2.1 SOURCES OF X AND GAMMA Rays AND THEIR INTERACTION WITH MATTER X-rays and Gamma rays are the most commonly used penetrating radiations for industrials radiography, 2.1.1 X-rays X-rays are produced when high-speed electron ike a metal target in a highly evacuated glass enclosure (vacuum = 10°" mm/Hg). A metal filament is scaled inside the enclosure, which is heated by a current of a few amperes to produce clectrons at its surface. At the other end of the glass enclosure, a high atomic number metal target is sealed, on which the fast moving electrons strike. To accelerate the electrons, high voltage of a few thousand volts is applied between the filament (cathode) and the metal target (anode). This arrangement is shown in Fig. 2.1. If the applied voltage is *V” and charge of the electron is ‘e’, the kinetic energy imparted to the electron is *Ve". Ifthe mass of the electrons is ‘m’ and acquired velocity is v, then the kinetic energy of the electron is equal to 4 mv? = Ve. Electrons approaching the target lose their energy in one or more of the following ways: Cathode electrons interact with free electrons of the target atom and, in the process, lose part of their energy, which is converted into heat and X-rays of low frequency. The corresponding wavelength of the emitted X-rays is given by a- —te © eV =V) @High voltage power supply J sot, a a3 ‘lament Ds Focal point Tube envelope Focusing cup X-ray beam, Fig. 2.1 Arrangement for Producing X-rays sbere h = Planck's constant c = Velocity of light (V1) e = Part of the electron’s energy converted into heat or X-rays. (@ Cathode electrons with sufficient energy may reach and be stopped by the heavy nucleus of the target. In the process, the entire energy of the electron is converted into X-rays of wavelength given by: Isa = x = ues [A substituting the value of h, c and e; A is the Angstrom unit fe This wavelength is minimum, corresponding to the maximum energy Ve acquired by the electrons. (a) tt may also happen that the cathode electron knocks out one of the orbital electrons of the target ‘atom and the atom is subsequently returning to its normal energy state when one of the electrons from an outer orbit falls into the vacancy. In this process, the X-rays of a definite wavelength, characteristic of the target material, are emitted. This is called characteristic radiation. These processes occur simultaneously and give rise to a spectrum of X-rays as shown in Fig. 2.2. Effect of Tube Voltage and Current on Intensity of X-rays The X-ray spectrum is significantly influenced by arate change in voltage between electrodes of the X-ray lube, Increased voltage leads to increase in generation of shorter wavelength compared to those that were Present at low voltage. Also, the intensity of the X- "y beam increases significantly and is given by the Telation: Intensity —> Continuous spectrum, ‘ I= kV Fn Wael ‘here K is a constant. : ne Fig. 2.2 Spectrums of X-rays14 Non-Destructive Test and Evaluation of Materials also rent i 5, (Tube current is the current that flows i be current increases. (TU n increases as the tu! ; cures i ee ‘and the anode and should not be con! fused with filament curren which he: between the cat a a ats the trons at its surface.) filament to produce elec! 4 oe, " Figure 33 shows the effect of increased voltage an The intensity J tube current on the X-ray spectrum. High voltage High current Additional wavelength Intensity ——— Intensity Low voltage Wavelength —>- Wavelength —>- (a) Effect of change in voltage (b) Effect of change in tube current Fig. 23 Effect of Change in Voltage and Tube Current on the X-ray Spectrum 2.1.2 Gamma Rays ‘The nucleus of an atom mainly consists of protons and neutrons bound to it. These particles exist in diserete energy levels similar to energy levels of orbital electrons of atoms. The nuclei exist in different nevzy sates. A transition of nuclear energy level from a higher state Eto a lower enerBy level Eis possible. In sucha transition of nuclear energy, gamma rays may be emitted according t0 the relation 7, —E; = hy, where his Planck's constant and Vis the frequency of emitted radiation. Gamma rays are similar to X-rays, except that they are emitted by the nucleus of the atom. Gamma rays consist of discrete wavelengths much shorter than that of X-rays, 2.1.3 Radioactivity The mass of a nucleus, consisting of protons and A-z neutrons, is found to be less than the sum of masses of z protons and A-z neutrons. This difference is called i is / : alled mass defect. i AM, then the energy equivalent of this mass ee E= AMC (where c is the velocity of light), is sai il , the nucleus bound together. ght, is said to be responsible for keeping the constituents of Coulomb forces between them te Pesca kat ibe cieleae i tiem tend to break the nucleus. However, for the nucleus to be stable, it is atomic numbers higher than sone overeome this repulsive fore, Ii found that in elements of repulsive forces are very high and s ‘These clements start disintegratis ry high and such elements le. Fae sineyatng a a stable elements of lower atomic number no longer stabl i clei of high atom pete 7 radioactivity. During the process of disi number, owing to repulsiv. , y- During the process of disintegration, alpha and i een Coulomb force, is called particles and gamma rays are emitted-Radiography 15 ‘Alpha particles (@) are positively-charged particles, with a mass of about four ‘times that of the Hydrogen sgom and carrying Wo units of positive charge They produce fluorescence, can ionize gases and are gasily absorbed by a thin sheet of paper. ‘They are deflected by magnetic fields. eta particles (f) are negatively charged particles, identified with electrons. It is believed that they are created during the radioactive decay process. They ionize gases and are deflected by magnetic fields ina direction opposite to the direction of alpha particles. They are casily absorbed by matter. ‘Gamma rays (7) are uncharged and not affected by magnetic fields. They are highly penetrating rays, emitted in discrete energy levels radioactive Series padioactive elements are divided into (1) naturally occurring radioactive elements and (2) artificially gradveed radioactive isotopes (elements having the same atomic number but different mass number are called isotopes). Naturally occurring radioactive elements decay by and emission. Radioactive isotopes formed by z sequence of transformations, constitute a radioactive series. A natural radioactive series starts with a Tong lived element and ends with a stable isotope. There are three such groups. Thorium series: Starting with o Th®? and ending with .)Pb*™* + Uranium series: Starting with o,U** and ending with gpPb°™ + Actinium series: Starting with U2 and ending with ,.Pb”” Bombarding materials with electrons, protons or neutrons produces artificial radioactive isotopes. Curent, artificial radioactive isotopes are produced by exposing materials contained in aluminum cylinders to a neutron flux in a channel in an atomic reactor core. The nucleus of the substance caprures “ome of the neutrons and is transformed into a radioactive substance. A few examples are: Na + gn! = 5 Na+ 0 no? + gn! — 50 +7 2 ltl! + gn! lt! + 7 Radioactive Decay Statistical studies have shown that radioactive disintegration occurring per unit time is proportional to the total number of radioactive atoms present. Mathematically this is represented by aN a = aw @ where = constant of proportionality N= Number of radioactive atoms present —ve sign indicates decrease in number with time Jf, initially, the number of atoms is No at ¢= 0, then Expression (1) can be expressed as N= Nye 2) This is the basic law of radioactivity transformation. The constant of proportionality 2 is called ioact dN radioactive decay constant. The quantity ea , called activity, decreases exponentially. It is seen from Bln on (2) that the time taken by any radioactive substance to decay completely (ie. N > 0) is finite. This information is of no significance. For this reason we consider “Half-life (T)", which is16 Non-Destructive Test ‘and Evaluation of ‘Materials jefined a nk for half the atom: initially present to under 0 transformation. Thus, afte defined as the ame required for half th ms initially pres derg Sf Sf as half-life we have ne 9.693 N 2 yew or 2= 2 1 calle the mean Tie ofa radioactive ator © paatelite (1) = 0.693 x mean life a / : From the viewpoint of industrial radiography, half-life is a convenient measure of the useful period for which a radioactive isotope can be used economically. radioactive substance is Curie, which is Units of Radiation ngth of a 10 disintegration per second. Curie is a The standard unit employed for measuring the stret defined as the quantity of radioactive material giving 3.7 x 10! large unit: therefore milli-curie and micro-curie are also used as units. 1 Curie = 1,000 milli-curie 1 milli curie = 1000 micro-curie The SI unit of radioactivity is Beequerel, whi 1 Becquerel = 1 disintegration/second 1 Curie = 3.7 x 10! Becquerels Another unit of radioactivity is Rutherford, which is defined as the amount of radioactive substance ‘ing out 10° disintegrations/second. ich is equal to 1 disintegration per second. Specific Activity Specific activity of a radioactive substance is defined as its activity per unit weight. It is expressed in Curie or milli-curie/gram. The specific activity of an irradiated substance increases with time of irradiation in the reactor. In practice, specific activity is found to be affected by: © Impurity of irradiated material © Variation in neutron flux 4 Loss of material due to conversion into radioactive material amma ray sources having high specific activity have ical di ama ray ‘hav small phy i f absorption. This aspect is significant from the radiographic foo an Season a Radioactive Isotopes for Industrial Radiography Gamma 1 ; nae risen rears radioactive materials like radium and radon were used for ‘nicrpiy aa bon widely wed fr instal radiogane The waite ecltoals avilable, Garmm™ . . rac . age. . eee BS radiography are Cobalt-60, Caesium-134, tography. The major artificially produced isotopes ; Yiterbium 169 and Tantalunael 82, |, Caesium-137, Iridium-192, Thulium-170, Sodiu™ Gamma rays emit discrete w: ion, with their own characteristic energy. The enereY avelength: ji it gy. ; Ct igths of radiati i istic pectra of some isotopes used for radiography are given in Fi S A, ee : ig. 2.4,Radiography 17 1 T ' 1 Thullum 170 r y oak 4 YT Terblum 169 oa 4 oot 3 oot} 4 oooiL_Lu_1__} WOO oot an oof 1 10 1 T ' 7 T—1 L Iridium 192 on oat 4 = 001r L 4 < oot a E oot oooh ene ‘oof 61 (10 z g 4 4 3 T T —T s Caesium 134 Cobalt 60 ZB oth 4 omy oot 4 oot 0-001 ad ey 107 tS Photon energy (MeV) 1 Sodium 24 of oo}| 1 |__ 000 On 10 Fig. 24 Gamma Ray Enersy Spectra Table 2.1 gives the mean energy, half-life and thickness/penetration of isotopes mentioned.Destructive Test and Evaluation of Materials 18 Nom TABLE 2.1 Half-life, mean energy and penetration in steel Isotopes Half-life ‘Mean Energy (MeV) Penetration in Steel (mm) "Co 5.26 yrs 14 230 IMCs 2.10 yrs 0.8 ICs 30 yrs 0.66 5 1p 74 days 0.5 75 VT 127 days 0.084 125 187 120 days 12 230 2Na 15 hrs 14 7 et) 31 days 0.15 3 Units of X-ray and Gamma Ray Measurement X-ray and gamma ray energy is usually expressed in ergs oF electron volts (eV). [1 eV = 1.6 x 10"? ergs}. From a practical point of view, X-rays or gamma rays energy is evaluated by the effect they produce after undergoing absorption in matter. The physical or biological effects produced by X-ray or Gamma ray absorption is based on the capability of these rays to produce ionization in materials. The unit for their quantitative measurement is based on their absorption and associated ionization in a standard substance. The standard substance selected is Icc of air at normal temperature and pressure (NTP). sev tis, the unit ‘Roentgen’ is defined as the quantity of X or gamma radiation thet ionizes 1 cc of din ergs, this is equivaleat air at NTP, to produce 1 electrostatic unit of electricity of either sign. Expresse to 87.7 ergs X and gamma rays are electromagnetic radiations and ‘Roentgen’ expresses the effect of absorption of these radiations. It is desirable to define units of measurement of all types of radiations that are absorbed by all types of material mediums and not only by air. In view of this, the following units are defined for a wide range of applications: REP (Roentgen equivalent physical) RAD (Radiation absorbed dose) RBE (Relative biological effectiveness) REM (Roentgen Equivalent man) REP is defined as that quantity of radiation that produces energy absorption of 94 ergs in soft tissue. Here, soft tissue is used asthe absorbing substance instead of | cc of air. The average energy for producing ion pair in tissue is 32.5 eV. __ RAD is defined as energy absomption of 100 ergs per unit mass of irradiated material at the location of niet aaa serie ‘rads’ is used as the unit of measurement, the material exposed also must be mento a Be ea 4 Heil Ch ea radi i ions does not produce the same biological effect. Therefore, eae A a factor of equivalence, The ‘Relative Biological Equivalent’ factors are eee Ser peau by irradiation with 250 KV X-rays to produce certain biological! ee pet o F eee biological effect. Table 2.2 gives biological effect oFRadiography 19 ABLE 2.2 Biological effect of ionizing radiation* Biological Effect Whole body irradiation (Blood forming organs) Remarks An exposure of 50-100 REM may lead to fatigue, diarrhea, nausea and death. The effect may develop within hours, days or week, depending on dose of exposure. Longer the dose, sooner a given effect occurs. The effect of radiation is delayed. It may be induced when dose exceeds approximately 200-300 REM. It may take months or years to appear. | 3, Cataract formation Studies show that there is potential risk associated with high dose exposure of| radiation. Radiation induced cancer may take years to appear. | 3. Cancer 7 Based on the studies of the National Academy of Sciences Committee on Biological Effects of Ionizing Radiation (BEIR) REM is defined as that amount of radiation that produces the same biological effect as one Roentgen of X and gamma radiation. Properties of X and Gamma Rays X-rays and gamma rays are a part of electromagnetic radiation. Figure 2.5 shows the electromagnetic spectrum. 3 2 2 ef 8 8 8 3 SB oI 2 2 S 68 < 8 3 3 x x x ke *& 6 ss = | 2 g = = =>] Radio L— Cosmic |«—___>| ‘SW. Gamma Soft Solar lo— Radio —=} ys eH e ‘communication ae 7"3e }<—— Hertzian waves ——*1 a x-ays—+} | 3 intrarea ~ Fig. 2.5 Electromagnetic Spectrum Chart Summary of the properties of X-ray and gamma rays: * Invisible, pass through space without transference of matter Not affected by electric and magnetic fields ; ; Propagate in a straight line, also exhibit wave properties and are reflected, refracted, diffracted and polarized, . , 10 om Transverse electromagnetic waves, velocity of propagation = 3x lo! cm/sec 5 Capable of ionizing gases and changing the electrical properties of liquids and solids Capable of blackening photographic film Produce fluorescence and phosphorescence in some substances Damage or kill living cells and produce genetic mutation a Liberate photoelectrons, recoil electrons, electron-positron pair and act photochemically Differentially absorbed by matter Produce characteristic spectra of chemical elements .20 Non-Destructive Test and Evaluation of Materials: i a it i ‘levant: far as industrial radiography is concemed, the following properties are rel Insofar as i © Rectilinear propagation © Differential absorption © Photographic effect and «Fluorescence effect Interaction of X-rays and Gamma Rays with Matter Penetrating radiation like X-rays or gamma rays passing through a material medium interact with matter in-a complex manner, The effect of interaction is attenuation of incident radiation. Attenuation takes place in two ways—absorption and scattering. Figure 2.6 shows complex interaction. Incident radiation “pt Material medium Pair production Heat radiation ‘Tronemited radiation i Compton Photoel . Unmodtied ompl eo) ‘Compton, electrons modified (incoherent) Fig. 2.6 Interaction of Radiation with Matter Attenuation resulti i ing from various interactions i io a ; 1,= Iye™. Wis more convenient to express nang er eee Mathematically as: ‘This relationship is expressed as: ress attenuation in terms of per gram of material irradiated-Radiography 21 “ }m = he le where /p = Intensity of incident radiation, J, = Reduced intensity of the emergent radiation after traversing a distance X, 11= Acconstant called linear absorption coefficient, which indicates decrease in intensity per unit length of the material traversed, p= Density of the material and E-| = Mass absorption coefficient Pp The absorption coefficient remains constant for mono-energetic radiation like gamma rays for a given material: however, it is not so for a spectrum of radiation. zis small for radiation of small wavelength (hard radiation) and large for long wavelength (soft radiation). For chemical compounds or mechanical mixtures (solids or liquids) it is expressed as: (t)-(2]e=(B]+ where a, and a, are the weight fractions of constituents 1, 2, ... Major factors for attenuation of incident X-rays or gamma rays are: «Photoelectric emission © Compton scattering «Pair production From the viewpoint of industrial radiography, itis of interest to know that: «Photoelectric emission dominates in the operating voltage range of 200 eV to 100 KeV (low energy range) Compton scattering dominates in the operating voltage ran; energy range) «At higher energy levels, Compton scattering slowly decreases and pair production sets in and dominates above 4 MeV (high-energy range) ge of 150 KeV to 300 MeV (medium 2.2 EQuipMeNT ————______ 2.2.1 X-ray Equipment X-ray radiographic equipment consists of: © X-ray tube * Arrangement to heat the tube filament to produce electrons © Arrangement to accelerate electrons to generate high impact energy * Accessories 10 rectify, regulate and measure current and voltage andl provision to measure X-ray exposure As mentioned earlier, X-rays are produced inside a highly evacuated glass envelope called an X-ray lube. Features ofthe X-ray tube are shown in Fig. 2.1. The power supply for heating the filament of the2 Non-Destruct¥ est and Bratton Materials ensions transformer The filament is heated by 0 CUrTENE OF a fey Xeray tube 1S at = s-S amp! Volts. oe e Trote ada wece tere the electrons produced on the eathog i lor k ing (unrectificd) power {9 supplies alternati PM puaity and quantity of X-ray outpy Mtems for low energy portable field fixed/mobile units, 4 control unil xd to improve th rectifier 8} plied te ts them to the stirmer. Revtifi X-ray units netant potential units f ray units are cl target surtte fers are U either halfawave for medium energy’ he sified as: Most of the it ' nits, oF fallow avy-duties ‘Commercial © cathode grounded | Unipolar Anode grounded «Center tap grounded = Bipolar snipolar and bipolar units. ave 60 ty available illustrates U Cathode LT. Transformer Anode Grounded IIIf > ent HT Transformer Cathode grounded unit (unipolar unit) a I= ‘ap grounded * ‘Anode grounded unit (unipotar unit) Cent ter tap grounded unit (bipolar unit r , ig. 2.7 Unipolar and Bipolar Units Unipolar units i operate in the ip range of 50-150 KV with tube operate between 200-400 KY, with tube ree i030 an rofS1S ipol rau mA. Bipolar units usually eed of X-ray Units -ray generation is “ny generation i a inefficient proc unit ° i¢ Input ener, a ted into X-ray Fa icone : depends on the design and conatiet be converted into Xenys as seen, ove Pn t es a on ofthe X-ray tube, foal - output oF X-rays from an xay , focal spot size, type of rectifi . Z rectifier, inherent filtration and mbit of the high voltage transformer. Table 2.3 gives a so SSi0s 8s voltage out idea of from commercial X-ray units, ie ig Table .3 gives X-ray 01| Radiography 23 TABLE 2.3 X-ray output from commercial X-ray units Maximum Focal Spot Size Tube Current X-ray Output KV (mm) (mA) Roentgens/min at | M 200 2.3%2.3 5 4 250 5x5 5 20 300 2.323 5 10 350 4x4 8 40 | i 400 1x6 5 25 ‘As mentioned earlier, most of the input electrical energy is converted into heat. The heat generated at the target surface is so high that it may melt the target ‘unless it is quickly conducted away. For this in Jon special arrangements are made for dissipating heat, The most ‘commonly used systems for heat dissipation are: + Circulation of oil behind the target and into the heat exchanger «Circulation of water in the tube target extension Figure 2.8 illustrates such cooling systems. The effectiveness of cooling system deter- liquid mines the extent to which an X-ray unit can be } operated continuously. S Milli 2.2.2 Selection of X-ray Units a Practical needs of radiography demand that the Air eooting T Co a following points be given due consideration before selecting a radiographic equipment: Fig. 2.8 Usual Methods of Cooling the Tube Target «© The degree of penetration must be sufficient to accommodate maximum thickness of material encountered + The exposure time must be short in order to cope with the volume of work The duty cycle (which indicates the extent to which a unit can be operated continuously) must be high enough to keep pace with the speed of production and inspection «The image quality must be such that all small flaws of interest are revealed, irrespective of their location in the specimen. Smaller focal size gives better image quality © The manoeuverability—for ease of movement and setting up of exposure © The reliability of operation X-ray Fluoroscopic Equipment When an X-ray impinges on a fluorescent surface like zine: et This fact is made use of in fluorescent X-ray radiography * A.50-300 KV range X-ray unit A radiation leak-proof enclosure audmium sulphide, it produces yellow-green The fluorescent radiographic unit consists24 Non-Destructive Test and Evaluation of Materials © A suitable window fitted with a fluorescent screen for viewing the radiographic image ; Amechanism for manoeuvering the test object position with respect to the X-ray beam ; The arrangement of a fluoroscopic unit is illustrated in Fig. 2.9. X-rays are differentially absorbed while traversing the object and the emergent beam forms an image of varying brightness on the fluorescent screen. The Fig. 2.9 X-ray Fluoroscopic Unit image is observed ina semi-dark enclosure. The quality of fluorescent pictures is generally poor compared to the quality of pictures obtained in film radiography. Usually a wire type penetrameter sensitivity of 5—10% is achieved as against a normal wire type penetrameter sensitivity of 2% in case of film radiography. The quality of a fluorescent radiographic image is improved by use of image Fluorescent Photo cathode intensifiers. The principle of an image Object. Screen Observation intensifier is illustrated in Fig. 2.10. Photo a An image intensifier system has a photo electrons cathode layer next to a fluorescent screen | mounted on an aluminum support and the ays \ assembly is sealed in an evacuated glass enclosure. The X-rays, emerging from the UO Glass envelope object, fall on the fluorescent screen and form L support 30 kV ible image of the object. The light from ai the fluorescent screen falls on the photo cathode layer and causes the emission of clectrons. The number of emitted electrons is proportional to the intensity distribution of the X-2Y lectrons are then accelerated under a p otential difference of about 30 KV and focused electt™ Statically on a fluorescent screen. The brightness of the image on this screen is increased to about 1000 times compared to a conventional fluoroscopic image, The intensified image is observed through a System, The improved sensitivity of the radiographic image is almost comparable to film radiograph Sensitivity. Figure 2.11 gives a comparative idea of improvement-of penetrameter sensitivity as a rst! of image intensification, Fluoroscopie X-ray units with ima time radiography. Figure 2.12 illuste X-rays after undergoing different X-ray beam Lead shield AO__» as Observer Fig. 2.10 Image Intensifier ge intensification are used in close circuit X-ray television for e! ‘ates the basic principle of the system. He aay Ing differential absorption in the object are converted in to visible light bY = Heed ee creams Where a radiographic image of the object is formed. This image is collected bY optical lens system and focused on a TV camera tube, ‘The information contained in the fluo image of the object is converted in to electrical signals, The electrical signals are suitably amplified a™? converted from analogue to digital image, which is Stored, enhanced and viewed on TV monitor. ae5 10 4 Fi SCE a aaa : = i af Bol F S| image intonsifier &1 & ap Image intensifier £ eee een g [secefaaer [oun 50453045 ~—«G0 SCO 0 2030-0 —= Aluminum (mm) Steel (mm) Fig. 211 Relative Sensitivity in Aluminum and Steel after Image Intensification Focusing lens system peer f Viewing TV camera system ; tube Fluoroscopic screen Fig. 2.12 Principle of Closed Circuit X-ray TV System ‘ource, object and image plane is similar to film radiography. Radiographic image is converted in to a digital image through image intensifier, optical lens “stem between intensifier and video camera. The analogue signal from video camera is digitized, Stored, enhanced and displayed on image monitor. Figure 2.12a shows schematic arrangement of the system The system worke in the range of 30 KV tc 300 KV. Real time radiographic systems are widely used in Aerospace, Pressure vessel, ‘Automotive, Electronic industries, ete. in real time, radiography arrangement of the s image ‘Analog to digital Xray tube intenatior | image convertor Focusing mage Test object fons system / TV | monitor ' camera y sie Platform for movement wang an Fotaton of te object int af Real Time Radiography Fig. 2.128 Schematic Arrang.ab NonDesirucnive Test and Evaluation of Materials vas advantage over film exdiography in following respects can be viewed simultancously as the X-rays pass hrough the test High sper tilted or moved, Entire object can be inspected in one set up, ajc a a pry is generally ess sensitive than film radiography. However ea ine ra olution is influenced by focal size, magnification and performance ree aaa ict 0 resolve small defects due to limitations of image intensifiers. Image of imaging eve resolution considerably. Normally itis possible to achieve a resolution of 0.1 p nos in aim radiography, in real time radiography with image proc ing, it could be oe to 0.25, (mmetsensivtyo 192 percent iseomnon in film radiography. In conventional ea time radiography, ia percent, With mage resin texniques and nero fol system contrast sensitivity can be improve 100.510 percent, Image quality indicators (wire or plaque type) are used in X-ray radiography, ir rcal time radiography line pair gauge is used. This shows how many lines can be discerned in a width ‘one millimeter. Resolving eapacity is indicated as “lines per mm’. ; ‘The system has advantage of presenting clear image at low intensity. The image can be shown as cither postive or negative. One or more TV receivers can be arranged in different locations for several viewers to observe and interpret the fluoroscopic image. Defects usually detected are: cracks, voids, misalignments, lack of fusion/penetration, slag inclusions, rinkages, forging bursts, laps, cold shuts, ete. Real time mudiographie system d ed of inspection, image abject ean be rotate scams, § High-energy X-ray Equipment High-energy X-ray units basically consist of a source of electrons, which is a hot filament, and a means to produce a high electric field to accelerate the electrons. These electrons acquire high energy and are finally made to give up their energy by striking a metal target, where X-rays are produced, Some of the high-energy X-ray units are given in Table 2.4. Efficiency of X-ray production at higher energies of electrons is approximately 30-40% as against 1~3% for X-ray units in the 100-300 KV range. Further, a small diameter of X-ray beam gives rise to fine focus (~0.1-0.25 mm). This ensures extreme image sharpness, This helps direct enlargement of the radiographic image of defects. Scattered Radiation, Filters and Screens An object in the path of X-rays or gamma rays gives rise to scattered radiation. This may be due to: * Secondary radiation produced as a result of interaction of the object with impinging radiation. Reflection of radiation from surrounding objects like nearby materials, floor walls, film cassettes, ete. Figure 2.13 illustrates this, TABLE 2.4 High-energy X-ray unit High-energy X-ray unit Range of Energy Generated _ Mande Graf generator (electrostatic generator) 6 MeV 5=100 MeV 6-50 MeV ee OL ea 4 MeV.Radiography 27 Wa Source ’ ._Blaphragm — =i ; ‘Ue Primary Srclation Undercut Fig. 2.13 Effect of Scattering on Image Quality Scattered radiation has low energy and less penetration compared to the primary beam. This makes radiographs hazy, with overall fogging, leading to poor image quality. In radiography of thicker objects, the amount of scattered radiation is significantly more intense than the primary radiation reaching the film. Most of the secondary radiation is generated within the specimen and is referred to as internal scatter, forward scatter or transmitted scatter. The undesired effect of scattered radiation on the radiographic image is minimized by keeping a thin lead foil between the object and the film and by collimating the radiation to the area of interest by using a lead diaphragm at the tube port. Another way is to cover the unwanted area of the film by lead sheets. Scattered radiation from all sources can be avoided by sandwiching the film between a pair of lead screens. Metallic sheets of high atomic number are used as filters to absorb ‘soft’ radiation (long wavelength) emanating from the tube port and allow comparatively hard radiation (short wavelength) to penetrate the specimen, The use of filters results in the following advantages: * Increased contrast around the specimen edge * Reduced undercut scatter at the edge of thinner sections * Record wide range of specimen thickness * Radiograph specimens with advantage wI complicated geometry Loss of intensity caused by the addition of filters is compensated either by increasing the time of €xposure or the KV. 1aGtely filters are made of aluminum, copper or lead A guideline for the use of filters is given in le 2.5, here satisfactory masking cannot be done due toian ey —— jot improve radiographic quality lity for specimens above 40 mm filter of thickness 3.0 mms thick steel, a lead filter of range. the use ofa filter at the tube window does n' erwean the film and the specimen improve image culty do not improve ality. A le: isthe a0 Prove esteel. Above 100 mm it the tube window to allow only ec improves image quality. | ae mm thickness a} ttering from walls or surrounding to use a diaphragm 0} 25 mm ms to reach the area of interest. This avoids heavy scal sure room. It is important useful primary beat objects in the expos posed to radiation, absorbs only a smal] amount of radiation. In nance the photographic effect, screens are used in combination | foil screens and fluorescent salt screens. screens, consisting of lead foil mounted on cardboard or them. One of the screens facing the Jhing the film between screen—which is placed behind the film. ‘Screens Radiographic film em order to fully utilize th ith films. Screens are of two types: meta etal foil screens are usually lead foil plastic. These are used in pairs BY sandwicl nce side is called the front screen and the other, the bac] ‘The main advantages of using lead screens are: hie aetion on the film by emission of photoelectrons and Enhancement of photograp! radiations generated in the lead «Soft seattered radiation is absorbed more than the primary radiation . Photerphic effect of the primary radiation is intensified at higher energies a : fs a radiation is reduced, thereby producing greater contrast and «Higher intensification reduces exposure time yulsion, when €X] .¢ radiation and ¢ secondary Besides lead intensifying screens, eens , other met s sess et inensing sees ol tal screens such as copper, tantalum and tungsten 5° ‘Table 2.6 gives screen materials and thickness of rae Lickn iat paul ate ss of front and back screens for various radiations: ying screen or salt screens Fluo i : of sponded orscnt materi such as valcinm tungeita or bovis led sul sa ufo Iyer sad with w suitable inde 0 "tungstate or barium lead sulfate, of which 9M meals vb ot wavilt bight when ERE past spp. The Ta ia avenge ght when exposed to X-rays or gamma rays i of cease cai ase eee dnadvantage of ust Te iene Ww lead intensifying steens. However the in time since the intensification fact every gh comes eo e ensifi . a : thc incnilleaion factor daies both ot love meas ve lower as W'Radingzaphy 2 TABLE 2.6 Screen material and recommended thickness Screen Material | Front Screen Thickness (mm) | Back Screen Thickness (mm) KV Lead =e 02. 04 730-250 KV_ | Lead 0.05-0.1 eri 750-400 KV Lead O1 Ol | mo KV Lead 15-20 10 5-10 MeV. Copper 1.5-2.0 15-20 | 15.30MeV | Tantalum/Tungsten Non | Gamma rays | Ther Lead 0.1 01-015 | C537 Lead 01 0.1-0.15 | | Cow Copper* 0.5-2.0 0.25-1.0 | * Copper screens produce better radiographs than lead screens but require a longer exposure approximately by a factor of 2. high energies. The use of these screens is limited in the field of industrial radiography because they produce poor definition and grainy images. Fluorescent screens are not used in gamma radiography, as intensification is poor with long exposures. usually common in gamma ray radiography. Foreign particles—dirt, dust, etc—should be prevented from accumulating between the screen and the film, as they tend to cast their shadow on the film. Further, the screen should not be exposed to the primary beam and should be stored away from chemicals and other sources of contamination. 2.2.3 Gamma Ray Equipment Gamma ray equipment consists of an isotopic source pencil and a container for positioning the source Pencil. Figure 2.14 shows the arrangement. x a Container is designed for safe handling, want Sues and for easy portability. The igs Sources are usually inthe form of @ aa one diameter equal to its length. These alanine ted ina container made of steel, diminam £f 2 alloy of magnesium and 4 mm whe size ofthe source ranges from Kaori tama 2 6 mm. The size of most of the 'Y Sources for routine radiography Lead plug Source capsule Stainless steel 2.14 Gamma Ray Source Pencil30 Now Deseret Test and Bratation ef Materials x 3mm. This size is comparable to the effective focal spot size of an X-ray tube, rained in the sonrce peri, made of stainless steel, The source pencil is placed ey the ether end is threaded for positioning the pencil inside the container is approximately The sourve capsule is com atone end of the container swith the help of a manipulator rod ON arenes ea rantion is allowed to be transmitted through the source pencil CO eMart source po should he coroson resistant and should nt react cheat with the maiation source (ii) The source peneil should be leak-proof (iv) The sourve pencil should be as small as possible a («) All radiation sourees should be sealed and marked for easy identification Sourve camera: The radiographic container (camera) is designed in such a way that there is no leakage of radiation above the permissible level when not in use. The camera is usually made of lead or depleted uranium. Depleted uranium makes it lighter, Further, the camera is designed to facilitate casy loading, exchange of the source and easy opening and closing of shutters, There are four types of gamma ray cameras as shown in Figs 2.15 to 2.18, Some cameras have the mechanism of lifting the source to position it as required, in the case of radiography of pipeline welding. Here, the source is attached to the lead backing, which in tum is connected to the camera-opening handle. A low strength source is used in this camera. The shutter is opened from the rear with the help of a hinge mechanism, It can be positioned for panoramic exposure with the help of a long handle. For panoramic exposure, source strength is not more than $ Curie of Ie!%2 Fig. 2.15 Gamma Ray Camera with Conical Shutter High-strength sources are kept in stationary cameras, which have a remote control mechanism for rotating and Source [source bringing out the source to the surface of XZ the camera for exposure. High energy, high strength sources employ cameras with source attached to Fig. 2.16 Gamma Ray Camera with Source a Aexibe cable. In storage, the source is Rotating Mechanism in the container, The ce capsule rere cance be extended to the required position through an extension manually oreleeically. The eres tached 10 a control box. This cable can be controled eithe Seen sion rod runs to a length of 20-25 meters. The flexible cable operation Hdling of the source with adequate distance shielding,Radiography 31 Control unit Lead shield Flexible cable source fifting mechanism. ‘Source gig, 217 Gamma Rav Camera Lifting ‘Mechanism Fig. 2.18 Gamma Ray Camera with Remote Handling Mechanism syith Sowree 2.3 GENERAL RADIOGRAPHIC ProceDuRE Radiography is essentially a technique of projecting a three-dimensional object on a plane, utilizing a few ihe properties of X-rays, gamma rays or any other penetration radiation. The properties used are: + Rectilinear propagation + Differential absorption «Photographic or fluorescence effects The projected image of the object is called a ‘radiograph’ and the process of obtaining the radiographic image and evaluating its contents is called ‘radiography”. The essential requirements for producing a radiograph are: « Asource of radiation + Object to be examined + Recording medium and «Processing chemicals In this section, the radiographic process using X-rays and gamma rays as the source of radiation and films as the recording medium is discussed. Irrespective of the type of component/assembly to be radiographed, the following steps are followed uring radiography: + Surface preparation + Selection of radiation source depending on density and thickness of the object + Optimizing exposure parameters and usage of exposure charts * Selection and processing of film + Ensuring appropriate radiographic sensitivity by using Image Quality Indicators iter) * Keeping image unsharpness to as low a value as possible 2.3.1 Surface Preparation Gross imperfection ofthe surface needs 10 be removed, a8 it creates confusion in the intepretation of intemal defects, In ease of castings, it is necessary 10 remove gates, risers, deep (etling marks and Te Sand fromm the surface, In ease of welding, deep surface cavities, slug and any other surface casi es that may hamper interpretation of internal defect hould be removed. The surface of raw i or welding need not have smooth surface finish. Sand particles adhering to the surface andPee oe 65 safety Aspects of Gamma Ray Cameras Radiography 63 sinon sources emit radiation continuously. Therefore, every precaution needs to be taken to safe storage SO that leakage radiation does not exceed the maximum permissible level. As the He mee strength increase the thickness of the container material also increases. It is necessary car nett the following: oo pose HE Tance of $ em from the surfice of the camera, the maximum radiation level should not (0 ney 100 mifhe; the average radiation level should not exceed 20 mafhr ee fistance of | meter from the source, the maximum radiation level should not exceed 10 mifhr; (i) Ma erage radiation Fevel shoul not exceed 1 merhr gi Lvking devices should be provided for eamers so that the camera canbe switched ‘on* and Hy only by authorized persons, The camera should be identified conspicuously with the nature and strength of radiation sources together with the radiation warning signal ja) The gamma camera when notin use, should be sored in a safe place in an isolated room or ina pit inside the exposure room with a radiation warning symbol at the entrance of the storage area. The maximum radiation leakage around such an area should not exceed 0.25 mr/hr (0) During transportation, radiation sources must be sealed in accordance with the relevant radiation regulation of the country 2.6.6 Protection Measures Against X-rays Indoor Radiography Tie walls of the exposure room should be of adequate thickness so that the radiation intensity outside theexposure room is well within the permissible limits. The doors of the exposure room should be lined with adequate thickness of lead sheets with proper overlapping. Interlock on the exposure room doors, alarm and warning lights should be connected through the control panel. The mechanism should be ‘0 ranged that if the exposure room door is opened inadvertently during exposure, the equipment is awonatically switched off and cannot be switched on while the door is open. The control panel should belocated in a separate room adjacent to the exposure room. The cable connecting the tube head and the contol panel should pass through a small duct at the floor level, at an angle to the wall thickness. Such tus should be lined and both the openings should be covered with lead sheets. Outdoor Radiography The out Moved loved to area around the X-ray unit is cordoned off and the area is radiation surveyed. No-one is 0 eter the unsafe area. Waming signals are kept around the cordoned-off area. may — to protection for radiographers, it is necessary to take steps to protect other personnel Tht S268 fo the radiographic site, Field radiography hardly evet eliminates exposure aos teed gy Posute should be planned and monitored. Survey meters and personnel dosimeters shoul ‘onitoring devices. p “sonnel Record s radiation workers. Durivg medical examination ot 1 meme utderg0 a medical examination before being employed as "ach person should undergo annual medical examination, Reports of the Ke. .i) ‘Cumulative dose for five-year block : 100 mSv 3) Anmual effective dose in any calendar year during fives, ° block not to execed 30 mSv ki car (iy Cumulative mfective dose exceeding 20 mSV to be invey ; Dose limit 2 mSv to the surface of women’s abdomen Tro" ofthe limit prescribed for radiation worker 2.6.2 Radiation Effect al effects caused by overdose of radiation, somatic effects and genetic two types of biologic area : The soma fle sow ihe physical effect on the body of the individual who receives radian Damage to blood cells, skin cells, tissue cells, etc. are considered somatic effects. Genetic effects are those that can be passed on to the next generation or to later descendents as hereditary characteristics, The exact nature of the effects of radiation on the future generation is still unknown, although cases of genetic effects have been observed on some animals. “The effects of overdose of radiation exposure are not necessarily detectable immediately after exposure. They are noticed after some delay. To avoid the ill effects of radiation exposure, it is necessary to keep the dosage within prescribed limits or less, by observing good safety practices. 2.6.3 Radiation Measuring Devices Some typical devices used for the measurement of radiation are: Ionization Chamber, Geiger Counte Proportional Counter, Scintillation Counter and Photographic Film. meperiecs el for monioring radiation areas are portable units to measure radiation levels at differs locaton in the vicinity of radiation sources. This is useful for immediate assessment of adequaty nee fon Prosection. Radiation survey meters, which are Ionization Chamber type of pe, are used to measure doses in terms of milli-Roentgen (mr) per hour at any ds0% from radiation sources. Readi i . Readings ranging from i ae area. These are pocket dosimeter, which gives © radiation exposure of personnel working it * oe radiation and films, wl cased gives an instant indication of accumulated dose of X oF ee Sed ina metal holder. The film holder is worn on the outer eothing gy be se falling i nergy bete radiation and thermal neutron falling on it. It is used to detect gamma rays, Fea Tinto each he fin eas. The metal holder contains an open window to alle evaluate X-ray exposure below 250 KV and evaluate gamma radiation, two copper filles 5 and the other to eva r so KV sa valuate & e above 250 KV uate thermal neutrons, a a The Or a peric Ca fe ij petiod of «fortnight or a month, it is removed and develoP™™ i sured density with a set of pre-est se. Figure 2.33 shows a film badge-Radiography 64 Plastic Plastic Fig. 2.33 Film Badge ‘The advantage ofa film badge is that it provides a permanent record of an individual's cumulative eggs 2.6.4 Radiation Protection ‘Tyee basic means are used as a protective measures to keep radiation exposure within allowable limits: time, distance and shielding. Exposure received by a person is directly proportional to the length of time peau inthe radiation area. The radiation exposure dose rate at any distance from the radiation source iscalculated using the standard dose rate of each isotope and radiation output of X-ray units. The wually expressed in terms of RHM value, the radiation level in sandard dose rate for an isotope is ust Roentgen/hr at 1 meter from 1 Curie of a gamma ray emitting isotope. ‘These values are fixed for each itp. Table 2.1 gives the RHM values of some important gamma ray isotopes. For X-ray sources, the radiation output depends on the KY, the tube current, the focal spot size, the spe of rectification and the amount of filtration. Usually, suppliers of X-ray units provide radiation cutput data for each X-ray unit. It is expressed as ‘Roentgen per minute at one meter from the X-ray source, TABLE 2.21 RHM value of isotopes [RHM Value (/hr/ci at 1 meter) Isotope cel 13 tr-192 05 Cs-137 ee 0.825 a ae ny the source. Therefore, the square of the distance ie : the radiation exposure. he c For radiation in PEN SPCR. Taw for calculating prot oncept of the inverse For closed enclosures, ection for closed The radiats treaye lation in er th tensity varies inversely as the s di et aw oe from the source, the lower ‘Sed ition protection is valid only f lose tion limits the validity of the inverse S88"ae Radiogrupny . . fet Linatons of ive in 2 ERAN) Cc esfetiveness and tive testing is most effective in 4 PF oF R22. S, ere mensional defects and the assembly of componer™ de sch method of non-destructive (TT vveasfully in detecting . A | ents fas been used successfull thods of testing, Table >) 3 gives the areas of efectiveapplicns areas normally nace jmportant to note that 1oeation, assembly condition, working environm, ». Here, it 1S . . of radiography ‘nd understanding, and the technique used contribute significantly to the efectvenes radiographer’s skill a of radiography TABLE 213. Effectiveness of radiography Limits of Detection Nature of Product Effective Detection (Approximate linear size) ; Condition | Limit of Detection (mm) J Gagors, castings. weldments | Cavities, porosity, inclusions, Laboratory 05 | - shrinkage, hot tears, cold shuts, | segregation, cracks (not very tigh!) Production 20 | Assembly Corrosion pits, cracks, entrapment | of materials, misalignments Service/field 5.0 | 2.4 RADIOGRAPHIC TECHNIQUE AND ACCEPTANCE STANDARD —————————_—______ In industries, one encounters a wide range of conditions in regard to component size, shape = composition. The objective of radiography is ‘to examine and evaluate these components as clearly 2 possible. This requires standardizing the most efficient way of projecting the object, depending on i shape. size, thickness and composition. The major stages involved in establishing 4 Stn radiographic technique are: 1. Study of drawing, alloy composition, part geometry and inspection requirements 2. Conducting radiographic experiments to optimize exposure parameters 3. Selective destructive tests to validate radiographic observations 4, Documentation Initially, necessary information such as areas of high stress, alloy composition, manufacturing proves and inspection requirements must be collected and the geometry of the component, studied. Radiographic experiments are conducted to optimize the following parameters: + Energy of penetration (KV) * Exposure factors (mA x time) * Radiographic coverage, which implies projecting every portion af the component 0} y involve one or more normal and angular exposures of the component rv the filRadiography $4 aramrorons need (0 ho adjuated In nich m way as to achieve IOL sensitivity better than 2% ’ ion at te De and nize of fim mut mate the requirements of the defect details and the sek yen ay CONCH Me al EH HO over aren oF hnlerent Hifi focus distanes, peomettic tneaharpnesa, filters and screens and chemical “ ape ' wo? pensill opunieation anwens of Hl uve eestretive fests ave 10 be conducted (0 extablivh the correlation of actual defects with raphic indication, Thiv inprovew the confidence level of radiographers and proves the vas af taiographic projection. ened experiments aie repented (Il w fiir degree of agreement is achieved between the he radiograph and the actual dixcontinulty in the component, Finally, optimized parameters he most acceptable radiography are documented for implementation, Thereafter, a periodic Javing of technique is done in the fight of feedback, information and experience gained. eat sep alter establishing the technique is to assess the influence of various defects and discontinuities on the mechanical property of the components and fix a realistic limit of acceptance of aes discontinuities, Information about the limit of acceptance of various defecv discontinuities jgusnally available in the component design or the inspection document; in case such information is not iaildble, defect-property correlation data is generated for fixing a eptance/rejection limit for various defectsiiscontinuitics, The limit of acceptance of defects is decided based on the functional and stress dhisifications of the component in coordination with the designer, Generally, components are classified into the following three categories + Class : These components are often subjected to high temperature, pressure, fatigue and impact sesses. The failure of such components can cause significant danger to operating personnel or ‘would result in serious operational penalties or loss of the entire system. One should be extremely careful in the examination and assessment of such components * Clays Il: These are stressed components whose failure may not have as drastic an effect as in case of Class | components, Failure of these components may lead to the damage of subassemblies that | can be replaced without causing serious damage to the system. One should be extremely careful | inthe examination and assessment of such components Clas II: These are low stressed or unstressed components, whose failure does not cause any pol nt damage to the system. Often, radiographic examination is not required for such omponents, sel Epo lo aft he me ination ot soapstone reve al I stration isgivent component area classification aceoding tothe distribution of stresses, + atl sng of reac limit of weceplance of « dees in ferent areas of the component ictal compositon/speificaion, which provides information about inherent susceptibility of alerial to some defects Machin Th, "allowance: ‘This information helps salvage and rework assessment NK . al puidel; 7 . Sng snes for acceptance tints for radiographically observed defeets for castings and Me deft the following table, Guidelines are give in fem of plate numbers of ASTM ‘ph ISS, = | ee