Lesson 1

Introduction

Radiographic testing is one of the primary nondestructive test methods widely practiced.

Application of radiography

Because of the penetrating ability of X and gamma (g) radiation and absorption characteristics in materials, radiography is used to test a variety of products such as welds, castings, forgings, and assemblies.

Conventional radiographic testing usually requires exposing a photographic film of specific characteristics to X or g rays that have penetrated a specimen and processing the exposed film. The resultant radiograph would then have to be interpreted in terms of quality parameters of the radiograph and the subject of the radiograph evaluated in terms of predetermined acceptance levels.

Advantages of radiography (RT)

1. Can be used with most solid materials.
2. Provides a permanent visual image.
3. Ability to detect internal flaws.
4. Discloses fabrication errors.
5. Reveals structural discontinuities.

Limitations of RT

1. Impracticable to use on specimens of complex geometry.
2. Both sides of the specimen must be accessible.
3. Detection of laminar type discontinues & planar defects may often be missed, unless they are favourably oriented.
4. Safety considerations imposed by X and g rays must be considered.
5. Relatively large capital costs and space requirement
6. Field inspection usually restricted to 75 mm of steel equivalent thickness.

Safety considerations

Experience, study & research as fall-out of the bombing of Hiroshima & Nagasaki and follow-up of many radiation accidents have conclusively proved that exposure to penetrating radiation have harmful effects on cells of living tissue. Also, as radiation cannot be detected by any of our five senses, strict compliance with the safety regulations is required.

Almost all countries that engage in production and/or use of radioactive materials have laid down specific safety regulations to control use of the radio-isotopes.

It is essential that radiographic testing personnel be continually aware of the radiation hazard and cognizant of safety regulations. Radiation monitoring equipment are addressed later.

Training and certification

It is recognised that the effectiveness of nondestructive testing depends on the capabilities of the personnel who are responsible for, and perform NDT. Thus all customers will require an assurance that NDT personnel whose specific jobs require appropriate knowledge of the technical principles underlying the nondestructive tests they perform, witness, monitor, or evaluate be qualified and certified. In addition every country may also have a program of training & certifying personnel responsible for performing radiography to be qualified & certified in safety aspects related use of radiation sources.

The American Society for Nondestructive Testing recommends the use of the documents “Recommended Practice no. SNT-TC-1A”. This document provides the employer with the necessary guidelines to properly qualify and certify the NDT technician in all methods. To comply with this document the employer must establish a “written practice” which describes in detail how the technician will be trained, examined and certified.

Current edition of SNT-TC-1A may be referred to determine the recommended number of hours of classroom instruction and months of experience necessary to be certified as a radiographic testing technician. Certification of NDT personnel always rests with the employer and is usually at three levels.

Level I - is qualified to perform specific calibrations, specific tests, and specific evaluations.

Level II - is qualified to set up and calibrate equipment and to interpret and evaluate results with respect to codes, standards and specifications. Must be able to prepare written instructions and report test results.

Level III - must be capable and responsible for establishing techniques, interpreting codes, and designating the test methods and techniques to be used . Must have a practical background in the technology and be familiar with other commonly used methods of NDT.

The SNT-TC-1A document recommends that level I and level II NDT technicians be examined in the following areas:

A. General examination.
B. Specific examination.
C. Practical examination.

The SNT-TC-1A document recommends that NDT level III personnel be examined in the following areas:

A. Basic examination.
B. Method examination.
C. Specific examination.

Lesson 2

Basic Radiography Physics

The basic particles - electron, proton, positron and neutron are described. Their places in nuclear and atomic structures are developed; an understanding of these will help in moving on to generation of electromagnetic radiation( X & g rays). The particles and the electromagnetic radiation (the fundamental probes) and their interaction with matter will then be discussed.

Structure of atom

Elements

An element is defined as a substance that cannot be broken chemically into simpler substances. There are over 100 elements known to man.

Elements may combine with one another to form molecules.

All unit blocks of an element (called atom) have a heavy, positively charged inner core (called nucleus) surrounded by electrons in discrete and specific orbits, called shells (K, L, M, N, ...). The nucleus is considered to be made up of protons and neutrons. For atoms to be neutral, the number of protons (called atomic number, Z) and the number of electrons in outer shells should be equal.

The total number of protons and neutrons in a nucleus is called the mass number A, (A = Z + N, N = number of neutrons).

The simplest of all elements is hydrogen with one proton and one orbital electron. The number of protons in a nucleus identifies the element and the number of electrons in the outermost shell decides the chemical properties of the element.

Usually, Z = N for light elements and N > Z for heavy elements.

Atom and its constituents

Elementary particles

The electron : The electron carries a negative charge. Some characteristics of the particle are given in Table 1-1.

The proton : The proton is a positively charged sub-atomic particle and is a constituent of the nucleus. Its charge is equal and opposite to that of the electron. It is heavier than the electron by about 1840 times. Some characteristics of the particle are described in Table 1-1.

The positron : The positron is identical to the electron in mass, charge, except that its charge is positive. It is extremely short lived and is capable of existence only in motion.

The neutron : Experiments indicated that there should be a sub-atomic particle that should have a neutral charge and a mass approximately that of proton. This particle was named neutron and today we know that it is a constituent of the nucleus. Some characteristics of the particle are described in Table 1-1.

Table 1-1 Characteristics of elementary particles

Particle	Charge, C, x 10 - 19	Rest mass, kg, x 10 - 31	Classical radius (m), x 10 - 15
Electron	-1.602	9.109	2.818
Proton	+1.602	1.673 x 10 +4	1.534 x 10+3
Neutron	Neutral	1.675 x 10 +4	1.532 x 10 +3

 

The number of protons in an atom determines the kind of atom or element.

All atoms that contains 2 protons are helium atoms.
All atoms that contain 4 protons are beryllium atoms.
All atoms that contain 8 protons are oxygen atoms.
All atoms that contain 26 protons are iron atoms, etc.

Basic elements may also be identified by their weight. Mass number or “A” number is a combination of protons and neutrons (heavy parts of the atom). Each atom is then assigned a number equal to the total number of protons and neutrons in the nucleus.

Radioactive materials

Isotopes : These are of the same element (i.e. same Z), but have different N. E.g. ²³⁵ U & ²³⁸ U. In this example the second isotope has three neutrons more than the first in its nucleus. Some of the isotopes are not stable. This is due to their nuclei having either too many or too few neutrons than required for stability. These nuclei reach a stable state by the emission of particles, either a ( He⁺⁺), or b- or b+ or by capturing an electron from the K shell and/or with the emission of photons, called g rays. These isotopes are termed radio-isotopes. On disintegration, the mass number of the nucleus may or may not change, but the atomic number changes.

Examples of radio-isotopes :

²³⁸ U ₉₂ ® ²³⁴ U ₉₂ + ⁴ He ₂ ++
²¹⁴ Pb ₈₂ ® ²¹⁴ Bi ₈₃ + b-

Notes :

1. Emission of alpha particle ( doubly ionised He nucleus) results in reducing Z by 2, A by 4.
2. Emission of b^- particle increase Z by 1, but does not change A.
3. Emission of b⁺ (positron) decreases Z by 1, but does not change A.
4. K capture results in decrease of Z by 1, but does not change A.

Thus a nucleus, on disintegration, changes to another element. Whenever a positron is emitted, it gets annihilated by an electron when it comes to rest, releasing 2 g photons of 0. 51 MeV each.

Ra - 226 is an example of natural isotope and Co-60, Ir-192, Tm-170 are artificial isotope. Cs-137 is derived by chemical separation of fission fragments.

Many isotopes of the various elements occur in nature, but artificial isotopes are now very common. Artificial isotopes are created by bombarding an element with an excess of neutrons. This is done in a nuclear reactor where the atomic fission process gives off large numbers of free neutrons. The basic element may capture some of the free neutrons. This increases the element’s “A” number or mass. Where these excess neutrons do not upset the balance of the nucleus, then this new isotope remain “stable”.

When these excess neutrons do upset the balance of the nucleus, the isotope is unstable and will disintegrate or decay into a more stable form. Unstable atoms are said to be radioactive. Some radioactive isotopes are found in nature, such as Radium and Uranium. Isotopes commonly used in radiography, such as Iridium 192 and Cobalt 60 are manmade.

When an element is made radioactive in the nuclear reactor, this process is known as “activation”.

When an unstable isotope is decaying or disintegrating, tiny particles traveling at high speeds are emitted and/or energy in the form of waves is given off. All radiation comes from the nucleus of the atoms.

The following particles and energies may be released from the radioactive atom:

“Alpha” particle - largest; radiation particle with 2 protons and 2 neutrons.
“Beta” particle - very light; high speed electron.
“Gamma” ray (not a particle); an energy wave.

No two radioactive isotopes have exactly the same decay pattern. A radioactive isotope can decay by any of the following modes:

1. Alpha emission only.
2. Beta emission only.
3. Alpha emission with associated gamma ray emission.
4. Beta emission with associated gamma ray emission.
5. Spontaneous fission.

Alpha particles usually are cut off by a sheet of paper and travel only a few inches in air, while beta particles can travel a few feet in air. Conventional radiography uses either gamma rays that are emitted by radio isotopes or those generated by X ray machines.

Unit of radioactivity

The basic unit of radioactive material is the “Curie”. When a radioactive material decays it is said to have an “activity” of one curie when 37 billion of its atoms disintegrate in one second. This is written 3.7 x 10¹⁰ disintegrations/second.

What is the activity of a radioactive source that has 185 billion disintegration/second ?

(Answer : Activity = 5 Curies)

It must be remembered that a source with a higher activity than another need not necessarily release more radiation.

When a Thulium 170 atom decays, ¼ of the atoms emit a beta particle and one gamma ray, and ¾ of the atoms emit beta particles with no gamma rays.

Specific activity of any radioactive source is measured in curies per gram. The maximum specific activity that a reactor facility can provide is limited by a neutron flux density of the reactor. We will learn later that the activity of the radio active isotope reduces with time. This does not mean, for a source of a given size, the specific activity reduces with time.

A reactor facility that can supply a given type of source with a high specific activity will usually be preferred. This is because a source with higher specific activity will be of smaller physical size for a given strength. We will learn later that smaller the source size, better the definition of radiograph.

Particulate radiation

There are three types of particulate radiation (alpha, beta and neutrons). While radiography using beta rays and neutrons are used as specialised techniques, the conventional radiography almost always uses x and gamma rays.

Particulate radiation is different from x and gamma rays as they have mass and do not travel at the speed of light.

However, particulate radiation will penetrate matter, will cause ionization and cannot be detected by human senses.

1. Alpha radiation has a positive charge and is slow and heavy. Alpha particles ionize atoms by removing electrons as they pass but they do not penetrate deeply.
2. Beta particles (high speed electrons) have a negative charge and because they are lightweight they are not as ionizing as alpha particles.
3. Neutron radiation has peculiar penetrating qualities. It penetrates many heavy elements with ease and is absorbed readily by many lighter elements, particularly hydrogen. This quality is just the reverse of x and gamma rays.

The neutron source is usually collimated and passes through the specimen to activate a conversion screen. When the activated conversion screen is exposed to X-ray film or some other image recorder, the image is transferred by the ionizing radiation from the conversion screen.

The half-life of a radio-isotope is the time it takes for ½ of the atoms to decay or disintegrate. There are radio isotopes with half-lives varying from a small fraction of second to hundreds of years. Lower the half-life, the specific activity, generally, will be high. Each radio isotopes is characterised by its emission (in terms of type, quantity and quality of radiation) and a specific half-life. Some isotopes decay rapidly ( short half-life ) therefore they have a high specific activity. Other isotopes decay slowly (long half life) and have a low specific activity.

Example - Cesium 137 has a half - life of 30 years. Whether you started with 1 gram or 10 pounds at the end of 30 years, you would only have ½ remaining.
After 150 years, what fraction of the original amount would you have left?
(Answer -1/20 th remaining)

Lesson 3

Penetration and Absorption

Electromagnetic radiation

Of the two types of radiation that are used in NDT (electromagnetic and particulate), x and gamma rays are called electromagnetic radiation. The following figure describes the electromagnetic spectrum of which x & gamma and visible light form the part. X and gamma rays are a family of waves that are called electromagnetic waves.



The spectrum is arranged in order by frequency of the wave. The waves with the lowest frequency are listed at the left end of the chart while the waves with the highest frequency are at the right.

Since visible light and X & gamma rays are members of the electromagnetic spectrum, they have many things in common.

1. Travel at the velocity of light (186,000 miles per second).
2. Travel in straight lines.
3. Not affected by magnetic or electric fields.
4. Will expose photographic film.

Wavelength is described as the distance between the peaks of the wave. The wavelength of x and gamma radiation is usually specified in Angstrom units (1 Angstrom unit is equal to 0.0001 microns or 10^--8 cm)). X and gamma rays that are used in industrial radiography have wavelengths 1 AU and smaller.

Frequency is described as the number of electromagnetic waves that pass a given point in one second. Remember, that all electromagnetic waves travel at the same velocity. Frequency is measured in “cycles per second”, a cycle being one complete wave, trough to trough or peak to peak.

The frequency and wavelength of electromagnetic waves are inversely proportional. This means that when one increases the other decreases by a proportionate amount. Double one and the other is reduced by one half. All X and gamma rays are considered to have the same amplitude or height or the same energy peak within each wave.

As x & gamma rays have such a high frequency and short wave length, they are able to penetrate opaque objects and expose photographic film. The higher energy is obtained from x and gamma rays with a high frequency and short wavelength.

X and gamma rays of the same energy are indistinguishable. While the gamma rays are emitted from the atomic nuclei of unstable isotopes, x rays result from the interactions between a rapidly moving stream of electrons and the atoms of a solid target materials.

Penetration and Absorption

X and Gamma rays possess the capability of penetrating materials.

In passing through matter, the amount of absorption at any point is dependent upon the thickness, atomic number and density. The depth of penetration depends upon the nature of materials that is exposed and the energy of the radiation used. This means that when an object is exposed to penetrating radiation part of the energy is absorbed and the rest transmitted. The intensity of transmitted radiation will vary depending upon the material thickness as well as the flaws that may be contained. When this variation is detected and recorded (usually on film), a means of imaging the internal structure of the material is available.

The latent image produced in the film becomes a shadow picture of the specimen when the film is processed.

It should be remembered x and gamma rays travel in a straight line. Therefore the latent image that is formed on a film is very much like the shadow of an object that is in front of a very strong source of light.



One characteristic of the penetrating radiation is its ability to ionise the materials through which they travel. This property is used to expose photographic films. When the radiation penetrates the film, the film is exposed because the rays ionize the tiny silver bromide grains in the film emulsion. The ionization of the film’s emulsion forms a “latent image” which is developed during later processing of the film. This process will be discussed later.

The specimen itself is an important consideration in making a radiograph. Enough rays must penetrate the objective to form an image but too many rays will overexpose the film. When the film is developed, the exposed portion turns dark while the unexposed portion will be clear.

The formation of an image on the film depends upon the amount of radiation received by different sections of the film. As shown below, a discontinuity such as a void represents a thickness difference in the specimen and will appear as a dark spot on the developed film.



If the discontinuity shown above had been an inclusion that was more dense than the specimen material (for example a tungsten inclusion in a weld) then the image on the film would have been lighter at that spot. The X-rays would have been absorbed more by the dense inclusion.