the target atoms. The deflection angle of scatter tends to be small, and decreasesas the energy of the incident electrons in-creases. Because high-energy interactionsduring electron penetration produce high-energy X-ray photons, the direction of these

emitted photons, like that of the scattered electrons, is also predominantly forward.Thus, the radiation intensity across the X-ray field is not uniform or flat; this effect istermed beaming and it increases with increased energy.

Target Thickness

When targets are slightly thicker than the electron path length of the most energeticelectron, a proportionally larger number of lower energy photons is produced, whencompared to the number of photons produced by a thin target. This thicker targetlessens the beaming effect, broadens field coverage and increases sensitivity ininspection of thin and low-density materials.

Compensators

In the very high-energy linacs and betatrons, the intensity of the X-ray beam is so muchgreater at its centerline than at small angles off-center that a compensator or fieldflattener may be employed to reduce the centerline intensity and produce a moreuniform intensity across the field. These compensators are usually made of aluminum.They are designed to be proportionally thicker in the center of the beam tocompensate for the higher beam intensity of the central ray; they are smoothly

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tapered and become thinner at the edges of the radiation field. As a consequence ofthe differential X-ray attenuation produced by the thickness variations, the resultingbeam profile is flatter. In some machines, the field flattener is located in the beamin such a way that it attenuates the X-rays after the output has been measured bythe ion chamber. In such cases, adjustments must be made to achieve properradiographic exposure.

Field Flattening

For the radiographer who requires a uniform field intensity in order to obtain auniform exposure across the radiograph, beaming can present a problem, and maybecome a controlling factor in applied radiography. As long as exposure times donot become excessively long, a lower X-ray energy source, with its flatter beam canprovide the more uniform field. Increasing the source-to-film distance reduces theeffect of the beaming for a fixed film size. Use of a higher energy, a more powerfulsource with compensator, and a large source-to-film distance, permits the use oflarger film areas, makes it possible to reduce overall inspection time, and in-creasesproduction rates.

The beaming characteristics of an X-ray machine can be useful in some cases. Forexample, when large, solid cylinders are radiographed diametrically, it can beadvantageous to have a more intense beam in the center than at the outer diameterof the cylinder. Large solid-propellant rocket motor radiography is an example of howthe use of optimum energy and field uniformity yields economical inspection. Specialfield flatteners may be constructed for special applications. When large numbers of itemsare radiographed with thickness variations greater than the combined latitudecapability of the ma-chine and film, a specially shaped compensator can beconstructed to flatten the radiographic field. An example of this application is theradiography of large caliber artillery shells.

Radiography of Propellants

Rocket Motors

Solid propellant rocket motors are made with diameters of 5 cm (2 in.) or less to 305cm(120 in.) or more, with a variety of bore configurations, some of which are shown inFigure below. These configurations have a designed burning surface area that produces apredictable pressure/flight curve. If this area is in-creased by the presence of a crack,void or separation, over pressurization of the case can occur, thus causing amalfunction.
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Typical Rocket Motor Configurations:(a) Longitudinal Section; (b) Transverse Sections.

Essentially, a solid propellant rocket motor consists of a rigid case, an internallybonded insulator and liner, and the solid propellant. One or more nozzles at the backend complete the basic motor. The case may be made of wound and epoxy-bonded glassor other fiber material, high strength steel, or titanium. The insulator and liner are oftenmade of an asbestos and rubber composition.

In general, the propellant in large motors is adhesively bonded to the liner toprovide structural sup-port and to restrict the burning to the bore surfaces. There aremany types of propellant; the two most common are a rubbery mixture of an organicfuel/ binder and an oxidizer, and a more rigid double-base compound made withplasticized nitroglycerin. By the nature of their design, solid propellant rocket motorsprovide low subject contrast when radiographed; therefore, every precaution must betaken to increase the radiographic contrast.

Radiography of Explosives

Explosive projectiles and Warheads

Explosives such as projectiles and warheads also require radiographic inspection formanufacturing defects and for defects that occur as a result of storage and handling.Warheads aboard aircraft that are subjected to repeated arrested landings on carriers cansustain substantial forces on crucial suspension and bearing points. Projectiles can becomedamaged as a result of the extreme handling and storage environments to which theymust be subjected in remote sites around the world.

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Complexity

High-energy radiography is used to examine and recertify most of these explosive items. Indoing so, some or all the complexities of the various types of radiography (i.e.,casting, welding and assembly test ing) must be addressed.

Some of these items are manufactured by pressing granulated powder into the containing

vessel. Some, however, are made by casting the explosive compound into the vessel. Inthis case, voids, cracks shrinkage and piping can be present just as in the case of castmetals. Each item has outer metal parts which can be welded, forged or extruded.Additionally, most of these items have using or other types of detonating devices which,must be examined while assembled in the explosive device. Figure below is aradiograph of a projectile showing some of the conditions found in this type ordnancematerial.

Radiograph of an Explosive-loaded, fused 5 in. (12.7cm) Projectile

Radiography of Assemblies

Assemblies such as jet engines, gas turbines valves, nuclear fuel elements and explosivedevices (bombs and fuses) are frequently radiographed with high-energy X-rays to showinternal conditions or dimensions. These assemblies may have material thicknesses thatvary by several HVLs at adjacent regions when projected on the film. Also, manyassemblies can have material and assembly characteristics that produce forward scatter,which obscures the sharpness of the radiographic image. In some instances, such as jetaircraft engines or gas utilized turbines, in-motion radiographic techniques are utilized

to determine dynamic dimensions between mating surfaces, gas seals, etc. Thus it isdifficult to prescribe radiographic techniques that are universally applicable to allassembly radiography, In each case other types of radiography, some experimentalradiographs must be taken before the technique can be finalized.

Radiographic Coverage

Radiography of the peripheral areas of the dome and cylindrical areas of a solid propellantrocket motor requires an exposure plan similar to that for the grain. In fact, the same

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layout and marking may be used for the tangential exposures. The central ray positionsdepend on the size of the motor and on the particular source to be used. When the X-raymachine has high output intensities and a large radiation cone, the grain exposure withsimultaneous bilateral tangential exposures can be made by directing the central rayradially. With a less powerful source or one with a small cone of radiation, only oneside of the motor can be exposed at a time. In general there is no need to use themulti-film technique, since one film may have sufficient latitude to show the criticalareas inside the case. As with all rocket motor radiography, some experimental

exposures may be needed to finalize the optimum technique.

RECENT TRENDS

With rapid developments in newer materials and requirements for their inspectionsat high flaw detection sensitivity, traditional methods are being continuously improved tomake them more rapid and reliable, with enhanced flaw detection sensitivity. There havebeen innovations in radiation sources, radiation detection systems, data processing, imageenhancement techniques, and interpretation methods. New applications include;

Applications of high energy radiation sources like linear accelerators, betatrons etc.for examination of thick welded and cast steel structures, civil engineering concretestructures, rocket propellants, explosives and special materials.

Use of microfocus X-ray systems for examination of thin sections for high resolutionradiography and for geometric enlargement projection radiography.

The newer trends in the use of flash X-ray systems for examination of dynamicsystems in petrochemical industries, ballistics, detonation phenomenon, biomedicalapplications and nuclear technology etc.

Use of new sources like Yb-169, Se-75 and Am-241 for testing of thin sections oflight metals and composites.

Gamma ray scattering NDE techniques for inspection of assemblies with one sideaccess.

Applications of Neutron sources for NDT of explosives, turbine blades, electronicdevices, assemblies and their use in metallurgy and nuclear industry.

Special radiography methods for inspection of radioactive objects and use ofrobotised X-ray systems.

Use of instant cycle radiographic paper in place of X-ray film for recording ofradiographic image for few applications.

DIGITAL RADIOGRAPHY

Digital radiography is an advanced technique which involves computerized methodsof investigation. In digital radiography the image may be directly acquired in digital form orbe converted into by means of digitising of an analogue, copied or transmitted to differentplaces without any loss of image information, digitally processed to enhance requiredfeatures or to eliminate interfering ones. The list of available processing procedures is largeand includes: functional transformations of intensity (brightness-contrast adjustments,

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histogram transformations), filtering of different kinds (noise reduction, sharpening),background linearization and lamination, and finally image segmentation, object detectionand interpretation.

There are several techniques of digital image acquisition:

Scanning of the traditional radiographic film is an obvious way to achieve digitalimage using conventional radiography systems. Because of the high maximum

optical density (Dmax> 5) of NDT films in comparison to films used in visible lightphotography and medical radiography, special scanners are designed for thispurpose. At present, this approach is unsurpassed in spatial resolution and signal-to-noise ratio, but requires film processing and is therefore time consuming and labourintensive.

Phosphor imaging plate technology is a replacement for conventional film whicheliminates necessity of dark room processing. They employ a coating ofphotostimulable storage phosphor on a flexible plate to capture image. Whenexposed to X-rays, radiation sensitive centre inside the phosphor crystals are excitedand electrons are trapped in a semi-stable higher energy state. A reading devicescans the plate by means of a laser beam. The laser energy releases the trapped

electrons, causing visible light to be emitted. This light is registered by aphotomultiplier and converted into a digital bit stream which encodes the digitalimage. After scanning the imaging plate can be erased with surplus light andrefused. The applicable dynamic range of imaging plates is even larger than NDTfilms, but resolution and signal-to-noise ration are inferior.

Fluorescent and scintillation screens coupled with photo diode matrices providemeans for instant detection (indirect flat panel detectors). Because of opticalscattering within the media, some spatial blurring and increased noise canencountered which degrades image quality as compared to film. However, thesesystems offer superior performance relatives to conventional radioscopy systems(image intensifiers or fluoroscopes), while exhibiting faster read-out times ascompared to digitized film and imaging plates.

Most progressive (at present) are direct registration detectors (direct flat paneldetectors). The detector consists of an amorphous selenium or cadmium telluride(CdTe) photoconductive layer coating a thin film transistor (TFT) array, X-rays areconverted directly into charge carriers. An electrical bias field is applied to separatethe charge carriers and to collect them (no photosensitive elements as in the indirectapproach). For such systems the resolution is only limited by elements size of theTFT matrix ( approx 100 m).

INDUSTRIAL COMPUTED TOMOGRAPHY

Although still only at an early stage of development, computed tomography (CT)systems clearly represent a breakthrough in industrial radioisotope and radiationapplications since they provide a range of cross-sectional views through materials,components and assemblies which would otherwise be opaque. CT imaging is anestablished technique in medical diagnostic radiology. Based on the same principle, butsignificantly different in operating parameters, a prototype Computed Tomography System(CITIS) has been indigenously developed using 7 curies of Cs - 137 source in the BARC.The gamma-ray based prototype unit is capable of scanning specimens of small diameters(upto 100 mm) and of varying densities. It has wider applications in the fields of nuclear,space and allied fields. A modified, X -ray based industrial CT system with PIN photo diodedetectors is presently being developed.
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Computed Tomography process uses collection of transmission data through anobject and subsequent mathematical reconstruction of an image corresponding to the crosssection of the object. In NDT, CT technique is used to obtain mapping of linear attenuationcoefficients inside an object. The design envisages high speed computers as an essentialpart of instrumentation for fast data processing and to display CT instrumentation for fastdata processing and to display CT images. The system in addition to ComputedTomography can also be used for Digital Radiography to serve as a powerful tool for NDTapplications. These systems find extensive applications in ndt of solid propellant rocket

motors, nuclear fueled assemblies, composite materials and ceramics. Industrial CTsystems demand capability to handle objects of wide range of density and size and tooperate in varying environmental conditions. The major limitations of ComputedTomography systems are the relatively high cost of equipment and limited throughput.

Applications of CT imaging.

The computed tomographic image is unobscured by other regions of specimen and ishighly sensitive to small density differences between the structures in the specimen. Thisdetection capability to present a density or linear attenuation coefficient map across a slicethrough the specimen enables to visualize many type of structures, flaws, voids &

inclusions, porosity, relative density distribution, residual core material in castings,machining defects etc., and is not restricted by the shape or composition of the objectbeing inspected. This ability to provide spatially specific structure and density informationenables to obtain three dimensional data representation of the physical components forcomputer aided design and engineering analysis. This system, in addition to computedtomographs can produce Digital Radiographs to serve as a powerful tool for ndtapplications.

CONCLUSIONS

The applications of conventional Industrial Radiography are now well established and

practiced by the industry. These applications will continue to expand in manufacturingindustry following substantial developments in supporting technologies, such as smallreliable instrumentation, new radiation detectors and rapid data processing. Thesedevelopments will make the applications more reliable, faster and cheaper. Modern,smaller, light-weight machines producing high-energy and high-intensity X-ray out-puthave eliminated the constraints of the conventional/older machines.

Modern manufacturing technology has presented requirements for radiography of largeassemblies and structures which cannot be moved to inspection facilities and whichcannot be inspected adequately using radioactive sources. High-energy X-raymachines, that can be transported to the test site, have provided a means ofaccomplishing these inspections.

When utilized in real-time radiography, high-energy X-ray machines provide instantimaging of thick, high-density parts. These and other features of high-energyradiography demonstrate the advantages of its use in non-destructive testing andassure continual progress in the applications of the method.

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GS Lecture -1 - 01_07_2013