RADIOGRAPHIC
INTERPRETATION
(RTI)
Course Notes
Course Reference
NDT 2/20
REVISION 3 May 2008
Subject Index: pages i to vii
Training notes & glossary: pages 1 to 153
Appendix 1: pages 154 to 158
Subject Index
Section Title Pages
1.0 INTRODUCTION TO NDT METHODS 1-5
2.0 INTRODUCTION & HISTORICAL BACKGROUND 6-7
2.1 PROPERTIES OF IONISING RADIATION 8-11
3.0 THE ELECTROMAGNETIC SPECTRUM 12-13
4.0 SIMPLE ATOMIC THEORY 14-15
5.0 IONISING RADIATION 15 6.0 X-RAYS OR BREMSTRAHLUNG 16-18 6.1 X-RAY EQUIPMENT 18 6.1.1 THE CATHODE 19 6.1.2 THE ANODE 20-21 6.1.3 X-RAY TUBES 22
6.1.4 X-RAY TUBE POWER SUPPLY 22-24
6.1.5 X-RAY TUBE CONTROLS 25
6.1.5.1 PENETRATING POWER OR
RADIATION QUALITY (kV)
25-26
6.1.5.2 QUANTITY OF RADIATION (mA) 26-27
6.1.6 HIGH ENERGY X-RAY SOURCES 27
6.1.6.1 BETATRONS 27
6.1.6.2 LINEAR ACCELERATORS 27
6.1.6.3 VAN der GRAAF GENERATORS 28
6.1.7 SPECIAL TYPES OF X-RAY UNIT 28
6.1.7.1 MICROFOCUS X-RAY SOURCES 28
6.1.7.2 ROD ANODE X-RAY TUBES 28
6.1.7.3 ROTATING ANODE X-RAY
EQUIPMENT
29
7.0 GAMMA RAYS 29
7.1 ALPHA AND BETA EMISSION 30
7.1.1 ALPHA PARTICLES 30
7.1.2 BETA PARTICLES 30
7.2 SEALED SOURCES 31-32
7.3 PENETRATING POWER OF GAMMA RADIATION 32
7.4 QUANTITY OF GAMMA RADIATION 32-34
7.5 RADIOISOTOPE CONTAINERS FOR INDUSTRIAL
RADIOGRAPHY
34-36
7.6 COMPARISON OF X-RAYS AND GAMMA RAYS 37
7.6.1 ENERGY AND OUTPUT OF RADIATION 37
7.6.2 RADIOGRAPHIC CONTRAST 37-38
7.6.3 FOCAL SPOT SIZE VERSUS SOURCE SIZE 38-39
7.6.4 EXPOSURE TIME (FILM RADIOGRAPHY) 39
7.6.5 POWER SUPPLY 39
7.6.6 PHYSICAL SIZE AND WEIGHT 39
8.1.4.5 DRYING 47
8.2 ADVANCED IMAGING TECHNIQUES 47-48
9.0 PRODUCTION OF A RADIOGRAPH (FILM RADIOGRAPHY) 48
9.1 RADIOGRAPHIC QUALITY 48-49
9.1.1 CONTRAST 49-50
9.1.1.1 FILM TYPE (AFFECTS FILM
CONTRAST)
50-51
9.1.1.2 FILM DENSITY (AFFECTS FILM
CONTRAST)
51-52
9.1.1.3 BASE FOG LEVEL (AFFECTS FILM
CONTRAST)
52
9.1.1.4 FILM PROCESSING (AFFECTS FILM
CONTRAST)
53
9.1.1.5 RADIATION QUALITY (AFFECTS
SUBJECT CONTRAST)
53
9.1.1.6 SCATTER (AFFECTS FILM AND
SUBJECT CONTRAST)
54
9.1.2 DEFINITION 54-55
9.1.2.1 GEOMETRIC UNSHARPNESS 55
9.1.2.2 INHERENT UNSHARPNESS 56
9.1.2.2.1 FILM (EFFECT ON INHERENT
UNSHARPNESS)
56
9.1.2.2.2 QUALITY OF RADIATION (EFFECT
ON INHERENT UNSHARPNESS)
56
9.1.2.2.3 INTENSIFYING SCREENS (EFFECT
ON INHERENT UNSHARPNESS)
57
9.1.2.3 RELATIVE MOVEMENT DURING
EXPOSURE
Subject Index
Section Title Pages
9.2 RADIATION SCATTER AND SCATTER CONTROL 57
9.2.1 SCATTERING MECHANISMS – THE CAUSES OF
SCATTER
58
9.2.1.1 THE PHOTOELECTRIC EFFECT 58
9.2.1.2 COMPTON SCATTERING
(INCOHERENT SCATTERING)
58-59
9.2.1.3 PAIR PRODUCTION 59
9.2.1.4 TOTAL SCATTER AT DIFFERENT
PRIMARY BEAM ENERGIES
60 9.2.2 TYPES OF SCATTER 60 9.2.2.1 SIDE SCATTER 60-61 9.2.2.2 BACK SCATTER 61 9.2.2.3 SELF-SCATTER 61 9.2.3 SCATTER CONTROL 62 9.2.3.1 COLLIMATION 62 9.2.3.2 DIAPHRAGMS 62 9.2.3.3 MASKING OR BLOCKING 62-63 9.2.3.4 GRIDS 63 9.2.3.5 FILTERS 63-64
9.2.3.6 METALLIC FOIL SCREENS 64
9.2.3.7 HIGHER RADIATION ENERGY 64
9.2.3.8 CHANGE FROM X-RAY TO GAMMA
RAY RADIOGRAPHY
65
9.2.3.9 REDUCING THE FOCUS OR SOURCE
TO FILM DISTANCE
65
9.4 DETERMINING THE CORRECT EXPOSURE: EXPOSURE
CHARTS
65-66
9.4.1 EXPOSURE CHARTS 66-71
9.4.1.1 USING EXPOSURE CHARTS (X-RAY) 71
9.4.1.1.1 FOCUS TO FILM DISTANCE 71-73
9.4.1.1.2 TUBE VOLTAGE 73-74
9.4.1.1.3 CHANGING THE FILM DENSITY 75
9.4.1.1.4 CHANGING THE FILM TYPE 75-76
9.4.1.1.5 RADIOGRAPHY OF OTHER
MATERIALS
76-77
9.4.1.1.6 COMPENSATING FOR THE USE OF A
FILTER
77
9.4.1.1.7 OTHER POSSIBLE CHANGES 77
11.4 RADIATION ENERGY 86
11.5 SOURCE TO FILM DISTANCE 86-87
11.6 SWSI TECHNIQUES 87
11.6.1 SINGLE WALL SINGLE IMAGE TECHNIQUE FOR
PLATE
87
11.6.2 SINGLE WALL SINGLE IMAGE TECHNIQUE:
SOURCE INTERNAL, PLACED CENTRALLY (PANORAMIC TECHNIQUE)
88
11.6.3 SINGLE WALL SINGLE IMAGE TECHNIQUE:
SOURCE INTERNAL, OFFSET
89
11.6.4 SINGLE WALL SINGLE IMAGE TECHNIQUE:
FILM INSIDE, SOURCE OUTSIDE
89-91
11.7 DWSI TECHNIQUE 91-93
11.8 DWDI TECHNIQUES 94-95
11.8.1 DOUBLE WALL DOUBLE IMAGE (ELLIPTICAL) 95-95
11.8.1 DOUBLE WALL DOUBLE IMAGE
(SUPERIMPOSED) 95-96 12.0 INTERPRETATION OF RADIOGRAPHS 97 12.1 INTRODUCTION 97 12.2 VIEWING CONDITIONS 97-98 12.3 REPORTING 98-99 12.4 FILM QUALITY 99 12.4.1 COMPONENT IDENTIFICATION 99 12.4.2 LOCATION MARKERS 99 12.4.3 FILM DENSITY 99-100 12.4.4 RADIOGRAPHIC SENSITIVITY 100
12.4.5 ARTEFACTS AND OTHER UNWANTED IMAGES 100-101
12.5 INTERPRETATION OF RADIOGRAPHIC IMAGES 101
12.6 ARTEFACTS 101
12.6.1 PRESSURE MARKS (CRIMP MARKS) 101
Subject Index
Section Title Pages
12.6.3 SCRATCHES: ON LEAD INTENSIFYING
SCREENS
101
12.6.4 DIRT: ON THE FILM OR SCREENS 102
12.6.5 STREAKINESS OR MOTTLING: POOR
DEVELOPMENT 102 12.6.6 DEVELOPER SPLASHES 102 12.6.7 FIXER SPLASHES 102 12.6.8 WATER SPLASHES 102 12.6.9 WATER MARKS 102 12.6.10 AIR BELLS 102 12.6.11 DIFFRACTION MOTTLING 103 12.6.12 STATIC MARKS 103 12.6.13 DICHROIC FOGGING 103 12.6.14 RETICULATION 103
12.6.15 FILM FOGGING BY X OR GAMMA RAYS 103
12.6.16 LIGHT FOGGING 104
12.6.17 FILM FOGGING DUE TO INADEQUATE
STORAGE CONDITIONS
104
12.6.18 SOLARISATION 104
12.6.19 A FINAL WORD ON ARTEFACTS 104
12.7 INTERPRETATION OF WELD RADIOGRAPHS 104
12.7.1 RADIOGRAPHIC INDICATIONS DUE TO
SURFACE GEOMETRY
104
12.7.1.1 EXCESSIVE ROOT PENETRATION 105
12.7.1.2 ROOT CONCAVITY 105
12.7.1.3 INCOMPLETELY FILLED GROOVE 106
12.7.1.4 LACK OF REINFORCEMENT 106
12.7.1.5 UNDERCUT 107
12.7.1.6 SPATTER 107
12.7.1.7 EXCESSIVE DRESSING (GRINDING MARKS) 108
12.7.1.8 HAMMER MARKS (TOOL MARKS) 108
12.7.1.9 TORN SURFACE 108
12.7.1.10 SURFACE PITTING 108
12.7.2 INTERNAL DEFECTS 109
12.7.2.1 CRACKS 109-111
12.7.2.2 LACK OF FUSION 111-113
12.7.2.3 INCOMPLETE ROOT PENETRATION 114
12.7.2.4 NON-METALLIC INCLUSIONS 114
12.7.2.5 METALLIC INCLUSIONS 114-115
12.7.2.6 GAS PORES: POROSITY 115-116
12.7.2.7 ELONGATED CAVITIES (HOLLOW BEAD) 116
12.7.2.8 WORMHOLES 116
12.7.2.9 CRATER PIPES & CRATER CRACKS 116-117
12.8 INTERPRETATION OF CASTING RADIOGRAPHS 117
12.8.1 VOIDS 117
12.8.1.1 MACROSHRINKAGE 117
12.8.2.2 FILAMENTARY SHRINKAGE (ALSO CALLED SPONGINESS)
14.1 IONISATION (EXPOSURE) 126
14.2 ABSORBED DOSE 126
14.3 MAN MAMMAL EQUIVALENT or RADIOBIOLOGICAL
EQUIVALENT
127
14.4 DOSE RATE 128
14.5 SOURCE STRENGTH OR ACTIVITY 128
14.6 SPECIFIC ACTIVITY 128
14.7 OUTPUT 128-129
15.0 RADIATION MONITORING DEVICES 129
15.1 SURVEY METERS 129
15.1.1 IONISATION CHAMBERS 129-130
15.1.2 PROPORTIONAL COUNTERS 130-131
15.1.3 GEIGER COUNTERS 131
15.1.4 SOLID STATE RADIATION DETECTORS 131
15.1.5 SCINTILLATION COUNTERS 131-132 15.2 PERSONAL MONITORS 132 15.2.1 FILM BADGES 132-133 15.2.2 THERMOLUMINESCENT DOSIMETERS (TLD) 133-134
15.2.3 QUARTZ FIBRE ELECTROMETER
(PERSONAL DOSIMETER)
134
Subject Index
Section Title Pages
15.0 RADIATION SAFETY 135
15.1 PRECAUTIONS 135
15.1.1 EXPOSURE BOOTHS 135
15.1.2 SITE WORK 136
15.1.3 SCATTER 136
15.2 EXPOSURE LIMITS FOR RADIATION WORKERS 136
15.2.1 DOSIMETERS 136
15.3 PERMITTED LEVELS 136
15.3.1 CLASSIFIED WORKERS 136
15.3.2 UNCLASSIFIED PERSONNEL, CONTROLLED
& SUPERVISED AREAS
137
15.4 ‘SAFE’ WORKING DISTANCES 137-138
15.4.1 SHIELDING 138
16.0 A GLOSSARY OF TERMS (USED IN RADIOGRAPHIC TESTING) 139-151
1.0 INTRODUCTION
Non destructive testing is the ability to examine a material (usually for discontinuities) without degrading it.
The five principal methods, other than visual inspection, are:
• Penetrant testing
• Magnetic particle inspection • Eddy current testing • Radiography
• Ultrasonic testing
In all the NDT methods, interpretation of results is critical. Much depends on the skill and experience of the technician, although properly formulated test techniques and procedures will improve accuracy and consistency.
1.1 Penetrant Testing
Penetrant testing locates surface breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of the excess surface penetrant the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used. Advantages
• Applicable to non-ferromagnetics
• Able to test large parts with a portable kit • Batch testing
• Applicable to small parts with complex geometry • Simple, cheap easy to interpret
• Sensitivity Disadvantages
• Will only detect defects open to the surface • Careful surface preparation required • Not applicable to porous materials • Temperature dependant
• Cannot retest indefinitely • Compatibility of chemicals
• Cheap rugged equipment • Direct test method Disadvantages
• Ferromagnetic materials only • Requirement to test in 2 directions • Demagnetisation may be required • Odd shaped parts difficult to test • Not suited to batch testing
• Can damage the component under test 1.3 Eddy Current Inspection
Eddy current inspection is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils' electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor.
Advantages
• Sensitive to surface defects • Can detect through several layers • Can detect through surface coatings • Accurate conductivity measurements • Can be automated
• Little pre-cleaning required • Portability
Disadvantages
• Very susceptible to permeability changes • Only on conductive materials
• Will not detect defects parallel to surface
• Not suitable for large areas and/or complex geometry's • Signal interpretation required
• No permanent record (unless automated) 1.4 Radiography
Radiography monitors the varying transmission of ionising radiation through a material with the aid of photographic film or fluorescent screens to detect changes in density and thickness. It will locate internal and surface breaking defects.
Advantages
• Gives a permanent record, the radiograph • Detects internal flaws
• Detects volumetric flaws readily • Can be used on most materials • Can check for correct assembly • Gives a direct image of flaws
• Fluoroscopy can give real time imaging Disadvantages
• There is a radiation health hazard
• Can be sensitive to defect orientation and so can miss planar flaws • Has limited ability to detect fine cracks
• Access is required to both sides of the object • Limited thickness of materials can be penetrated • Skilled radiographic interpretation is required • Is a relatively slow method of inspection • Has a high capital cost
• Has a high running cost 1.5 Ultrasonic Testing
Ultrasonic Testing measures the time for high frequency (0.5MHz - 50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound reflects back to the probe in a time other than that appropriate to good material.
• Unsuited to coarse grained materials • Reliant upon defect orientation 1.6 Choice of Method
Before deciding on a particular NDT inspection method it is advantageous to have certain information.
• Reason for inspection. (To detect cracks, to sort between materials, to check assembly, etc.)
• Likely orientation of planar discontinuities, if they are the answer to the
above question.
• Type of material.
• Likely position of discontinuities.
• Geometry and thickness of object to be tested. • Accessibility
This information can be derived from:
• Product knowledge
• Previous failures
Accuracy of critical sizing of indications varies from method to method.
1.6.1 Liquid Penetrant Inspection
• The length of a surface breaking discontinuity can be determined readily, but the depth dimensions can only be assessed subjectively by observing the amount of 'bleed out'.
1.6.2 Magnetic Particle Inspection
• The length of a discontinuity can be determined from the indication, but no assessment of discontinuity depth can be made.
1.6.3 Eddy Current Inspection
• The length of a discontinuity can be determined. The depth of a discontinuity or material thinning can be determined by amplitude measurement, phase measurement or both, but the techniques for critical sizing are somewhat subjective.
1.6.4 Ultrasonic Testing
• The length and position of a discontinuity can be determined. Depth measurements are more difficult but crack tip diffraction or time of flight techniques can give good results.
1.6.5 Radiography
• The length and plan view position can be determined. Through thickness positioning requires additional angulated exposures to be taken. The through thickness dimension of discontinuities cannot readily be determined.
rays were passing from the tube and through the plates. He found that the new ray could pass through most substances casting shadows of solid objects. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects. One of Roentgen's first experiments late in 1895 was a film of the hand of his wife, Bertha.
Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896, French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Henri Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with his photographic plates. Later when he developed these plates, he discovered that they were fogged (exhibited exposure to light). Becquerel questioned what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light. In addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium.
One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with his photographic plates. Later when he developed these plates, he discovered that they were fogged (exhibited exposure to light). Becquerel questioned what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light. In addition, he noticed that only the plates that were in the
drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitchblende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radioactive element in pitchblende, and named it 'polonium' in honor of Marie Curie's native homeland. Later that year, the Curies discovered another radioactive element which they named radium, or shining element. Both polonium and radium were more radioactive than uranium. Since these discoveries, many other radioactive elements have been discovered or produced. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitchblende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radioactive element in pitchblende, and named it 'polonium' in honor of Marie Curie's native homeland. Later that year, the Curies discovered another radioactive element which they named radium, or shining element. Both polonium and radium were more radioactive than uranium. Since these discoveries, many other radioactive elements have been discovered or produced. In 1946, man-made gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and were much less expensive. The manmade sources rapidly replaced radium, and use of gamma rays grew quickly in industrial radiography. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During World War II, industrial radiography grew tremendously as part of the Navy's shipbuilding program. In 1946, man-made gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and were much less expensive.
The manmade sources rapidly replaced radium, and use of gamma rays grew quickly in industrial radiography.
William D. Coolidge's name is inseparably linked with the X-ray tube-popularly called the 'Coolidge tube.' This invention completely revolutionized the generation of X-rays and remains to this day the model upon which all X-ray tubes for medical applications are patterned. He invented ductile tungsten, the filament material still used in such lamps. He was awarded 83 patents during his lifetime.
and atomic number).
(iii) The wavelength or “photon energy” of the radiation itself.
(3) Penetrating radiation can be detected using a photographic emulsion or by other means. The system used to detect the radiation must be capable of differentiating between different intensities of radiation.
Figure 1A. The radiograph that would result from the set-up in figure 1 above
(Note that in film radiography thin sections appear darker while thicker sections appear lighter. The opposite is true if a fluorescent screen rather than a photographic film is used as a radiation detector)
(1) A radiograph is a two-dimensional image of a three-dimensional object: The through thickness position and size of an object producing a
radiographic image cannot be determined solely from the information given by a single radiograph (this is demonstrated in figures 2 and 2A).
(2) A defect will only appear as an image in a radiograph if: (a) the defect causes a local difference in radiation absorption and
(b) the method used for detecting the radiation is capable of detecting the difference in radiation intensity so caused.
For example, suppose that a chosen radiographic technique is capable of detecting a thickness difference of say 0.5 mm in 50 mm of steel. If we use this technique to radiograph the weld shown in figure 3 then:
(1) The gas pore will readily be detected because A - (B + C) = 3 mm.
(2) The lack of side fusion will not appear as an image on the radiograph because A - (D + E) = 0.01 mm which is much too small to be detected by the
Maxwell predicted a speed of travel for such waves that was equal to the then known speed of light. It soon became clear that light was in itself a form of electromagnetic radiation.
All types of electromagnetic radiation travel at the same velocity (v), the velocity of light, which is about 2.998 x 108 ms-1 (186,000 miles per second in old money), but differ in terms of their wavelength (λ) and frequency (f). Wavelength can be defined as the distance travelled during one complete field oscillation while frequency is the total number of oscillations occurring in one second.
v = f
λ
As scientific knowledge advanced it became clear that in some circumstances light behaved not so much like a waveform, but more like a particle. Considering such behaviour in 1900 a scientist called Max Planck first put forward the theory that light had, what he called, a ‘quantum’ nature. Planck postulated that electromagnetic energy could not exist in amounts (‘quantum’ being Latin for ‘amount’) smaller than a given very small amount of energy and that all larger amounts of electromagnetic energy were exact multiples of this amount to which he gave the name “photon”. Planck believed that the photon energy of any form of electromagnetic radiation would be equal to a constant multiplied by its frequency. In later years Planck’s hypothesis was proved to be true and the constant in question became known as ‘Planck’s Constant’, usually abbreviated as ‘h’.
E = hf
Where ‘h’ is Planck’s constant ( = 6.63 x 10 –23 Js) and ‘E’ is the photon energy of electromagnetic radiation of frequency ‘f’.
The properties of electromagnetic radiation, especially in the way it interacts with matter are largely determined by its wavelength. Figure 6 is a schematic of the
electromagnetic spectrum.
Figure 6. The electromagnetic spectrum
Notes on figure 5:
1. The electron volt (eV) is a unit of energy which is equal to the kinetic energy that an
electron obtains when it accelerates through an electric field of 1 volt. A Mega electron volt (MeV) is equal to the kinetic energy of an electron that has accelerated through an electric field of 1 million volts. On electron volt is equal to 1.6 x 10-19 Joules.
2. The relationship between wavelength and photon energy on which the diagram above has
orbiting negatively charged electrons.
Figure 4. Simple model of atomic structure
In the equilibrium state the number of orbital electrons is equal to the number of protons and there is no net electrical charge. When there is inequality between the numbers of protons and electrons then there is a net electrical charge and the atom is said to be ionised. Ions may be negatively charged if the number of electrons exceeds the number of protons or positively charged if the converse is true. So called electropositive elements, a group which includes all metals, ‘like’ to form positive ions while the electronegative elements such as oxygen, phosphorous, chlorine and sulphur ‘like’ to form negative ions.
The orbital electrons exist in fixed energy levels or shells. Each shell can contain a fixed maximum number of electrons. The shells are identified by letters – K, L, M, N and so on. The lowest energy level is represented by the K-shell; this is the innermost of the
electron shells and it can contain a maximum of 2 electrons. The L-shell can contain up to 8 electrons while the M-shell contains a maximum of 18 and the N-shell contains a maximum of 32. The maximum total number of electrons in each shell is equal to 2n2 where ‘n’ is the shell number counting the K-shell as 1, L-shell as 2 etc. Within the M, N and other shells certain groupings of electrons produce greater stability, elements having an even number of electrons tend to less chemically reactive than those which have an odd number. A group of 8 electrons in the M or N shells produces an element which is the most chemically inert of all elements – an inert gas. In electropositive elements the orbital electrons are relatively loosely bound and there is a tendency to form positive ions. In electronegative elements the
orbital electrons are relatively tightly bound and there is a tendency to form negative ions. The inert gasses such as neon, argon, xenon and krypton either have an outer shell that is completely full or one which contains a very stable grouping of electrons. Based on their chemical properties the elements can be organised into a ‘periodic table’ as shown below. Elements falling in the same vertical column share very similar chemical properties.
(The numbers above and below each chemical symbol are the atomic number and the atomic weight of each element. Note that the atomic weight differs slightly from the atomic mass number)
Figure 5. Periodic table of elements
X-rays result from ionisation and de-ionisation events: when a positively charged ion captures a free electron the atom descends into a lower energy state and the left over energy may be released in the form of an x-ray ‘photon’. X-rays can also result as a negative ion loses a captured electron, because again there is a reduction in the energy stored in the atom as it returns to a state of zero electrical charge.
Each element has its own characteristic number of protons in the nucleus. This number is the ‘atomic number’, usually abbreviated as ‘Z’. It is the atomic number that determines the chemical properties of a given substance. However, each element can exist as any one of a number of ‘nuclides’ or ‘isotopes.’ Each isotope of a given element has the same atomic number, the same number of protons and the same chemical properties, but each isotope has a different ‘atomic mass number’. The difference in atomic mass number is due to a difference in the number of neutrons in the nucleus. The atomic mass number is equal to the total of protons + neutrons in the nucleus. Most elements can exist in nature as any one of several stable isotopes. Some isotopes, however, are not stable – these are the so called ‘radioactive isotopes’. The following notation is typically used:
1 2 H He 1.008 4.003 3 4 5 6 7 8 9 10 Li Be B C N O F Ne 6.940 9.012 10.81 12.01 14.01 16.00 19.00 20.17 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar 22.99 24.30 26.98 28.09 30.97 32.06 35.45 39.95 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 39.10 40.08 44.96 47.90 50.94 52.00 54.94 55.85 58.93 58.71 63.55 65.38 69.74 72.59 74.92 78.96 79.90 83.80 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 85.47 87.62 88.91 91.22 92.91 95.94 98.91 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 132.9 137.3 138.9 178.5 181.0 183.9 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222) 87 88 89 Fr Ra Ac (223) 226.0 (227) 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Lanthanide Series 140.1 140.9 144.2 (145) 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw Actinide Series 232.0 231.0 238.0 237.1 (244) (243) (249) (247) (251) (254) (257) (258) (259) (260)
million million million), the constant probability of decay gives rise to a constant ‘half-life.’ With such a large number the random nature of decay just ‘averages out’.
Gamma rays are an occasional by product of this process of nuclear fission. Fission means ‘splitting’. There are several routes by which nuclear fission can take place and two of these are of importance in the production of gamma rays. These will be discussed in greater detail in the section on gamma ray sources.
5.0 IONISING RADIATION
The 2 types of penetrating radiation most used in industrial radiography, x-rays and gamma rays, are often referred to as “ionising radiation”. This is because the nature of their interaction with matter is to cause ionisation. Ionisation is caused by loss of an orbiting
electron which leaves the atom in a electrically positively charged state (+ Ion). Alpha
particles and beta particles, which are products of radioactive fission also cause ionisation and are therefore included within the term ‘ionising radiation’. Neutron radiation is a hazard in the nuclear power industry, it can [indirectly] cause ionisation, and it is therefore often included within this group of types of radiation referred to as ionising. Alpha and beta particle radiation are covered in greater detail in the sections below.
6.0 X-RAYS or BREMSTRAHLUNG
The term ‘x-ray’ is applied to ionising radiation produced when a beam of high velocity (i.e. high kinetic energy) electrons collides with the atoms of a ‘target’ material. The ‘photon energy’ of the x-radiation thereby produced depends two factors:
(i) the kinetic energy of the electron at the point of collision and
(ii) the relative efficiency of the process of stopping the incident electron – does this occur in a single large event or in a series of events of varying
Figure 7. X-ray production
The maximum energy of the x-ray photons produced is determined by the maximum kinetic energy of the high velocity electrons impacting upon the target material. There is no minimum to the energy of the x-ray photons produced. This is because there is wide
variation in the amount of energy which the electron loses on collision with an atom. Some of the electrons will score only a glancing hit on the atom, in so doing interacting with the loosely bound electrons in the outermost electron shells. This causes the impacting electron to be deflected and it loses part of its velocity. The reduced energy electron may then interact with another atom and in so doing produce another photon of x-rays of variable although reduced energy. In addition to this the x-ray photons produced by electron collisions can themselves interact with adjacent atoms and thereby produce reduced energy photons. X-ray spectra are commonly termed to be “continuous, white or polychromatic” (this is because there is no minimum energy). Figure 8 shows the form of a typical x-ray spectrum. Note that higher energy x-rays have shorter wavelengths.
penetrating power of the x-rays can be controlled by increasing or decreasing the accelerating voltage. The greater the accelerating voltage, the more penetrating the radiation. In an x-ray set the accelerating voltage is the tube voltage.
The total number of photons produced at all wavelengths is directly related to the number of high velocity electrons arriving at the target. The total number of electrons is directly proportional to the magnitude of the electric current passing through the accelerating field. This current in an x-ray set is referred to as tube current. Radiation intensity is directly proportional to tube current.
The two characteristic peaks shown in figure 8 are caused by target material inner shell electrons jumping to a higher energy level, then falling back to their equilibrium state. Characteristic radiation generally occurs at relatively low energy, long wavelength and is of no great importance in the industrial radiography of metallic components although it can cause a problem known as diffraction mottling (see the section on artefacts). As the name suggests, each element produces its own specific characteristic peaks, and measurement of these can be used to perform chemical analysis (x-ray fluoroscopy). Low energy x-rays can be ‘diffracted’ by crystalline materials such as metals. In the diffraction process radiation is deflected from its original path at an angle that is determined by its wavelength and the spacing of the atoms in the crystalline material. This effect can be used to produce the mono-wavelength x-rays that are used in “x-ray crystallography”.
6.1 X-RAY EQUIPMENT
In order to produce x-rays three things are required: 1. A source of electrons.
2. A target, constructed from a suitable high melting point material. 3. A means of accelerating electrons toward the target.
High velocity electrons cannot travel far in air, therefore the process of acceleration must take place in a high vacuum.
The source of electrons is called the cathode. In a conventional x-ray tube it
consists of a tungsten filament which is heated by passing a small current through it. Heating the filament produces a cloud of loosely bound, low kinetic energy electrons in close
proximity to the filament. This process is known as “thermionic emission.” Electrons are negatively charged and can be accelerated toward the target by making it positively charged with respect to the source of electrons.
This makes it a relatively efficient material for converting kinetic energy to x-ray energy which in turn helps to reduce the amount heat produced as a proportion of the total output of energy. Sometimes the target is constructed from Tantalum (melting point 2996°C) and less frequently from other refractory metals.
Nearly all anodes are ‘hooded’ (see figures 10 and 11) the hood is a high
conductivity copper shroud which is designed to intercept stray electrons and to prevent them from hitting the tube walls. The hood has a ‘window’ in the form of a beryllium insert or a thinned section of copper which permits x-rays to exit without unduly increasing ‘inherent filtration’. Inherent filtration is the term used to describe removal of x-rays from the primary beam due to absorption by the materials used in x-ray head construction. The reason that a beryllium window is used in many x-ray heads is that beryllium has a very low absorption factor and this minimises inherent filtration whilst still affording the tube walls protection from stray electrons.
Anodes maybe directional as in figure 10 or panoramic as in figure 11. In either case anode design is such that the effective focus size in the direction of the useful beam is much smaller than the actual focus size. This arrangement is called a “line” or “Benson” focus and it serves to maximise anode life without unduly compromising image quality.
Figure 11. Panoramic anode
The target is generally set at an angle of about 70° to the electron beam as shown in figures 10 and 11. This produces a small effective focus size whilst maintaining a large actual focal spot size. The large actual focus size helps to dissipate the heat generated more efficiently. Therefore higher tube currents can be sustained without the risk of damaging the target. This design feature is known as “Benson” or “Line” focusing. See figure 12 below.
Figure 13. Directional x-ray tube (metal – ceramic type)
The x-ray beam produced is filtered by the wall of the glass (or metal-ceramic) envelope. This reduces the useful quantity of x-rays produced, with the low energy components of the spectrum being particularly effected. Therefore it is common in glass tubes that the tube wall will be ground thinner in the region of the useful beam in order to minimise the x-ray energy lost due to self-filtration. Metal-ceramic x-ray tubes (and low kilovoltage glass tubes) may have Beryllium inserts (usually called windows) in order to minimise the filtration effect of the tube wall. Beryllium is used for this purpose because it has a very low x-ray absorption coefficient and because it is mechanically strong enough to contain the necessary vacuum.
X-ray tubes are invariably mounted inside some form of ‘tank’. This is usually a metal cylinder that may be fitted with a beryllium or plastic window in order to minimise self-filtration of the x-rays produced. The tank contains a coolant which may be oil or some type of gas. It provides high voltage insulation and mechanical protection. In portable equipment the high voltage transformer is mounted inside the tank.
6.1.4 X-RAY TUBE POWER SUPPLY
In order to produce a beam of electrons from the filament in the tube it is necessary to make the anode positive with respect to the cathode. If an AC supply is connected across the tube then the beam of electrons will pass only when the anode is positive and the tube will act as a half wave rectifier.
Figure 14. Current flow across a half-wave self-rectified x-ray tube
Most older type portable x-ray sets were half-wave self-rectified. This produced a considerable weight saving compared with the earlier types of constant potential unit. Most modern portable units are constant potential and use lightweight solid state rectifiers to produce what is effectively DC current.
The metal ceramic tubes used in modern equipment are able to safely withstand a greater potential difference between the anode / cathode and the tube wall. This permits the use of “grounded anode” type circuitry which in turn permits direct water cooling of the anode. In an older type unit operating at say 200 kV the cathode voltage would have been minus100 kV while the anode voltage would have been plus 100 kV, giving a maximum potential difference between the electrodes and the glass tube wall of 100 kV. With modern grounded anode circuitry it is safe to hold the cathode at minus 200 kV with the anode at zero volts to produce the same 200 kV potential difference. An anode held at zero volts can be safely cooled by water.
Water is a very efficient coolant and direct water cooling of the anode permits operation at greatly increased tube currents. For example the maximum tube current for an older type 200 kV oil cooled head was typically 5 mA self-rectified. With modern portable equipment maximum constant potential tube currents of 15 or 20 mA are not unusual for a 200 kV head.
Older type constant potential industrial x-ray units were extremely heavy and bulky and were suitable for use only in fixed installations. Much of the weight and bulk involved came from the rectification circuitry used, the so called “Greinacher Circuit” and the large external oil cooling system that was necessary in order to dissipate the large amount of heat
Figure 15. Greinacher circuit voltage
The total quantity of x-rays produced by an x-ray unit is directly proportional to the area under the line showing voltage against time. Thus for the same tube current and peak voltage, an x-ray tube using a smoothed and fully rectified supply (i.e. a constant potential unit) will produce more x-rays than a self rectified tube. In fact the output of x-rays is more than doubled for the same tube current. In a self-rectified unit the tube voltage varies from zero to the peak voltage and back again with each cycle. In a constant potential unit the tube voltage is close to constant. Thus, looking at the spectrum of x-rays produced, a self-rectified unit produces proportionally more low energy radiation than does a similar constant potential unit.
6.1.5 X-RAY TUBE CONTROLS
The radiation produced by the x-ray tube can be varied in quantity and penetrating power (or quality) by controlling the electrical supplies to the tube.
6.1.5.1 PENETRATING POWER OR RADIATION QUALITY (kV)
The penetrating power of x-rays depends on the magnitude of the accelerating voltage which is applied between the cathode and the anode. The higher the voltage - the higher the kinetic energy of the accelerated electrons - the higher the photon energy of x-rays produced. The higher the photon energy - the shorter the wavelength - the greater the
penetrating power. Thus the penetrating power or quality of x-rays is controlled by the tube voltage.
Conventional x-ray tubes, as used in industrial radiography, are capable of being operated in the range from below 50 to 400 kV. If greater penetrating power is required high energy x-ray sources such as betatrons, linear accelerators or Van der Graaf generators can be used to provide x-ray energies of up to 30 or even 40 MeV.
National codes and standards such as ASME V (pre-1996 revisions) and BS EN 1435 relate the maximum kilovoltage which may be used to the material thickness which is to be examined. Table 2 gives the approximate limiting maximum economically penetrable thicknesses of steel for various kilovoltages. The figures given are typical for film
radiography using lead intensifying screens and portable self rectified equipment . Constant potential units can be used economically on greater thicknesses than can self-rectified units.
Table 2. Approximate penetrating power in mm of steel for various kilovoltages.
6.1.5.2 QUANTITY OF RADIATION (mA)
The quantity of radiation produced by the x-ray tube per unit time depends on the number of electrons released by the cathode filament. The number of electrons per second reaching the anode multiplied by the charge on the electron is equal to the tube current. The tube current is not controlled directly. It is increased or decreased by controlling the size of the heating current supplied to the cathode filament. The higher the heating current, the hotter the filament, the greater the thermionic emission of electrons and hence the greater the tube current. Tube current will also be increased for the same heating current if the tube voltage is increased. This is because higher voltages can draw more electrons from the filament even though the filament temperature does not change. So if the tube voltage is altered it will be necessary to adjust the heating current if the same value of tube current is to be maintained. The use of too high a tube current would cause damage to the anode due to overheating, therefore x-ray equipment always incorporates a safety cut-out switch in order to prevent the use of a too-high value of tube current.
The total quantity of radiation produced by the x-ray set is directly proportional to the product of the exposure time (i.e. the time for which the x-ray tube is energised) and the tube current; therefore x-ray exposures are usually given in milliampere minutes (mAmin) at a given tube voltage.
The standard controls on the x-ray set are:
(1) Voltage control: This alters the tube voltage (kV) by varying the low voltage supply to the high voltage transformer. Note that high voltage is not generated in the control panel. This minimises the hazard to personnel.
(2) Milliampere control: An ammeter incorporated into the control panel measures (albeit indirectly) the current flowing across the tube. This is proportional to the number of electrons flowing from the cathode to the anode per unit time. In order to increase the supply of electrons the heating current to the filament is increased using the
milliampere control. Note that the ammeter measures the current flowing between
the anode and the cathode, not the current flowing in the filament (i.e. the heating current).
(3) Timer: Since the quantity of x-rays produced is proportional to the length of time during which the tube is energised it is convenient to incorporate a time-switch into the control panel of the x-ray set which automatically terminates the exposure at a preset time.
As explained above it is convenient to refer to x-ray exposures in terms of milliampere minutes. For example an exposure which produces an acceptable radiograph may have been determined to be, 36 mAmins at 200 kV. If this was the case then any of the following exposures should give an identical acceptable result:
(a) 9 mA for 4 mins at 200 kV. (b) 18 mA for 2 mins at 200 kV. (c) 2 mA for 18 mins at 200 kV.
This is because the amount of radiation produced is the same in each case. Obviously it would be desirable to use a high value of mA, in order to reduce the exposure time, but as explained above the use of high tube currents can severely damage the anode of the x-ray tube and thus reduce its service life. Therefore it is usual to operate at a value of mA which is well within the tube’s specified capabilities.
The reciprocal relationship between time and tube current is sometimes referred to as the reciprocity law or the Bunsen Roscoe reciprocity law.
6.1.6 HIGH ENERGY X-RAY SOURCES
6.1.6.1 BETATRONS
Betatrons are used to produce ultra-hard extremely penetrating radiation with photon energies in the range 1-30 MeV. The efficiency with which the kinetic energy of the
accelerated electrons is converted to x-rays is much better at these high voltages than at those experienced in conventional x-ray tubes. Consequently betatrons usually benefit from quite small focal spots. In betatrons electrons are accelerated in a spiral path of perhaps 1,000,000 revolutions by means of alternating magnetic fields before being deflected towards the target. The radiation produced by betatrons can penetrate 300 mm or more of steel. They are
primarily used for the radiography of castings or large section welds in fixed installations but portable units are available. These are sometimes used on site for the inspection of
reinforcing bars in heavy concrete sections. Up to around 10 MeV betatrons are usually preferred to linear accelerators because they are more compact and less expensive to manufacture.
6.1.6.2 LINEAR ACCELERATORS
Linear accelerators (often called linacs) accelerate electrons to very high velocities along a straight path by means of an electromagnetic waveform generated by a device called a magnetotron. The particle velocities are similar to those achieved in betatrons but a much higher output of radiation is achievable. For radiation energies above 10 MeV linear
mm. This is small enough to provide adequate image quality for most standard techniques. Microfocus x-ray equipment may have an effective focus size as small as 0.1 mm. Using such a small focus size geometric enlargement techniques are possible whilst still producing an adequately sharp image.
Figure 18. Image enlargement using standard and microfocus x-ray equipment
6.1.7.2 ROD ANODE X-RAY TUBES
In a rod anode tube the target is at the end of a copper or aluminium tube which is usually less than 20 mm outside diameter and may be up to a metre long. The target is invariably of the panoramic variety. Grounded anode circuitry is essential for this type of tube. The anode can be positioned inside small diameter pipes in order to carry out panoramic radiography of girth welds; it can also be positioned in many other otherwise inaccessible locations. Rod anode tubes are most often used in aerospace applications.
Figure 19. Rod-anode x-ray tube
6.1.7.3 ROTATING ANODE X-RAY EQUIPMENT
In medical radiography a very large tube current is generally desirable – a high tube current permits a very short exposure time which in turn helps to eliminate or reduce
unsharpness caused by relative movement during exposure. In order to maximise tube current some medical equipment is fitted with a rotating anode. In a rotating anode x-ray tube the anode rotates at high speed and the focus area of the target is therefore constantly changing. Each section of the tungsten target is ‘in use’ for a short time followed by a slightly longer period of ‘resting’. This helps to prevent overheating so the tube current can be greatly increased.
7.0 GAMMA RAYS
‘Gamma (γ) ray’ is the term applied to the electromagnetic radiation which is sometimes produced when the atomic nuclei of a radioactive isotope disintegrate in the process known as atomic fission. Alpha (α) and beta (β) particles may also be produced during the disintegration process; in fact gamma emission is always a by-product of alpha or beta emission. Of the three main types of radiation produced by fission alpha is by far the most hazardous to health; alpha and beta radiation must be taken into consideration when assessing safety. Except as a health hazard, alpha and beta particle radiation have no
significance for industrial radiography since they are easily absorbed by very thin materials. The disintegration process is fixed for each radioactive isotope and as a result the gamma ray energies produced are also fixed.
Thus in alpha emission there is a loss of 4 amu from the nucleus and a reduction in atomic number of 2 (see the example above). Alpha particle radiation cannot penetrate more than a thin sheet of paper or a few centimetres of air, it is, however, very strongly ionising. The great danger to health with alpha emitters is that they may be ingested – radioactive contamination. Once within the human body they will in most cases cause cancer.
7.1.2 BETA PARTICLES
Beta particles may be emitted during radioactive decay. A beta particle consists of a very high velocity electron emitted from the nucleus of a radioactive atom when a neutron converts to a proton. It is important to note that although the beta particle is an electron it has very much higher kinetic energy than a free electron which has resulted from an ionisation event.
Thus in beta emission there is no loss from the atomic mass number whilst the atomic number increases by 1 (see the example above). Beta radiation is more penetrating than alpha. It can penetrate the outer layers of the skin and lead to fatal skin burns. The damage caused is very similar to sunburn, but much more severe. Many of the early victims of the Chernobyl disaster died as a result of skin burns caused by exposure to high intensities of beta radiation. If beta emitters are ingested they will often lead to cancer.
7.2 SEALED SOURCES
The first gamma ray emitting radioisotopes to be used in industrial radiography were naturally occurring radioactive materials such as Radium. Such sources were not sealed and therefore there was a danger of exposure to alpha (α) and beta (β) particles, both of which are extremely damaging to human tissue. Coupled with this was the even greater hazard of radioactive contamination by which radioactive materials might find their way into the human body.
All gamma sources in use today are man-made. They are manufactured by neutron bombardment of non-radioactive raw materials in the core of a small nuclear reactor. The sources in use are all beta emitters, gamma rays being produced as a by-product of beta emission. In order to prevent beta emission or contamination hazard the sources used in industrial radiography are invariably sealed sources. The fissile material is encapsulated in a high integrity titanium or stainless steel shell. Beta radiation is not capable of penetrating the walls of the capsule, and the capsule further precludes any possible contamination hazard so long as it remains intact.
Isotope Half-Life Principle Emissions (MeV) Equivalent x-ray Kilovoltage (kV) Penetrating Power in mm of Steel Iridium (Ir) 192 74.4 days 0.31,0.47,
0.60 400 75
Cobalt (Co) 60 5.3 years 1.17,1.33 1200 200 Thulium (Tm) 170 127 days 0.052,0.084 80 4 Ytterbium (Yb) 169 32 days 0.17,0.20 145 10
Selenium (Se) 75 118.5 days 0.121, 0.136, 0.265, 0.28, 0.401
217 (low energy beam
components improve sensitivity)
30
Table 3. Gamma emissions for commonly used isotopes.
Figure 22. Sealed source
Figure 22 shows the typical encapsulation arrangement for Iridium 192 and Cobalt 60. Some isotopes such as Caesium 137 are double encapsulated. In the case of Caesium 137 this is because the caesium is in the form of caesium chloride which is highly corrosive and highly water soluble (but this is still an improvement on caesium metal which causes an explosion on contact with water).
7.3 PENETRATING POWER OF GAMMA RADIATION
The penetrating power is fixed for each isotope because the spectrum of gamma radiation emitted is fixed. If a material thickness is too great to produce a radiograph using, say, Ir192 then an isotope which produces higher energy gamma radiation such as Co60 must be used.
7.4 QUANTITY OF GAMMA RADIATION
The amount of gamma radiation – the number of photons, produced by an isotope is controlled by the number of disintegrations (atomic fissions) per unit time. The “source strength” of an isotope is usually expressed in curies (Ci) or becquerels (Bq). “Source strength” may also be referred to as “source activity.”
1 Ci = 3.7 x 1010 disintegrations per second 1 Bq = 1 disintegration per second
The becquerel, which is the SI unit of radioactivity, is a very small unit in terms of what is required for industrial radiography. The curie is therefore generally preferred. If the becquerel is used at all then it is usually in the form of gigabecquerels (GBq). One
gigabecquerel is equal to one thousand million (109) becquerels. One curie is equal to 37 gigabecquerels (37 GBq). In the great majority of cases gamma ray exposures are expressed in curie-hours, curie-minutes or curie-seconds; this in each case being the product of source strength measured in curies multiplied by exposure time measured in hours minutes or seconds.
Example:
A steel section 50 mm thick requires an exposure of 700 curie-minutes using Iridium 192 with a source to film distance of 1 metre using Kodak CX film and lead intensifying screens.
All other factors being equal the exposure time would therefore be: (a) 1 hour 10 minutes with a source strength of 10 Curies.
or
(b) 20 minutes with a source strength of 35 Curies. or
(c) 7 minutes with a source strength of 100 Curies.
Gamma rays are produced by a disintegration process. Atoms having unstable nuclei decay with a fixed probability to form other atoms having stable nuclei. Therefore the source strength of the radioactive isotope will reduce with time. The probability decay for a large number of unstable atoms is fixed and proportional to the number of unstable atoms present. This means that the strength of a radioactive source will always reduce
exponentially: i.e. the strength of a given source will reduce by 50% in a fixed time. This fixed time is referred to as ‘HALF LIFE’.
The half life of various commonly encountered isotopes is given in table 3.
If the half life of an isotope is known then the source activity at a given time can be calculated if at some point previously the source activity was measured.
Suppose that an isotope having a half life, h had an activity, S0 at time, t = 0.
Then at time, t the source strength or activity, St can be calculated using :
S
t= S
02
-(t/h)Alternatively the activity of a source can be estimated using a decay chart. Figure 23 shows the decay chart for Iridium 192.
Figure 23. Iridium 192 decay
7.5 RADIOISOTOPE CONTAINERS FOR INDUSTRIAL RADIOGRAPHY
Radioactive isotopes emit gamma rays continuously. The decay process cannot be switched off or in any way slowed down. Gamma radiation is extremely harmful to human body tissues so radioactive isotopes must be shielded when not in use. The shielding materials used in isotope containers are always dense materials such as lead, tungsten or (more commonly) depleted uranium. Most modern containers use depleted uranium shielding because uranium is an extremely efficient absorber of gamma radiation. Uranium shielded isotope containers are much lighter and more portable than their lead shielded counterparts. A uranium shielded container having a weight of about 20 kg can safely store 100 Ci of Iridium 192. A lead shielded container of the same weight would be capable of safely containing only 20 Ci of Iridium 192.
Radioactive isotope containers are designed to fulfil two important functions : (1) To contain the radioactive isotope and reduce the emitted intensity of radiation to a level which allows for safe transportation and storage.
(2) To allow the radioactive isotope to be safely exposed in order that it may be used for radiography.
In addition, radioactive isotope containers have to be capable of withstanding possible accidents involving impact or fire.
All modern isotope containers are designed to be operated by cable (see figure 26). They are of two basic types (see figures 24 & 25). Of the two types depicted the “S” tube type is intrinsically safer but around 30% heavier than the equivalent shutter type. Older types of isotope container did not provide for remote operation.
Figure 24. S-tube type radioactive source container
Figure 26. Remote control isotope delivery system
7.6 COMPARISON OF X-RAYS AND GAMMA RAYS
7.6.1 ENERGY AND OUTPUT OF RADIATION
X-ray equipment produces a continuous range of photon energies up to a threshold level dependent upon the tube voltage setting. The threshold photon energy level can be adjusted from 50 keV or less up to a maximum (for high energy equipment) of perhaps 30 MeV. The photon energy of gamma ray sources is fixed.
The output of radiation per unit time is variable for x-ray equipment up to the maximum mA rating of the tube. The output of radiation from a radioactive isotope is fixed by the source activity. The output of radiation produced by x-ray equipment is generally much greater than that produced by radioactive isotopes.
The penetrating power of ionising radiation is controlled by its maximum photon energy and the photon energy distribution. Table 4 gives an indication of the maximum steel thickness that can practically be radiographed using conventional x-ray equipment and the commonly encountered isotopes. The penetrating power of x-rays produce by self rectified equipment is less than that of x-rays produced by constant potential equipment operating at the same tube voltage. This is because the constant potential equipment produces a larger proportion of high energy radiation than does the self rectified.
Source of Radiation Useful Thickness Range/mm of Steel
x-ray 100 kV (peak) Maximum 6 mm
Self- 150 kV (peak) Maximum 20 mm
Rectified 200 kV (peak) Maximum 30 mm
300 kV (peak) Maximum 60 mm x-ray 100 kV Maximum 10 mm Constant 150 kV Maximum 32 mm Potential 200 kV Maximum 45 mm 300 kV Maximum 100 mm Thulium 170 Maximum 4 mm
Gamma Ray Selenium 75 4-30 mm
Ytterbium 169 2-8 mm
Iridium 192 10-75 mm
Cobalt 60 40-200
Table 4. Useful thickness range for various sources of radiation.
Note: Steel sections of 500 or 600 mm can be radiographed using x-rays generated by linear
accelerators or betatrons.
7.6.2 RADIOGRAPHIC CONTRAST
A radiograph produced using small effective source size will usually be of higher quality than one produced with a larger effective source size. The average focal spot size an x-ray tube is similar to the average physical size of the gamma ray sources which are
commonly used. Most x-ray tubes have a fixed effective focal spot of between 1 and 4 mm. With some x-ray equipment the focal spot size can be varied. Microfocus x-ray tubes may have an effective focal spot of less than 0.1 mm. The size of the focal spot in an x-ray tube tends to be larger for the higher maximum kilovoltage tubes. This is due to the need to dissipate the increased amount of heat generated at high kV. The practical source size for a radioactive isotope is determined by the maximum economically achievable specific activity. Specific activity is usually expressed in curies (or becquerels) per gram. Table 5 below gives typical practical achievable maximum specific activity for 4 common isotopes.
Isotope
Practically achievable maximum specific
activity (Curies per gram)
Density (g/cm3) Maximum practically achievable activity for 3 mm diameter, 3 mm long cylindrical pellet (Curies) Cobalt 60 50 8.9 10 Iridium 192 350 22.4 166 Caesium 137 25 3.5 (note 1) 2 Thulium 170 1,000 4 (note 2) 85
Note 1. Density is for compressed caesium chloride (CsCl) Note 2. Density is for thulium oxide (Tm2O3)
Table 5. Specific activity for common radioisotopes
Note that the maximum activity of a gamma ray source is limited by it’s physical size. The most useful isotopes are those which have a high value of practically achievable specific activity. In an iridium 192 source at the maximum achievable activity about 2.5 atoms per 100 million are radioactive. In a cobalt 60 source the figure is only about 1 atom in every 10,000 million.
The output of radiation from a typical x-ray machine is much greater than the output of radiation from a typical gamma source. This means that in x-radiography the use of long
focal to film distances is more economically feasible than is the case in gamma radiography. Thus, even though the focus is similar in physical size when compared with the average gamma source, it is generally the case that geometric unsharpness is better for x-ray techniques than for gamma.
7.6.4 EXPOSURE TIME (FILM RADIOGRAPHY)
An exposure time of between 1 & 5 minutes is usual for x-ray radiography. A conventional self-rectified x-ray set operating at maximum kilovoltage and tube current will generally be capable of continuous use with an exposure time of up to 5 minutes followed by a rest period between successive exposures of around 1 or 2 minutes. If the exposure time is extended beyond 5 minutes then overheating will generally occur if the rest period is not considerably extended. Constant potential equipment intended for fixed installation usage will usually be capable of continuous operation at its maximum output rating. However, even with such equipment, exposure times in exceeding 10 minutes will generally be avoided.
The exposure time for gamma radiography tends to be longer. This is because the output of radiation (in photons per second) is generally much less. Gamma ray exposure times are usually in the range from about 30 seconds to 1 hour, but exposure times exceeding 24 hours are not unheard of. The required exposure time for a gamma ray source increases as the source activity reduces with time.
7.6.5 POWER SUPPLY
X-ray sets require power from a mains supply or mobile generator. Usually a 4.5 kW generator will provide sufficient power to operate a 300 kV self-rectified set. Gamma radiography can in general be carried out without the need for a power supply.
7.6.6 PHYSICAL SIZE AND WEIGHT
An Iridium 192 isotope with a source activity of up to 100 curies can safely be stored in a container weighing approximately 15-20 kg which has outside dimensions of approximately 200 x 400 x 100 mm. Such isotopes are useful for the radiography of steel sections of up to 75 mm thick. Gamma ray sources can be used to make exposures in situations where access is extremely limited.
A typical self-rectified 300 kV rated x-ray set (which is useful for the radiography of steel sections of up to 60 mm thick) is on the other hand considerably less portable and less manoeuvrable. A typical 300 kV SR tube head could weigh 55 kg and measure 300 x 300 x 750 mm while the associated control panel might weigh as much as 30 kg and measure 450 x 350 x 250 mm. Low kilovoltage equipment offers improved portability and manoeuvrability but this has to be offset against the reduced penetrating power.
7.6.7 EQUIPMENT COST
8.0 METHODS OF PRODUCING A RADIOGRAPHIC IMAGE 8.1 RADIOGRAPHIC FILM
Radiographic film is essentially the same as that used in photography in that it consists of a suspension of silver halide grains in a gelatine binder on an acetate or polyester base. Radiographic film, however, differs from photographic film in the following respects: (i) The acetate or polyester base material is considerably thicker than is the case
with photographic film.
(ii) The emulsion is applied to both sides of the film. This effectively doubles the film density (i.e. degree of darkness) for the same exposure to radiation and thereby doubles the film speed.
(iii) The emulsion tends to be thicker (usually around 0.025 mm) than that used in photographic films, in order to further increase the film speed.
Two types of radiographic film are used for industrial radiography, these being:
Direct type film, where the principal cause of image formation is the ionising radiation itself.
This may be coupled with the effect of “secondary electrons” emitted from metallic foil intensifying screens.
and
Screen type film, where the principal cause of image formation is light emitted from
fluorescent image intensifying screens under the action of ionising radiation.