Postgraduate Study: Radiography, NDT M.Sc. Materials - PDF

Postgraduate Study Radiography NDT M.Sc. Materials 1. Introduction 1-1 History of Radiography X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845- 1923), a Professor at Wurzburg Univ...

Postgraduate Study Radiography NDT M.Sc. Materials 1. Introduction 1-1 History of Radiography X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845- 1923), a Professor at Wurzburg University in Germany. Working with a cathode-ray tube in his laboratory, Roentgen observed a fluorescent glow of crystals on a table near his tube. The tube that Roentgen was working with consisted of a glass envelope (bulb) with positive and negative electrodes. The air in the tube was evacuated, and when a high voltage was applied, the tube produced a fluorescent glow. Roentgen shielded the tube with heavy black paper and discovered a green-colored fluorescent light generated by a material located a few feet away from the tube. He concluded that a new type of ray was being emitted from the tube. This ray could pass through the heavy paper covering and excite the phosphorescent materials in the room. He found that the new ray could pass through most substances, casting shadows on 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, which was conducted late in 1895, was a film about the hand of his wife, Bertha. Interestingly, the first use of X-rays was for an industrial (not medical) application. Roentgen produced a radiograph of a set of weights in a box to show his colleagues. Roentgen's discovery was a scientific bombshell and was received with extraordinary interest by scientists and laymen. Scientists everywhere could duplicate his experiment because the cathode tube was well-known during this period. Many scientists dropped other lines of research to pursue the mysterious rays. Newspapers and magazines of the day provided the public with numerous stories, some true, others fanciful, about the properties of the newly discovered rays. Public fancy was caught by this invisible ray, which could pass through solid matter and provide a picture of bones and interior body parts in conjunction with a photographic plate. The demonstration of a wavelength shorter than light captured scientific fancy. This generated new possibilities in physics and in investigating the structure of matter. Much enthusiasm was generated about the potential applications of rays as an aid in medicine and surgery. Within a month after the announcement of the discovery, several medical radiographs had been made in Europe and the United States, which surgeons used to guide them in their work. In June 1896, only 6 months after Roentgen announced his discovery, X-rays were being used by battlefield physicians to locate bullets in wounded soldiers. Asst. Prof. Dr. Nasri S. M. Namer 1 Postgraduate Study Radiography NDT M.Sc. Materials Before 1912, X-rays were used little outside medicine and dentistry, though some X-ray pictures of metals were produced. X-rays were not used in industrial applications before this date because the X-ray tubes (the source of the X-rays) broke down under the voltages required to produce rays of satisfactory penetrating power for industrial purposes. However, that changed in 1913 when the high vacuum X-ray tubes designed by Coolidge became available. The high vacuum tubes were an intense and reliable X-ray source, operating at energies up to 100,000 volts. In 1922, industrial radiography took another step forward with the advent of the 200,000- volt X-ray tube that allowed radiographs of thick steel parts to be produced in a reasonable amount of time. In 1931, General Electric Company developed 1,000,000-volt X-ray generators, providing an effective tool for industrial radiography. That same year, the American Society of Mechanical Engineers (ASME) permitted X-ray approval of fusion welded pressure vessels, opening the door to industrial acceptance and use. A Second Source of Radiation 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 new research showed that certain types of atoms disintegrate by themselves. Henri Becquerel discovered this phenomenon while investigating the properties of fluorescent minerals. Becquerel was researching the principles of fluorescence, wherein certain minerals glow (fluoresce) when exposed to sunlight. He utilized photographic plates to record this fluorescence. 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 compounds in a drawer with his photographic plates. Later, when he developed these plates, he discovered 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 in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the radiation source was uranium. Unlike the X-rays, Becquerel’s discovery was virtually unnoticed by laymen and scientists alike. Relatively few scientists were interested in Becquerel's findings. It was not until the discovery of radium by the Curies two years later that interest in radioactivity became widespread. Asst. Prof. Dr. Nasri S. M. Namer 2 Postgraduate Study Radiography NDT M.Sc. Materials 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 called 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 radiographed castings up to 10 to 12 inches thick. 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 the use of gamma rays grew quickly in industrial radiography. Health Concerns The science of radiation protection, or "health physics" as it is more properly called, grew out of the parallel discoveries of X-rays and radioactivity in the closing years of the 19th century. Experimenters, physicians, laymen, and physicists set up X-ray-generating apparatuses and proceeded about their labors without concern regarding potential dangers. Such a lack of concern is quite understandable, for there was nothing in previous experience to suggest that X-rays would in any way be hazardous. Indeed, the opposite was the case, for who would suspect that a ray similar to light but unseen, unfelt, or otherwise undetectable by the senses would damage a person? More likely, or so it seemed to some, X-rays could benefit the body. Inevitably, the widespread and unrestrained use of X-rays led to serious injuries. Injuries were often not attributed to X-ray exposure, partly because of the slow onset of symptoms and because there was simply no reason to suspect X-rays as the cause. Some early experimenters did tie X-ray exposure and skin burns together. The first warning of possible adverse effects of X-rays came from Thomas Edison, William J. Morton, and Nikola Tesla, who each reported eye irritations from experimentation with X-rays and fluorescent substances. Today, it can be said that radiation ranks among the most thoroughly investigated causes of disease. Although much remains to be learned, more is known about the mechanisms of radiation damage on the molecular, cellular, and organ systems than for most other health- stressing agents. Indeed, it is precisely this vast accumulation of quantitative dose-response data that enables health physicists to specify radiation levels so that medical, scientific, and industrial uses of radiation may continue at levels of risk no greater than, and frequently less than, the levels of risk associated with any other technology. Asst. Prof. Dr. Nasri S. M. Namer 3 Postgraduate Study Radiography NDT M.Sc. Materials X-rays and Gamma rays are electromagnetic radiation of the same nature as light but of much shorter wavelength. The wavelength of visible light is 6000 angstroms, while the wavelength of x-rays is in the range of one angstrom, and that of gamma rays is 0.0001 angstrom. This very short wavelength gives x-rays and gamma rays their power to penetrate materials that light cannot. These electromagnetic waves are highly energetic and can break chemical bonds in materials they penetrate. If the irradiated matter is living tissue, breaking chemical bonds may result in altered structure or a change in the function of cells. Early exposure to radiation resulted in the loss of limbs and even lives. Men and women researchers collected and documented information on the interaction of radiation and the human body. This early information helped science understand how electromagnetic radiation interacts with living tissue. Unfortunately, much of this information was collected at great personal expense. 1-2 Present State of Radiography In many ways, radiography has changed little from the early days of its use. We still capture a shadow image on film using similar procedures and processes technicians used in the late 1800's. Today, however, we can generate images of higher quality and greater sensitivity through higher-quality films with a larger variety of film grain sizes. Film processing has evolved to an automated state, producing more consistent film quality by removing manual processing variables. Electronics and computers allow technicians to capture images digitally. "filmless radiography" provides a means of capturing an image, digitally enhancing it, sending it anywhere in the world, and archiving an image that will not deteriorate with time. Technological advances have provided the industry with smaller, lighter, portable equipment that produces high-quality X-rays. Using linear accelerators provides a means of generating extremely short wavelength, highly penetrating radiation, a concept dreamed of only a few short years ago. While the process has changed little, technology has evolved, allowing radiography to be widely used in numerous inspection areas. Radiography has seen expanded usage in the industry to inspect not only welds and castings but also to inspect items such as airbags and canned food products radiographically. Radiography is used in metallurgical material identification and security systems at airports and other facilities. Gamma-ray inspection has also changed considerably since the Curies discovered radium. Man-made isotopes of today are far stronger and offer the technician a wide range of energy levels and half-lives. The technician can select Co-60, which will effectively penetrate very thick materials, or select a lower energy isotope, such as Tm-170, which can be used to inspect plastics and very thin or low-density materials. Gamma rays have wide applications in industries such as petrochemicals, casting, welding, and aerospace. Asst. Prof. Dr. Nasri S. M. Namer 4 Postgraduate Study Radiography NDT M.Sc. Materials Addressing Health Concerns In the Manhattan District of the US Army Corps of Engineers, the name "health physics" was born, and great advances were made in radiation safety. From the onset, the leaders of the Manhattan District recognized that a new and intense source of radiation and radioactivity would be created. In the summer of 1942, the leaders asked Ernest O. Wollan, a cosmic ray physicist at the University of Chicago, to form a group to study and control radiation hazards. Thus, Wollan was the first to bear the title of health physicist. Carl G. Gamertsfelder soon joined him, recently graduating with a physics baccalaureate, and Herbert M. Parker, the noted British-American medical physicist. By mid-1943, six others had been added. These six include Karl Z. Morgan, James C. Hart, Robert R. Coveyou, O.G. Landsverk, L.A. Pardue, and John E. Rose. Within the Manhattan District, the name "health physicist" seems to have been derived in part from the need for secrecy (and hence a code name for radiation protection activities) and the fact that it was a group of mostly physicists working on health-related problems. Activities included developing appropriate monitoring instruments, physical controls, administrative procedures, monitoring radiation areas, personnel monitoring, and radioactive waste disposal. In the Manhattan District, many modern concepts of protection were born, including the rem unit, which took into account the biological effectiveness of the radiation. In the Manhattan District, radiation protection concepts realized maturity and enforceability. 1-3 Future Direction of Radiographic Education Although many methods and techniques developed over a century ago remain in use, computers are slowly becoming a part of radiographic inspection. The future of radiography will likely see many changes. As noted earlier, companies are performing many inspections without film aid. Radiographers of the future will capture images in digitized form and e-mail them to the customer when the inspection has been completed. Film evaluation will likely be left to computers. Inspectors may capture a digitized image, feed it into a computer, and wait for a printout of the image with an accept/reject report. Systems will be able to scan a part and present a three-dimensional image to the radiographer, helping him or her to locate the defect within the part. In the future, inspectors can peel away layer after layer of a part to evaluate the material in greater detail. Color images, much like computer-generated ultrasonic C-scans of today, will interpret indications much more reliably and less time- consuming. Asst. Prof. Dr. Nasri S. M. Namer 5 Postgraduate Study Radiography NDT M.Sc. Materials Educational techniques and materials must be updated to keep pace with technology and meet the industry's requirements. These needs may well be met with computers. Computer programs can simulate radiographic inspections using a computer-aided design (CAD) model of a part to produce physically accurate simulated x-ray radiographic images. Programs allow the operator to select different parts to inspect, adjust the placement and orientation of the part to obtain the proper equipment/part relationships and adjust all the usual X-ray generator settings to arrive at the desired radiographic film exposure. Computer simulation will likely have its greatest impact in the classroom, allowing the student to see results in almost real-time. Simulators and computers may become the primary tools for instructors and students in the technical classroom. 2. Physics of Radiography 2-1 Nature of Penetrating Radiation X-rays and gamma rays differ only in their source of origin. An X-ray generator produces X-rays, and gamma radiation is the product of radioactive atoms. They are both part of the electromagnetic spectrum. They are waveforms like light rays, microwaves, and radio waves. X-rays and gamma rays cannot be seen, felt, or heard. They possess no charge and no mass, are not influenced by electrical and magnetic fields, and generally travel in straight lines. However, they can be diffracted (bent) like light. Asst. Prof. Dr. Nasri S. M. Namer 6 Postgraduate Study Radiography NDT M.Sc. Materials Both X-rays and gamma rays can be characterized by frequency, wavelength, and velocity. However, they act somewhat like particles at times in that they occur as small "packets" of energy and are referred to as "photons." Due to their short wavelength, they have more energy to pass through matter than other forms of energy in the electromagnetic spectrum. As they pass through matter, they are scattered and absorbed, and the degree of penetration depends on the kind of matter and the energy of the rays. Properties of X-rays and Gamma-rays They are not detected by human senses (cannot be seen, heard, felt, etc.). They travel in straight lines at the speed of light. Electrical or magnetic fields cannot change their paths. They can be diffracted to a small degree at interfaces between two materials. They pass through matter until they have a chance encounter with an atomic particle. Their degree of penetration depends on their energy and the matter they are traveling through. They have enough energy to ionize matter and damage or destroy living cells. 2-2 X-Radiation X-rays are just like any other kind of electromagnetic radiation. They can be produced in parcels of energy called photons, just like light. Two different atomic processes can produce X-ray photons. One is called Bremsstrahlung, a German term meaning "breaking radiation." The other is called K-shell emission. They can both occur in the heavy atoms of tungsten. Tungsten is often the material chosen for the target or anode of the X-ray tube. Both ways of making X-rays involve a change in the state of electrons. However, Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when the velocity of the electron shot at the tungsten changes. The negatively charged electron slows down after swinging around the nucleus of a positively charged tungsten atom. This energy loss produces X-radiation. Electrons are scattered elastically and inelastically by the positively charged nucleus. The inelastically scattered electron loses energy, which appears as Bremsstrahlung. Elastically scattered electrons (which include backscattered electrons) are generally scattered through larger angles. In the interaction, Asst. Prof. Dr. Nasri S. M. Namer 7 Postgraduate Study Radiography NDT M.Sc. Materials many photons of different wavelengths are produced, but none have more energy than the electron had to begin with. After emitting the X-ray radiation spectrum, the original electron is slowed down or stopped. Bremsstrahlung Radiation X-ray tubes produce X-ray photons by accelerating a stream of electrons to energies of several hundred kilovolts with velocities of several hundred kilometers per hour and colliding them into a heavy target material. The abrupt acceleration of the charged particles (electrons) produces Bremsstrahlung photons. X-ray radiation with a continuous spectrum of energies is produced with a range from a few keV to a maximum of the energy of the electron beam. Target materials for industrial tubes are typically tungsten, which means that the wave functions of the bound tungsten electrons are required. The inherent filtration of an X- ray tube must be computed, which is controlled by the amount that the electron penetrates the surface of the target and by the type of vacuum window present. The bremsstrahlung photons generated within the target material are attenuated as they pass through typically 50 microns of the target material. The aluminum or beryllium vacuum window further attenuates the beam. The results are an elimination of the low energy photons, one keV through l5 keV, and a significant reduction in the portion of the spectrum from 15 keV through 50 keV. The spectrum from an X-ray tube is further modified by the filtration caused by the selection of filters used in the setup. K-shell Emission Radiation Remember that atoms have their electrons arranged in closed "shells" of different energies. The K-shell is the lowest energy state of an atom. An incoming electron can give a K-shell electron enough energy to knock it out of its energy state. About 0.1% of the electrons produce K-shell vacancies; most produce heat. Then, a tungsten electron of higher energy (from an outer shell) can fall into the K-shell. The energy lost by the falling electron shows up in an emitted X- ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, and so on. K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray photon comes out at a single wavelength. Asst. Prof. Dr. Nasri S. M. Namer 8 Postgraduate Study Radiography NDT M.Sc. Materials When outer-shell electrons drop into inner shells, they emit a quantized photon "characteristic" of the element. The energies of the characteristic X-rays produced are only very weakly dependent on the chemical structure in which the atom is bound, indicating that the non-bonding shells of atoms are the X-ray source. The resulting characteristic spectrum is superimposed on the continuum, as shown in the graphs below. An atom remains ionized for a very short time (about 10-14 seconds), and thus, an atom can be repeatedly ionized by the incident electrons, which arrive about every 10-12 seconds. 2-3 Gamma Radiation Gamma radiation is one of the three types of natural radioactivity. Gamma rays are electromagnetic radiation, like X-rays. The other two types of natural radioactivity are alpha and beta radiation, which are particles. Gamma rays are the most energetic form of electromagnetic radiation, with a short wavelength of less than one-tenth of a nanometer. Gamma radiation is the product of radioactive atoms. Depending on the ratio of neutrons to protons within its nucleus, an isotope of a particular element may be stable or unstable. When the binding energy is not strong enough to hold the nucleus of an atom together, the atom is said to be unstable. Atoms with unstable nuclei constantly change due to the energy imbalance within the nucleus. Over time, the nuclei of unstable isotopes spontaneously disintegrate or transform, known as radioactive decay. Various types of penetrating radiation may be emitted from the nucleus and/or its surrounding electrons. Nuclides which undergo radioactive decay are called radionuclides. Any material which contains measurable amounts of one or more radionuclides is radioactive. Asst. Prof. Dr. Nasri S. M. Namer 9 Postgraduate Study Radiography NDT M.Sc. Materials Types of Radiation Produced by Radioactive Decay When an atom undergoes radioactive decay, it emits one or more forms of radiation with sufficient energy to ionize the atoms with which it interacts. Ionizing radiation consists of high-speed subatomic particles ejected from the nucleus or electromagnetic radiation (gamma-rays) emitted by the nucleus or orbital electrons. Alpha Particles Certain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by the emission of alpha particles. These alpha particles are tightly bound units of two neutrons and two protons each (He4 nucleus) and have a positive charge. Emitting an alpha particle from the nucleus decreases two units of atomic number (Z) and four units of mass number (A). Alpha particles are emitted with discrete energies characteristic of the particular transformation from which they originate. All alpha particles from a particular radionuclide transformation will have identical energies. Asst. Prof. Dr. Nasri S. M. Namer 10 Postgraduate Study Radiography NDT M.Sc. Materials Beta Particles A nucleus with an unstable ratio of neutrons to protons may decay through the emission of a high-speed electron called a beta particle. This results in a net change of one unit of atomic number (Z). Beta particles have a negative charge, and the beta particles emitted by a specific radionuclide will range in energy from near zero up to a maximum value, which is characteristic of the particular transformation. Gamma-rays An excited nucleus may emit one or more photons (packets of electromagnetic radiation) of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead moves the nucleus from a higher to a lower energy state (unstable to stable). Gamma-ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes. 2-4 Activity (of Radionuclide) The quantity that expresses the degree of radioactivity or the radiation-producing potential of a given amount of radioactive material is activity. The curie was originally defined as the amount of any radioactive material that disintegrates at the same rate as one gram of pure radium. The curie has since been defined more precisely as a quantity of radioactive material with 3.7 x 1010 atoms disintegrating per second. The International System (SI) unit for activity is the Becquerel (Bq), which is the quantity of radioactive material in which one atom is transformed per second. The radioactivity of a given amount of radioactive material does not depend upon the mass of material present. For example, two one-curie sources of Cs-137 might have very different masses depending on the relative proportion of non-radioactive atoms in each source. Radioactivity is the number of curies or becquerels per unit mass or volume. The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called "specific activity." Specific activity is the number of curies or becquerels per unit mass or volume. Each gram of Cobalt-60 will contain approximately 50 curies. Iridium-192 will contain 350 curies for every gram of material. The shorter the half-life, the less material will be required to produce a given activity or curies. The higher specific activity of Iridium results in physically smaller sources. This Asst. Prof. Dr. Nasri S. M. Namer 11 Postgraduate Study Radiography NDT M.Sc. Materials allows technicians to place the source closer to the film while maintaining geometric unsharpness requirements on the radiograph. These unsharpness requirements may not be met if a source with a low specific activity were used at a similar source to film distances. 2-5 Isotope Decay Rate (Half-Life) Each radionuclide decays at its unique rate, which no chemical or physical process can alter. A useful measure of this rate is the half-life of the radionuclide. Half-life is defined as the time required for the activity of any particular radionuclide to decrease to one-half of its initial value. In other words, one-half of the atoms have reverted to a more stable state material. Half-lives of radionuclides range from microseconds to billions of years. The half-life of two widely used industrial isotopes is 74 days for Iridium-192 and 5.3 years for Cobalt-60. More exacting calculations can be made for the half-life of these materials. However, these times are commonly used. The applet below offers an interactive representation of the radioactive decay series. The four series represented are Th232, Ir192, Co60, Ga75, and C14. Use the radio buttons to select the series you want to study. Note that Carbon-14 is not used in radiography but is one of many useful radioactive isotopes used to determine the age of fossils. To learn more about Carbon-14 Dating, follow this link: Carbon-14 Dating. The Sequence Info button displays a chart that depicts the path of the series with atomic numbers indicated on the vertical axis on the left and the number of neutrons shown along the bottom. Colored arrows represent alpha and beta decays. To return to the main user interface, click the "Dismiss" button. Initially, a selected series contains all parent material, and the amount is represented by a colored bar on a vertical logarithmic scale. Each line represents a factor of ten. To step forward through the sequence by a specified number of years, you may type the appropriate number into the "Time Step" field and hit "Enter." A negative time step will backtrack through the sequence. You may choose a step interval in years and progress through each step by pressing the "Enter" key. The "Animate" button will automate the progress through the series. You can Asst. Prof. Dr. Nasri S. M. Namer 12 Postgraduate Study Radiography NDT M.Sc. Materials either choose a time step before you animate or leave it at zero. If the time step is left at zero, the system will choose time steps to optimize viewing performance. 2-6 Ionization As penetrating radiation moves from point to point in matter, it loses its energy through various interactions with the atoms it encounters. The rate at which this energy loss occurs depends upon the type and energy of the radiation and the density and atomic composition of the matter through which it is passing. The various types of penetrating radiation impart their energy to matter primarily through excitation and ionization of orbital electrons. The term "excitation" describes an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of x-rays while returning to a lower energy state. "ionization" refers to the complete removal of an electron from an atom following the transfer of energy from a passing charged particle. The term "specific ionization" is often used to describe the intensity of ionization. This is defined as the number of ion pairs formed per unit path length for a given type of radiation. Because of their double charge and relatively slow velocity, alpha particles have a high specific ionization and a relatively short range in matter (a few centimeters in air and only fractions of a millimeter in tissue). Beta particles have a much lower specific ionization than alpha particles and, generally, a greater range. For example, the relatively energetic beta particles from P32 have a maximum range of 7 meters in air and 8 millimeters in tissue. On the other hand, the low- energy betas from H3 are stopped by only 6 Asst. Prof. Dr. Nasri S. M. Namer 13 Postgraduate Study Radiography NDT M.Sc. Materials millimeters of air or 6 micrometers of tissue. Gamma-rays, x-rays, and neutrons are referred to as indirectly ionizing radiation since, having no charge; they do not directly apply impulses to orbital electrons as do alpha and beta particles. Electromagnetic radiation proceeds through matter until there is a chance of interaction with a particle. If the particle is an electron, it may receive enough energy to be ionized, whereupon it causes further ionization by direct interactions with other electrons. As a result, indirectly ionizing radiation (e.g. gamma, x-rays, and neutrons) can cause the liberation of directly ionizing particles (electrons) deep inside a medium. Because these neutral radiations undergo only chance encounters with matter, they do not have finite ranges but rather are attenuated exponentially. In other words, a given gamma ray has a definite probability of passing through any medium of any depth. Neutrons lose energy in matter by collisions which transfer kinetic energy. This process is called moderation and is most effective if the matter the neutrons collide with has about the same mass as the neutron. Once slowed down to the same average energy as the matter being interacted with (thermal energies), the neutrons have a much greater chance of interacting with a nucleus. Such interactions can result in material becoming radioactive or can cause radiation to be given off. 2-7 Newton's Inverse Square Law Any point source that spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius (r) is the source strength divided by the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound, and radiation obey the inverse square law. In inverse square law, a point radiation source can be characterized by the diagram above whether you are talking about Roentgens, rads, or rems. All measures of exposure will drop off by the inverse square law. For example, if the radiation exposure is 100 mR/hr at 1 inch from a source, the exposure will be 0.01 mR/hr at 100 inches. Asst. Prof. Dr. Nasri S. M. Namer 14 Postgraduate Study Radiography NDT M.Sc. Materials The applet below shows a radioactive source. The distance to the green source is shown below. You can also drag the little person and his Geiger counters around to a distance of your choice. When the mouse button is released, a point is plotted on the graph. The person's dosage at a particular distance is shown numerically and graphically. The graph allows you to confirm Newton's Inverse Square Law. If the distance is too small, the dosage will be too high, and our brave technician will face severe medical effects. To clear the graph, select a new material or the same one again. Moving the mouse from the white area to the gray will turn off the sound! What dosage in mR/hr is considered safe? Better find out! The red dosage lines represent 2, 5, and 100 mR/hr levels. Exercise: Assume you are standing three feet from a 15 Curie Cobalt-60 source. How many mR/hr dosages are you getting? Roentgens: A roentgen (R) measures the radiation intensity of X-rays or gamma rays. Rads: A rad is a unit of absorbed radiation in terms of the energy deposited in the tissue. The rad is defined as absorbed by 0.01 joules/kg of tissue. Rems: The biologically effective effects of radiation in rems are multiplied by a “quality factor,” which assesses the effectiveness of the particular type and energy of radiation. Asst. Prof. Dr. Nasri S. M. Namer 15 Postgraduate Study Radiography NDT M.Sc. Materials 2-8 Interaction Between Penetrating Radiation and Matter When x-rays or gamma rays are directed into an object, some of the photons interact with the particles of the matter, and their energy can be absorbed or scattered. This absorption and scattering is called attenuation. Other photons travel completely through the object without interacting with the material's particles. The number of photons transmitted through a material depends on the thickness, density, atomic number of the material, and the energy of the individual photons. When x-rays or gamma rays are directed into an object, some of the photons interact with the particles of the matter, and their energy can be absorbed or scattered. This absorption and scattering is called attenuation. Other photons travel completely through the object without interacting with the material's particles. The number of photons transmitted through a material depends on the thickness, density, atomic number of the material, and the energy of the individual photons. The formula that describes this curve is: The factor that indicates how much attenuation will occur per cm (10% in this example) is the linear attenuation coefficient, m. The above equation and the linear attenuation coefficient will be discussed in more detail on the following page. Asst. Prof. Dr. Nasri S. M. Namer 16 Postgraduate Study Radiography NDT M.Sc. Materials 2-9 Transmitted Intensity and Linear Attenuation Coefficient For a narrow beam of mono-energetic photons, the change in x-ray beam intensity at some distance in a material can be expressed in the form of an equation: Where: dI = the change in intensity I = the initial intensity n = the number of atoms/cm3 a proportionality constant that reflects the total probability of a photon  = being scattered or absorbed dx = the incremental thickness of material traversed When this equation is integrated, it becomes: The number of atoms/cm3 (n) and the proportionality constant () are usually combined to yield the linear attenuation coefficient (). Therefore, the equation becomes: Where: I = the intensity of photons transmitted across some distance x I0 = the initial intensity of photons a proportionality constant that reflects the total probability of a  = photon being scattered or absorbed  = the linear attenuation coefficient x = distance traveled The Linear Attenuation Coefficient () The linear attenuation coefficient () describes the fraction of a beam of x-rays or gamma rays absorbed or scattered per unit of absorber thickness. This value accounts for the number of atoms in a cubic cm of material and the probability of a photon being scattered or absorbed from the nucleus or an electron of one of these atoms. The linear attenuation Asst. Prof. Dr. Nasri S. M. Namer 17 Postgraduate Study Radiography NDT M.Sc. Materials coefficients for various materials and x-ray energies are available in various reference books. Using the transmitted intensity equation above, linear attenuation coefficients can be used to make several calculations. These include: the intensity of the energy transmitted through a material when the incident x-ray intensity, the material, and the material thickness are known. the intensity of the incident x-ray energy when the transmitted x-ray intensity, material, and material thickness are known. the thickness of the material when the incident and transmitted intensity and the material are known. the material can be determined from the m value when the incident, transmitted intensity, and material thickness are known. 2-10 Half-Value Layer The thickness of any given material where 50% of the incident energy has been attenuated is known as the half-value layer (HVL). The HVL is expressed in units of distance (mm or cm). Like the attenuation coefficient, it is photon energy dependent. Increasing the penetrating energy of a stream of photons will increase a material's HVL. The HVL is inversely proportional to the attenuation coefficient. If an incident energy of 1 and a transmitted energy of 0.5 is plugged into the equation introduced on the preceding page, it can be seen that the HVL multiplied by must equal 0.693. If x is the HVL, then times HVL must equal 0.693 (since the number 0.693 is the exponent value that gives a value of 0.5). Asst. Prof. Dr. Nasri S. M. Namer 18 Postgraduate Study Radiography NDT M.Sc. Materials Therefore, the HVL and m are related as follows: The HVL is often used in radiography because it is easier to remember values and perform simple calculations. In a shielding calculation, such as illustrated to the right, it can be seen that if the thickness of one HVL is known, it is possible to quickly determine how much material is needed to reduce the intensity to less than 1%. Approximate HVL for Various Materials when Radiation is from a Gamma Source Half-Value Layer, mm (inch) Source Concrete Steel Lead Tungsten Uranium Iridium-192 44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11) Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27) Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source Half-Value Layer, mm (inch) Peak Voltage (kVp) Lead Concrete 50 0.06 (0.002) 4.32 (0.170) 100 0.27 (0.010) 15.10 (0.595) 150 0.30 (0.012) 22.32 (0.879) 200 0.52 (0.021) 25.0 (0.984) 250 0.88 (0.035) 28.0 (1.102) 300 1.47 (0.055) 31.21 (1.229) 400 2.5 (0.098) 33.0 (1.299) 1000 7.9 (0.311) 44.45 (1.75) Note: The values presented on this page are intended for educational purposes. Other sources of information should be consulted when designing shielding for radiation sources. Asst. Prof. Dr. Nasri S. M. Namer 19 Postgraduate Study Radiography NDT M.Sc. Materials 2-11 Sources of Attenuation The attenuation resulting from the interaction between penetrating radiation and matter is not a simple process. A single interaction event between a primary X-ray photon and a particle of matter does not usually result in the photon changing to another form of energy and effectively disappearing. Several interaction events are usually involved, and the total attenuation is the sum of the attenuation due to different types of interactions. These interactions include the photoelectric effect, scattering, and pair production. The figure below shows an approximation of the total absorption coefficient, (µ), in red, for iron plotted as a function of radiation energy. The four radiation-matter interactions contributing to the total absorption are shown in black. The four types of interactions are photoelectric (PE), Compton scattering (C), pair production (PP), and Thomson or Rayleigh scattering (R). Since most industrial radiography is done in the 0.1 to 1.5 MeV range, it can be seen from the plot that photoelectric and Compton scattering account for the majority of attenuation encountered. Summary of different mechanisms that cause attenuation of an incident x-ray beam Photoelectric (PE) absorption of X-rays occurs when the X-ray photon is absorbed, resulting in the ejection of electrons from the outer shell of the atom and, hence, the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of an x-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image-making process. Photoelectron absorption is the dominant process for x-ray absorption up to energies of about 500 KeV. Photoelectron absorption is also dominant for atoms of high atomic numbers. Asst. Prof. Dr. Nasri S. M. Namer 20 Postgraduate Study Radiography NDT M.Sc. Materials Compton scattering (C) occurs when the incident X-ray photon is deflected from its original path by interacting with an electron. The electron gains energy and is ejected from its orbital position. The X-ray photon loses energy due to the interaction but continues to travel through the material along an altered path. Since the scattered X-ray photon has less energy, it has a longer wavelength than the incident photon. The event is also known as incoherent scattering because the photon energy change resulting from an interaction is not always orderly and consistent. The energy shift depends on the angle of scattering and not on the nature of the scattering medium Pair production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV but only becomes significant at energies around 10 MeV. Pair production occurs when an electron and positron are created with the annihilation of the x-ray photon. Positrons are short-lived and disappear (positron annihilation) by forming two photons of 0.51 MeV energy. Pair production is particularly important when high-energy photons pass through materials of a high atomic number. Below is another interaction phenomenon that can occur. Under special circumstances, these may need to be considered but are generally negligible. Thomson scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with the whole atom so that the photon is scattered with no change in internal energy to the scattering atom nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy. Scattering is mainly in the forward direction. Photodisintegration (PD) is the process by which the X-ray photon is captured by the nucleus of the atom with the ejection of a particle from the nucleus when all the X-ray energy is given to the nucleus. Because of the enormously high energies involved, this process may be neglected for the energies of X-rays used in radiography. Effect of Photon Energy on Attenuation Asst. Prof. Dr. Nasri S. M. Namer 21 Postgraduate Study Radiography NDT M.Sc. Materials Absorption characteristics will increase or decrease as the energy of the x-ray is increased or decreased. Since attenuation characteristics of materials are important in the development of contrast in a radiograph, understanding the relationship between material thickness, absorption properties, and photon energy is fundamental to producing a quality radiograph. A radiograph with higher contrast will provide a greater probability of detecting a given discontinuity. Understanding absorption is also necessary when designing X-ray and gamma-ray shielding, cabinets, or exposure vaults. 2-12 Geometric Unsharpness Geometric unsharpness refers to the loss of definition resulting from geometric factors of the radiographic equipment and setup. It occurs because the radiation does not originate from a single point but rather over an area. Consider the images below, which show two sources of different sizes, the paths of the radiation from each edge of the source to each edge of the feature of the sample, the locations where this radiation will expose the film, and the density profile across the film. In the first image, the radiation originates at a very small source. Since all the radiation originates from the same point, very little geometric un- sharpness is produced in the image. In the second image, the source size is larger, and the different paths that the rays of radiation can take from their point of origin in the source cause the edges of the notch to be less defined. The three factors controlling un-sharpness are source size, source-to-object distance, and object-to-detector distance. The source size is obtained by referencing manufacturers’ specifications for a given X-ray or gamma-ray source. Industrial X-ray tubes often have focal spot sizes of 1.5 mm squared, but microfocus systems have spot sizes in the 30- micron range. As the source size decreases, the geometric un-sharpness also decreases. For a given size source, the un-sharpness can also be decreased by increasing the source-to- object distance, but this comes with a reduction in radiation intensity. The object-to-detector distance is kept as small as possible to help minimize un-sharpness. However, there are situations, such as when using geometric enlargement, when the object is separated from the detector, which will reduce the definition.. Asst. Prof. Dr. Nasri S. M. Namer 22 Postgraduate Study Radiography NDT M.Sc. Materials Codes and standards used in industrial radiography require that geometric un-sharpness be limited. Generally, the allowable amount is 1/100 of the material thickness up to a maximum of 0.040 inches. These values refer to the degree of penumbra shadow in a radiographic image. Since the penumbra is not nearly as well defined as shown in the image to the right, it isn't easy to measure it in a radiograph. Therefore, it is typically calculated. The source size must be obtained or measured from the equipment manufacturer. Then, the un-sharpness can be calculated using the setup measurements. For the case, such as that shown to the right, where a sample of significant thickness is placed adjacent to the detector, the following formula is used to calculate the maximum amount of un-sharpness due to specimen thickness: Ug = f * b/a f = source focal-spot size a = distance from the source to the front surface of the object b = the thickness of the object For the case when the detector is not placed next to the sample, such as when geometric magnification is being used, the calculation becomes: Ug = f* b/a f = source focal-spot size. a = distance from the x-ray source to the front surface of material/object b = distance from the front surface of the object to the detector Asst. Prof. Dr. Nasri S. M. Namer 23 Postgraduate Study Radiography NDT M.Sc. Materials 2-13 Filters in Radiography At x-ray energies, filters consist of material placed in the useful beam to absorb, preferentially, radiation based on energy level or to modify the spatial distribution of the beam. Filtration is required to absorb the lower-energy X-ray photons emitted by the tube before they reach the target. Filters produce a cleaner image by absorbing the lower energy X-ray photons that tend to scatter more. The total filtration of the beam includes the inherent filtration (composed of part of the X- ray tube and tube housing) and the added filtration (thin sheets of metal inserted in the X- ray beam). Filters are typically placed at or near the x-ray port in the direct path of the x-ray beam. Placing a thin sheet of copper between the part and the film cassette has also proven an effective filtration method. The filters added to the x-ray beam are often constructed of high atomic number materials such as lead, copper, or brass for industrial radiography. Filters for medical radiography are usually made of aluminum (Al). The amount of inherent and the added filtration is stated in mm of Al or mm of Al equivalent. The amount of filtration of the X-ray beam is specified by and based on the voltage potential (keV) used to produce the beam. The filter materials' thickness depends on atomic numbers, kilovoltage settings, and the desired filtration factor. Gamma radiography produces relatively high energy levels at essentially monochromatic radiation. Therefore, filtration is not a useful technique and is seldom used. 2-14 Secondary (Scatter) Radiation and Undercut Control Secondary (Scatter) Radiation Secondary or scatter radiation must often be considered when producing a radiograph. The scattered photons create a loss of contrast and definition. Often, secondary radiation is considered to strike the film reflected from an object in the immediate area, such as a wall, or from the table or floor where the part is resting. Side scatter originates from walls or objects on the source side of the film. Control of side scatter can be achieved by moving objects in the room away from the film, moving the X-ray tube to the center of the vault, or placing a collimator at the exit port, thus reducing the diverging radiation surrounding the central beam. Asst. Prof. Dr. Nasri S. M. Namer 24 Postgraduate Study Radiography NDT M.Sc. Materials It is often called backscatter when it comes from objects behind the film. Industry codes and standards often require that a lead letter "B" be placed on the back of the cassette to verify the control of backscatter. If the letter "B" shows as a "ghost" image on the film, a significant amount of backscatter radiation reaches the film. The image of the "B" is often nondistinctive, as shown in the image to the right. The arrow points to the area of backscatter radiation from the lead "B" on the film's backside. The control of backscatter radiation is achieved by backing the film in the cassette with a sheet of lead that is at least 0.010 inches thick. It is a common practice in industry to place a 0.005" lead screen in front and a 0.010" screen behind the film. Undercut Undercut is another condition that must often be controlled when producing a radiograph. Parts with holes, hollow areas, or abrupt thickness changes are likely to suffer from undercut if controls are not put in place. The undercut appears as a darkening of the radiograph in the area of the thickness transition. This results in a loss of resolution or blurring at the transition area. Undercut occurs due to scattering within the film. At the edges of a part or areas where the part transitions from thick to thin, the intensity of the radiation reaching the film is much greater than in the thicker areas of the part. The high radiation intensity reaching the film results in a high level of scattering within the film. It should also be noted that the faster the film speed, the more undercut that is likely to occur. Scattering from within the walls of the part also contributes to undercut, but research has shown that scattering within the film is the primary cause. Masks are used to control undercut. Sheets of lead are cut to fill holes or surround the part, and metallic shot and liquid absorbers are often used as masks. Asst. Prof. Dr. Nasri S. M. Namer 25 Postgraduate Study Radiography NDT M.Sc. Materials 2-15 Radiation Safety Ionizing radiation is an extremely important NDT tool, but it can harm human health. For this reason, special precautions must be observed when using and working around ionizing radiation. Producing radioactive materials and using radiation-producing devices in the United States are governed by strict regulatory controls. The primary regulatory authority for most types and uses of radioactive materials is the Federal Nuclear Regulatory Commission (NRC). However, more than half of the states in the US have entered into an "agreement" with the NRC to assume regulatory control of radioactive material use within their borders. As part of the agreement process, the states must adopt and enforce regulations comparable to those found in Title 10 of the Code of Federal Regulations. Regulations for control of radioactive material used in Iowa are found in Chapter 136C of the Iowa Code. For most situations, the types and maximum quantities of radioactive materials possessed, how they may be used, and the individuals authorized to use radioactive materials are stipulated in the form of a "specific" license from the appropriate regulatory authority. In Iowa, this authority is the Iowa Department of Public Health. However, for certain institutions that routinely use large quantities of numerous types of radioactive materials, the exact quantities of materials and details of use may not be specified in the license. Instead, the license grants the institution the authority and responsibility for setting the specific requirements for radioactive material use within its facilities. These licensees are termed "broadscope" and require a Radiation Safety Committee and usually a full-time Radiation Safety Officer. Asst. Prof. Dr. Nasri S. M. Namer 26 Postgraduate Study Radiography NDT M.Sc. Materials 3. Equipment’s & Materials 3-1 X-ray Generators The major components of an X-ray generator are the tube, the high-voltage generator, the control console, and the cooling system. As discussed earlier in this material, X-rays are generated by directing a stream of high-speed electrons at a target material, such as tungsten, which has a high atomic number. When the electrons are slowed or stopped by the interaction with the atomic particles of the target, X-radiation is produced. This is accomplished in an X-ray tube such as the one shown here. The X-ray tube is one of the components of an X-ray generator; tubes come in various shapes and sizes. The image below shows a portion of the Roentgen tube collection of Grzegorz Jezierski, a professor at Opole University of Technology. The tube cathode (filament) is heated with a low- voltage current of a few amps. The filament heats up, and the wires' electrons become loosely held. The high-voltage generator creates A large electrical potential between the cathode and the anode. Electrons that break free of the cathode are strongly attracted to the anode target. The stream of electrons between the cathode and the anode is the tube current. The tube current is measured in milliamps and is controlled by regulating the low-voltage, heating current applied to the cathode. The higher the temperature of the filament, the larger the number of electrons that leave the cathode and travel to the anode. The milliamp or current setting on the control console regulates the filament temperature, which relates to the intensity of the X-ray output. The high voltage between the cathode and the anode affects the speed at which the electrons travel and strike the anode. The higher the kilovoltage, the more speed and energy the electrons have when they strike the anode. Electrons striking with more energy results in X- rays with more penetrating power. The high-voltage potential is measured in kilovolts, controlled with the voltage or kilovoltage control on the control console. An increase in the kilovoltage will also increase the intensity of the radiation. A focusing cup concentrates the stream of electrons to a small area of the target called the focal spot. The focal spot size is an important factor in the system's ability to produce a sharp image. See the information on image resolution (see section 4-1) and geometric un- sharpness (see section 2-12) for more information on the effect of the focal spot size. Asst. Prof. Dr. Nasri S. M. Namer 27 Postgraduate Study Radiography NDT M.Sc. Materials Much of the energy applied to the tube is transformed into heat at the focal spot of the anode. As mentioned above, the anode target is commonly made from tungsten, which has a high melting point and atomic number. However, cooling of the anode by active or passive means is necessary. Water or oil-recirculating systems are often used to cool tubes. Some low-power tubes are cooled simply using thermally conductive materials and heat-radiating fins. It should also be noted that to prevent the cathode from burning up and arcing between the anode and the cathode, all of the oxygen is removed from the tube by pulling a vacuum. Some systems have external vacuum pumps to remove any oxygen that may have leaked into the tube. However, most industrial X-ray tubes require a warm-up procedure to be followed. This warm-up procedure raises the tube current and voltage to slowly burn any available oxygen before the tube is operated at high power. The other important component of an X-ray-generating system is the control console. Consoles typically have a keyed lock to prevent unauthorized use of the system. They will have a button to start the generation of X-rays and a button to stop the generation of X-rays manually. The three main adjustable controls regulate the kilovolts tube voltage, millivolts' amperage, and the exposure time in minutes and seconds. Some systems also have a switch to change the focal spot size of the tube. X-ray Generator Options Kilovoltage - X-ray generators come in a large variety of sizes and configurations. There are stationary units for lab or production environments and portable systems that can be easily moved to the job site. Systems are available in a wide range of energy levels. When inspecting large steel or heavy metal components, systems capable of producing millions of electron volts may be necessary to penetrate the full thickness of the material. Alternately, small, lightweight components may only require a system capable of producing only a few tens of kilovolts. Focal Spot Size - Another important consideration is the focal spot size of the tube since these factors affect the geometric un-sharpness of the image produced. Generally, the smaller the spot size, the better. However, as the electron stream is focused on a smaller area, the tube's power must be reduced to prevent overheating at the tube anode. Therefore, the focal spot size becomes a tradeoff between resolving capability and power. Generators can be classified as conventional, mini-focus, and microfocus Asst. Prof. Dr. Nasri S. M. Namer 28 Postgraduate Study Radiography NDT M.Sc. Materials systems. Conventional units have focal spots larger than about 0.5 mm, minifocus units have focal spots ranging from 50 to 500 microns (.050 mm to.5 mm), and microfocus systems have focal spots smaller than 50 microns. Smaller spot sizes are especially advantageous when the magnification of an object or region of an object is necessary. The cost of a system typically increases as the spot size decreases and some microfocus tubes exceed $100,000. Some manufacturers combine two filaments of different sizes to make a dual-focus tube. This usually involves a conventional and a mini focus spot size, adding flexibility to the system. AC and Constant Potential Systems – AC X-ray systems supply the tube with sinusoidal varying alternating current. They produce X-rays only during one-half of the 1/60th second cycle. This produces bursts of radiation rather than a constant stream. Additionally, the voltage changes over the cycle, and the X-ray energy varies as the voltage ramps up and back down. Only some of the radiation is useable, and low-energy radiation must usually be filtered out. Constant potential generators rectify the AC wall current and supply the tube with DC. This results in a constant stream of relatively consistent radiation. Most newer systems now use constant potential generators. Flash X-Ray Generators Flash X-ray generators produce short, intense bursts of radiation. These systems are useful when examining objects in rapid motion or when studying transient events such as the tripping of an electrical breaker. In these situations, high-speed video rapidly captures images from an image intensifier or other real-time detector. Since the exposure time for each image is very short, a high level of radiation intensity is needed to get a usable output from the detector. The generator supplies microsecond bursts of radiation to prevent the imaging system from becoming saturated from continuous exposure to high-intensity radiation. The tubes of these X-ray generators do not have a heated filament; instead, electrons are pulled from the cathode by the strong electrical potential between the cathode and the anode. This process is known as field emission or cold emission and can produce electron currents in the thousands of amperes. 3-2 Radio Isotope (Gamma) Sources Manmade radioactive sources are produced by introducing an extra neutron to atoms of the source material. As the material rids itself of the neutron, energy is released in the form of gamma rays. Two more common industrial gamma-ray sources for industrial radiography are iridium-192 and cobalt-60. These isotopes emit radiation in a few discreet wavelengths. Cobalt-60 will emit a 1.33 and a 1.17 MeV gamma ray, and iridium-192 will emit 0.31, 0.47, and 0.60 MeV gamma rays. Compared to an X-ray generator, cobalt- 60 produces energies comparable to a 1.25 MeV X-ray system and iridium-192 to a 460 keV X-ray system. These high energies make penetrating thick materials with a relatively Asst. Prof. Dr. Nasri S. M. Namer 29 Postgraduate Study Radiography NDT M.Sc. Materials short exposure time possible. This and the fact that sources are very portable are why gamma sources are widely used for field radiography. Of course, the disadvantage of a radioactive source is that it can never be turned off, and safely managing the source is a constant responsibility. The physical size of isotope materials varies between manufacturers, but generally, an isotope material is a pellet that measures 1.5 mm x 1.5 mm. Depending on the level of activity desired, a pellet or pellets are loaded into a stainless-steel capsule and sealed by welding. The capsule is attached to a short flexible cable called a pigtail. The source capsule and the pigtail are housed in a shielding device called an exposure device or camera. Depleted uranium is often used as a shielding material for sources. The exposure device for iridium-192 and cobalt-60 sources will contain 45 pounds and 500 pounds of shielding materials, respectively. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. When the source is not used to make an exposure, it is locked inside the device. Asst. Prof. Dr. Nasri S. M. Namer 30 Postgraduate Study Radiography NDT M.Sc. Materials To make a radiographic exposure, a crank-out mechanism and a guide tube are attached to opposite ends of the exposure device. The guide tube often has a collimator at the end to shield the radiation except in the direction necessary for the exposure. The end of the guide tube is secured where the radiation source needs to be to produce the radiograph. The crank-out cable is stretched as far as possible to put as much distance as possible between the exposure device and the radiographer. To make the exposure, the radiographer quickly cranks the source out of the exposure device and into position in the collimator at the end of the guide tube. At the end of the exposure time, the source is cranked back into the exposure device. A series of safety procedures, including several radiation surveys, must be accomplished when exposing oneself to a gamma source. See the radiation safety material for more information. Asst. Prof. Dr. Nasri S. M. Namer 31 Postgraduate Study Radiography NDT M.Sc. Materials 3-3 Radiographic Film X-ray films for general radiography consist of an emulsion- gelatin containing radiation-sensitive silver halide crystals, such as silver bromide or silver chloride, and a flexible, transparent, blue-tinted base. The emulsion differs from those used in other photography films to account for the distinct characteristics of gamma rays and X-rays, but X-ray films are sensitive to light. Usually, the emulsion is coated on both sides of the base in layers about 0.0005 inches thick. Putting emulsion on both sides of the base doubles the amount of radiation-sensitive silver halide and thus increases the film speed. The emulsion layers are thin enough, so developing, fixing, and drying can be done reasonably. A few of the films used for radiography only have emulsion on one side, producing the greatest detail in the image. When x-rays, gamma rays, or light strike the grains of the sensitive silver halide in the emulsion, some of the Br- ions are liberated and captured by the Ag+ ions. This change is of such a small nature that it cannot be detected by ordinary physical methods and is called a "latent (hidden) image." However, the exposed grains are now more sensitive to the reduction process when exposed to a chemical solution (developer), and the reaction results Asst. Prof. Dr. Nasri S. M. Namer 32 Postgraduate Study Radiography NDT M.Sc. Materials in the formation of black, metallic silver. This silver, suspended in the gelatin on both sides of the base, creates an image. Film Selection When radiographing any particular component, the selection of a film depends on several factors. Below are some factors that must be considered when selecting a film and developing a radiographic technique. 1. Composition, shape, and size of the part being examined and, in some cases, its weight and location. 2. Type of radiation used, whether x- rays from an x-ray generator or gamma rays from a radioactive source. 3. Kilovoltages available with the x-ray equipment or the intensity of the gamma radiation. 4. Relative importance of high radiographic detail or quick and economical results. Selecting the proper film and developing the optimal radiographic technique usually involves arriving at a balance between several opposing factors. For example, if the high resolution and contrast sensitivity are important, a slower and finer-grained film should be used instead of a faster film. Film Packaging Radiographic film can be purchased in several different packaging options. The most basic form is as individual sheets in a box. In preparation for use, each sheet must be loaded into a cassette or film holder in the darkroom to protect it from exposure to light. The sheets are available in various sizes and can be purchased with or without interleaving paper. Interleaved packages have a layer of paper that separates each piece of film. The interleaving paper is removed before the film is loaded into the film holder. Many users find the interleaving paper useful in separating the sheets of film and offers some protection against scratches and dirt during handling. Asst. Prof. Dr. Nasri S. M. Namer 33 Postgraduate Study Radiography NDT M.Sc. Materials Industrial X-ray films are also available in a form in which each sheet is enclosed in a light- tight envelope. The film can be exposed from either side without being removed from the protective packaging. A rip strip makes removing the film in the darkroom easy to process. This form of packaging eliminates the process of loading the film holders in the darkroom. The film is completely protected from finger marks and dirt until the time the film is removed from the envelope for processing. Packaged film is also available in rolls, which allows the radiographer to cut the film to any length. The ends of the packaging are sealed with electrical tape in the darkroom. In applications such as the radiography of circumferential welds and the examination of long joints on an aircraft fuselage, long lengths of film offer great economic advantage. The film is wrapped around the outside of a structure, and the radiation source is positioned on the axis inside, allowing for examining a large area with a single exposure. The envelope-packaged film can be purchased with the film sandwiched between two lead oxide screens. The screens function to reduce scatter radiation at energy levels below 150 keV and as intensification screens above 150 keV. Film Handling X-ray film should always be handled carefully to avoid physical strains, such as pressure, creasing, buckling, friction, etc. Whenever films are loaded in semi-flexible holders, and external clamping devices are used, care should be taken to ensure the pressure is uniform. If a film holder bears against a few high spots, such as on an underground weld, the pressure may be great enough to produce desensitized areas in the radiograph. This precaution is particularly important when using envelope-packed films. Marks resulting from contact with fingers that are moist or contaminated with processing chemicals and crimp marks are avoided if large films are always grasped by the edges and allowed to hang free. A supply of clean towels should be kept close to the hand as an incentive to dry the hands often and well. Using envelope-packed films avoids many problems until the envelope is opened for processing. Another important precaution is avoiding rapidly drawing film from cartons, exposure holders, or cassettes. Such care will help to eliminate circular or treelike black markings in the radiograph that sometimes result from static electric discharges. Asst. Prof. Dr. Nasri S. M. Namer 34 Postgraduate Study Radiography NDT M.Sc. Materials 3-4 Exposure Vaults & Cabinets Exposure vaults and cabinets allow personnel to work safely in the area while exposures occur. Exposure vaults tend to be larger walk-in rooms with shielding provided by high-density concrete blocks and lead. Exposure cabinets are often self-contained units with integrated X-ray equipment and are typically shielded with steel and lead to absorb X-ray radiation. Exposure vaults and cabinets have protective interlocks that disable the system if anything interrupts the enclosure's integrity. Additionally, walk-in vaults are equipped with emergency "kill buttons" that allow radiographers to shut down the system if it should accidentally be started while they are in the vault. Asst. Prof. Dr. Nasri S. M. Namer 35 Postgraduate Study Radiography NDT M.Sc. Materials 4. Techniques & Calibrations 4-1 Image Considerations The usual objective in radiography is to produce an image showing the most detail possible. This requires careful control of several different variables that can affect image quality. Radiographic sensitivity is a measure of the quality of an image in terms of the smallest detail or discontinuity that may be detected. Radiographic sensitivity depends on the combined effects of two independent sets of variables. One set of variables affects the contrast , and the other set of variables affects the image's definition. Radiographic contrast is the degree of density difference between two areas on a radiograph. The contrast makes distinguishing features of interest, such as defects, from the surrounding area easier. The image to the right shows two radiographs of the same step wedge. The upper radiograph has a high contrast level, and the lower radiograph has a lower contrast level. While they are both imaging the same change in thickness, the high-contrast image uses a larger change in radiographic density to show this change. There is a small circle of equal density in both of the two radiographs. It is much easier to see in the high-contrast radiograph. The factors affecting contrast will be discussed in more detail on the following page. Asst. Prof. Dr. Nasri S. M. Namer 36 Postgraduate Study Radiography NDT M.Sc. Materials The radiographic definition is the abrupt change from one area of a given radiographic density to another. In contrast, the definition also makes it easier to see features of interest, such as defects, but differently. In the image to the right, the upper radiograph has a high level of definition, and the lower radiograph has a lower level of definition. In the high-definition radiograph, it can be seen that a change in the thickness of the step wedge translates to an abrupt change in radiographic density. It can be seen that the details, particularly the small circle, are much easier to see in the high-definition radiograph. It can be said that the detail portrayed in the radiograph is equivalent to the physical change present in the step wedge. In other words, a faithful visual reproduction of the step wedge was produced. In the lower image, the radiographic setup did not produce a faithful visual reproduction. The edge line between the steps is blurred. The gradual transition between the high and low-density areas on the radiograph evidences this. The factors affecting the definition will be discussed in more detail on the following page. Since radiographic contrast and definition are not dependent upon the same set of factors, it is possible to produce radiographs with the following qualities: Low contrast and poor definition High contrast and poor definition Low contrast and good definition High contrast and good definition 4-2 Radiographic Contrast As mentioned on the previous page, radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image, and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector (film) contrast. further exposes the film and increases contrast. Asst. Prof. Dr. Nasri S. M. Namer 37 Postgraduate Study Radiography NDT M.Sc. Materials Contrast Subject Factors Film Factors Absorption differences in Type of film (grain size) the subject. Chemistry of film processing chemicals. Wavelength of the primary Concentrations of film radiation. processing chemicals. Development film. Amount of film agitation. Secondary radiation from Overall film density scatter. Subject Contrast Subject contrast is the ratio of radiation intensities transmitted through different areas of the evaluated component. It depends on the absorption differences in the component, the wavelength of the primary radiation, and the intensity and distribution of secondary radiation due to scattering. Unsurprisingly, absorption differences within the subject will affect the contrast level in a radiograph. The larger the difference in thickness or density between two areas of the subject, the larger the difference in radiographic density or contrast. However, it is also possible to radiograph a particular subject and produce two radiographs having entirely different contrast levels. Generating X-rays using a low kilovoltage will generally result in a radiograph with high contrast. This occurs because low-energy radiation is more easily attenuated. Therefore, the ratio of photons transmitted through a thick and thin area will be greater with low-energy radiation. This will result in the film being exposed to a greater and lesser degree in the two areas. There is a tradeoff, however. Generally, as contrast sensitivity increases, the latitude of the radiograph decreases. Radiographic latitude refers to the range of material thickness that can be imaged This means that more areas of different thicknesses will be visible in the image. Therefore, the goal is to balance radiographic contrast and latitude so that there is enough contrast to identify the features of interest and ensure the latitude is great enough to inspect all areas of interest with one radiograph. In thick parts with a large range of thicknesses, multiple radiographs will likely be necessary to get the necessary density levels in all areas. Asst. Prof. Dr. Nasri S. M. Namer 38 Postgraduate Study Radiography NDT M.Sc. Materials Film Contrast Film contrast refers to density differences that result from the type of film used, how it was exposed, and how it was processed. Since there are other detectors besides film, this could be called detector contrast, but the focus here will be on film. Exposing a film to produce higher film densities will generally increase the contrast in the radiograph. A typical film characteristic curve, which shows how a film responds to different amounts of radiation exposure, is shown to the right. (More information on film characteristic curves is presented later in this section.) From the shape of the curves, it can be seen that when the film has not seen many photon interactions (which will result in a low film density), the slope of the curve is low. In this region of the curve, a large change in exposure produces a small change in film density. Therefore, the sensitivity of the film is relatively low. It can be seen that changing the log of the relative exposure from 0.75 to 1.4 only changes the film density from 0.20 to about 0.30. However, at film densities above 2.0, the slope of the characteristic curve for most films is at its maximum. In this region of the curve, a relatively small change in exposure will result in a relatively large change in film density. For example, changing the log of relative exposure from 2.4 to 2.6 would change the film density from 1.75 to 2.75. Therefore, the film's sensitivity is high in this curve region. Generally, the highest overall film density that can be conveniently viewed or digitized will have the highest contrast level and contain the most useful information. Lead screens in the thickness range of 0.004 to 0.015 inches typically reduce scatter radiation at energy levels below 150,000 volts. Above this point, they will emit electrons to provide more film exposure to ionizing radiation, thus increasing the density and contrast of the radiograph. Fluorescent screens produce visible light when exposed to radiation and this light. 4-3 Definition As mentioned previously, the radiographic definition is the abruptness of change from one density to another. Geometric factors of the equipment, the radiographic setup, and film and screen factors affect the definition. Geometric factors include the size of the area of origin of the radiation, the source-to-detector (film) distance, the specimen-to-detector (film) distance, movement of the source, specimen, or detector during exposure, the angle between the source and some feature and the abruptness of the change in specimen thickness or density. Asst. Prof. Dr. Nasri S. M. Namer 39 Postgraduate Study Radiography NDT M.Sc. Materials Geometric Factors The effect of source size, source-to-film distance, and specimen-to-detector distance were covered in detail on the geometric unsharpness (see sections 2-12) page. But briefly, to produce the highest level of definition, the focal spot or source size should be as close to a point source as possible, the source-to-detector distance should be as great as practical, and the specimen-to-detector distance should be as small as practical. This is shown graphically in the images below. Asst. Prof. Dr. Nasri S. M. Namer 40 Postgraduate Study Radiography NDT M.Sc. Materials The angle between the radiation and some features will also affect the definition. If the radiation is parallel to an edge or linear discontinuity, a sharp distinct boundary will be seen in the image. However, if the radiation is not parallel with the discontinuity, the feature will appear distorted, out of position, and less defined in the image. Abrupt changes in thickness and/or density will appear more defined in a radiograph than will areas of gradual change. For example, consider a circle. Its largest dimension will be a cord that passes through its centerline. The thickness gradually decreases as the cord moves away from the centerline. It is sometimes difficult to locate the edge of a void due to this gradual change in thickness. Lastly, any movement of the specimen, source, or detector during the exposure will reduce definition. Similar to photography, any movement will result in blurring of the image. Vibration from nearby equipment may be an issue in some inspection situations. Film and Screen Factors The last set of factors concerns the film and fluorescent screens. A fine-grain film can produce an image with a higher level of definition than a coarse-grain film. The wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased film development will increase the radiograph's apparent graininess. The use of fluorescent screens also results in lower definition. This occurs for a couple of different reasons. Fluorescent screens are sometimes used because incident radiation causes them to give off light that helps to expose the film. However, the light they produce spreads in all directions, exposing the film in adjacent areas and areas that are in direct contact with the incident radiation. Fluorescent screens also produce screen mottle on radiographs. Screen mottle is associated with the statistical variation in the number of photons interacting with the screen from one area to the next. Asst. Prof. Dr. Nasri S. M. Namer 41 Postgraduate Study Radiography NDT M.Sc. Materials 4-4 Radiographic Density Photographic, radiographic, or film density measures the degree of film darkening. Technically it should be called "transmitted density" when associated with transparent-base film since it measures the light transmitted through the film. Density is a logarithmic unit that describes a ratio of two measurements. Specifically, it is the log of the intensity of light incident on the film (I0) to the intensity of light transmitted through the film (It). Similar to the decibel, using the log of the ratio allows ratios of various sizes to be described using easy-to-work numbers. The following table shows the relationship between the amount of transmitted light and the calculated film density. Film Transmittance Percent Density (I0/It) Transmittance Log(I0/It) 1.0 100% 0 0.1 10% 1 0.01 1% 2 0.001 0.1% 3 0.0001 0.01% 4 0.00001 0.001% 5 0.000001 0.0001% 6 0.0000001 0.00001% 7 From this table, it can be seen that a density reading of 2.0 is the result of only one percent of the incident light making it through the film. At a density of 4.0, only 0.01% of transmitted light reaches the far side of the film. Industrial codes and standards typically require a radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common film viewers. For an evaluation above 4.0, extremely bright viewing lights are necessary. Contrast within a film increases with increasing density, so the higher the density the better. When digitizing radiographs, densities above 4.0 are often used since digitization systems can capture and redisplay for easy viewing information from up to 6.0. Film density is measured with a densitometer. A densitometer has a photoelectric sensor that measures the amount of light transmitted through a piece of film. The film is placed between the light source and the sensor, and the instrument produces a density reading. Asst. Prof. Dr. Nasri S. M. Namer 42 Postgraduate Study Radiography NDT M.Sc. Materials 4-5 Film Characteristic Curves In film radiography, the number of photons reaching the film determines how dense the film will become when other factors, such as the developing time, are held constant. The number of photons reaching the film is a function of the intensity of the radiation and the time that the film is exposed to the radiation. The term used to describe controlling the number of photons reaching the film is “exposure.” Film Characteristic Curves Different types of radiographic film respond differently to a given amount of exposure. Film manufacturers commonly characterize their film to determine the relationship between the applied exposure and the resulting film density. This relationship commonly varies over film densities, so the data is presented as a curve such as the one for Kodak AA400 shown to the right. The plot is called a film characteristic curve, sensitometry curve, density curve, or H and D curve (named for developers Hurter and Driffield). "Sensitometry" is the science of measuring the response of photographic emulsions to light or radiation. A log scale is used, or the values are reported in log units on a linear scale to compress the x-axis. Also, relative exposure values (unitless) are often used. Relative exposure is the ratio of two exposures. For example, if one film is exposed at 100 keV for 6mAmin and a second film is exposed at the same energy for 3mAmin, the relative exposure would be 2. The image directly to the right shows three film characteristic curves with the relative exposure plotted on a log scale. In contrast, the image below and to the right shows the log relative exposure plotted on a linear scale. Asst. Prof. Dr. Nasri S. M. Namer 43 Postgraduate Study Radiography NDT M.Sc. Materials Using the logarithm of the relative exposure scale makes it easy to compare two sets of values, which is the primary use of the curves. Film characteristic curves can adjust the exposure used to produce a radiograph with a certain density to an exposure that will produce a second radiograph of higher or lower film density. The curves can also be used to relate the exposure produced with one type of film to the exposure needed to produce a radiograph of the same density with a second type of film. Adjusting the Exposure to Produce a Different Film Density Suppose Film B was exposed to 140 keV at 1mA for 10 seconds, and the resulting radiograph had a density in the region of interest 1.0. Specifications typically require a density above 2.0 for reasons discussed on the film density page. The film characteristic curve determines the relative exposures for the actual density and desired density. The ratio of these two values is used to adjust the actual exposure. This first example will use a plot with log relative exposure and a linear x-axis. From the graph, first, determine the difference between the relative exposures of the actual and the desired densities. A target density 2.5 is used to ensure the exposure produces a density above the 2.0 minimum requirement. The log relative exposure of a density of 1.0 is 1.62, and the log of the relative exposure when the density of the film is 2.5 is 2.12. The difference between the two values is 0.5. Take the anti-log of this value to change it from log relative exposure to simply the relative exposure, which is 3.16. Therefore, the exposure to produce the initial radiograph with a 1.0 density must be multiplied by 3.16 to produce a radiograph with the desired density of 2.5. The exposure of the original x-ray was 10 mAs, so the new exposure must be 10 mAs x 3.16 or 31.6 mAs at 140 keV. Adjusting the Exposure to Allow Use of a Different Film Type Another use of film characteristic curves is to adjust the exposure when switching film types. The location of the characteristic curves of different films along the x-axis relates to the film speed of the films. The farther to the right that a curve is on the chart, the slower the film speed. It must be noted that the two curves being used must have been produced with the same radiation energy. The shape of the characteristic curve is largely independent of the wavelength of the x-ray or gamma radiation. Still, the location of the curve along the x-axis, concerning the curve of another film, does depend on radiation quality. Asst. Prof. Dr. Nasri S. M. Namer 44 Postgraduate Study Radiography NDT M.Sc. Materials Suppose an acceptable radiograph density of 2.5 was produced by exposing Film A for 30 seconds at 1mA and 130 keV. Now, it is necessary to inspect the part using Film B. The exposure can be adjusted by following the above method if the two film characteristic curves are produced with roughly the same radiation quality. For this example, the characteristic curves for Film A and B are shown on a chart showing relative exposure on a log scale. The relative exposure that produced a density of 2.5 on Film A is found to be 68. The relative exposure that should produce a density of 2.5 on Film B is 140. The relative exposure of Film B is about twice that of Film A, or 2.1, to be more exact. Therefore, to produce a 2.5-density radiograph with Film B, the exposure should be 30 mAs times 2.1 or 62 mAs. 4-6 Exposure Calculations Properly exposing a radiograph is often a trial-and-error process, as many variables affect the final radiograph. Some of the variables that affect the density of the radiograph include: The spectrum of radiation produced by the x-ray generator. The voltage potential used to generate the x-rays (KeV). The amperage used to generate the x-rays (mA). The exposure time. The distance between the radiation source and the film. The material of the component being radiographed. The thickness of the material that the radiation must travel through. The amount of scattered radiation reaching the film. The film being used. The concentration of the film processing chemicals and the contact time. The current industrial practice is developing a procedure that produces an acceptable density by trial for each X-ray generator. This process may begin using published exposure charts to determine a starting exposure, which usually requires refinement. However, it is possible to calculate the density of a radiograph to a fair degree of accuracy when the spectrum of an X-ray generator has been characterized. The calculation cannot completely account for scattering, but otherwise, the relationship between many of the variables and their effect on film density is known. Therefore, the change in film density can be estimated for any given variable change. For example, Newton's Inverse Square Asst. Prof. Dr. Nasri S. M. Namer 45 Postgraduate Study