Unit III radiographic methods

UNIT-III
RADIOGRAPHIC
METHOD
By- SHIVAM SHARMA
04-06-2021 SHIVAM SHARMA 1

CONTENT
• Introduction to electromagnetic waves and radioactivity,
• various decays,
• Attenuation of electromagnetic radiations,
• Photo electric effect,
• Rayleigh’s scattering (coherent scattering),
• Compton’s scattering (Incoherent scattering),
• Pair production,
• Beam geometry and Scattering factor.
• X-ray radiography: principle, equipment & methodology, applications,
• Types of radiations and limitations
• γ-ray radiography – principle, equipment.,
• Source of radioactive materials & technique
• Advantages of γ-ray radiography over X-ray Radiography
• Precautions against radiation hazards.
• Case Study - casting and forging.

INTRODUCTION
• Radiography is used in a very wide range of applications
including medicine, engineering, forensics, security etc.
• In NDT, radiography is one of the most important and widely
used method.
• Radiographic testing offers a number of advantages over other
NDT methods, however, one of its major disadvantages is the
health risk associated with the radiation.
• In general, RT is method of inspecting materials for hidden
flaws by using the ability of short wavelength electromagnetic
radiation (high energy photons) to penetrate various materials.
• The intensity of the radiation that penetrates and passes through
the material is either captured by a radiation sensitive film
(Film Radiography) or by a planer array of radiation sensitive
sensors (Real-time Radiography).
• Film radiography is the oldest approach, yet it is still the most
widely used in NDT.

Basic Principles
• In radiographic testing, the part to be inspected is placed
between the radiation source and a piece of radiation
sensitive film.
• The radiation source can either be an X-ray machine or
a radioactive source (Ir-192, Co-60, or in rare cases Cs-
137).
• The part will stop some of the radiation where thicker
and more dense areas will stop more of the radiation.
• The radiation that passes through the part will expose
the film and forms a shadowgraph of the part.
• The film darkness (density) will vary with the amount of
radiation reaching the film through the test object where
darker areas indicate more exposure (higher radiation
intensity) and lighter areas indicate less exposure (lower
radiation intensity).
• This variation in the image darkness can be used to
determine thickness or composition of material and
would also reveal the presence of any flaws or
discontinuities inside the material.

Advantages
• Both surface and internal discontinuities can be detected.
• Significant variations in composition can be detected.
• It has a very few material limitations.
• Can be used for inspecting hidden areas (direct access to surface is not
required)
• Very minimal or no part preparation is required.
• Permanent test record is obtained.
• Good portability especially for gamma-ray sources.

Disadvantages
• Hazardous to operators and other nearby personnel.
• High degree of skill and experience is required for exposure and
interpretation.
• The equipment is relatively expensive (especially for x-ray sources).
• The process is generally slow.
• Highly directional (sensitive to flaw orientation).
• Depth of discontinuity is not indicated.
• It requires a two-sided access to the component.

PHYSICS OF RADIATION
Nature of Penetrating Radiation
• Both X-rays and gamma () rays are electromagnetic
waves and on the electromagnetic spectrum they
occupy frequency ranges that are higher than ultra
violate radiation.
• In terms of frequency, gamma rays generally have
higher frequencies than X-rays as seen in the figure.
• The major distinction between X-rays and gamma
rays is the origin where X-rays are usually artificially
produced using an X-ray generator and gamma
radiation is the product of radioactive materials.
• Both X-rays and gamma rays are waveforms, as are
light rays, microwaves, and radio waves. X-rays and
gamma rays cannot been seen, felt, or heard.
• They possess no charge and no mass and, therefore,
are not influenced by electrical and magnetic fields
and will generally travel in straight lines. However,
they can be diffracted (bent) in a manner similar to
light.

Conti…
• Electromagnetic radiation act somewhat like a particle at times in that
they occur as small “packets” of energy and are referred to as “photons”.
• Each photon contains a certain amount (or bundle) of energy, and all
electromagnetic radiation consists of these photons.
• The only difference between the various types of electromagnetic
radiation is the amount of energy found in the photons.
• Due to the short wavelength of X-rays and gamma rays, they have more
energy to pass through matter than do the 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.

Various Decay:- Alpha particle
• Certain radioactive materials of high atomic mass (such as
Ra-226, U-238, Pu-239), decay by the emission of alpha
particles.
• These alpha particles are tightly bound units of two
neutrons and two protons each (He-4 nucleus) and have a
positive charge.
• Emission of an alpha particle from the nucleus results in a
decrease of 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.

Various Decay:- Beta particle
• A nucleus with an unstable ratio of neutrons to
protons may decay through the emission of a high
speed electron called a beta particle.
• In beta decay a neutron will split into a positively
charged proton and a negatively charged electron.
• This results in a net change of one unit of atomic
number (Z) and no change in the mass number
(A).
• Beta particles have a negative charge and the beta
particles emitted by a specific radioactive material
will range in energy from near zero up to a
maximum value, which is characteristic of the
particular transformation.
•

Various Decay:- Gamma particle
• A nucleus which is in an excited state (unstable nucleus) may emit one
or more photons of discrete energies. The emission of gamma rays
does not alter the number of protons or neutrons in the nucleus but
instead has the effect of moving 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.

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.
• Their paths cannot be changed by electrical or magnetic fields.
• They can be diffracted, refracted to a small degree at interfaces between
two different materials, and in some cases be reflected.
• They pass through matter until they have a chance to 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 can damage or destroy
living cells.

Characteristics of Radiation
• There are certain unique characteristics relative to radiation that must be understood in order to
realize the physics and variables involved with producing a radiograph with the use of a radiation
source.
• These characteristics apply to both x-radiation and gamma radiation.
• Recall that the only difference between x- and gamma radiation is their origin.
• X- Rays are produced by an x-ray tube and gamma rays come from a radioactive source that is
disintegrating.
• The following characteristics apply to the radiation that will be used in the nondestructive
examination of materials.
1. Radiation is Absorbed and Scattered by Material
There are four common absorption processes that influence the amount of radiation that passes
through a part. The four absorption processes are:
1. Photoelectric effect
2. Rayleigh scattering
3. Compton scattering
4. Pair production

photoelectric effect
• The photoelectric effect is that process in which a photon of low
radiation energy (less than 500 kV) transfers all of its energy to an
electron in some shell of the material atom.
• The energy may simply move an electron from one shell to another, or
if there is energy above that required to interact with the orbital
electron in the material, it will impart kinetic energy and the electron
will be ejected from the atom.

Rayleigh scattering
• Another name for Rayleigh scattering is “coherent scattering” and it is
a direct interaction between the photon and the orbital electrons in the
atom of the material.
• However, in this case the photon is deflected without any change or
reduction of the kinetic energy of the photon or of the energy of the
material atoms.
• In this process, there are no electrons released from the atom. It is
estimated that Rayleigh scattering accounts for no more than about
20% of the total attenuation.

Compton scattering
• Compton scattering is a direct interaction between photons in the 0.1–
3.0 MeV energy range and an orbital electron.
• In this case, when the electron is ejected from the material atom, only
a portion of the kinetic energy of the photon is used.
• The photon then scatters in a different direction than the direction
from which it came and actually emerges with a reduced energy and,
therefore, a lower wavelength.
• Compton scattering varies with the atomic number of the material and
varies, roughly, inversely with the energy of the photon.

Pair production
• Pair production is an attenuation process that results in the creation of
two 0.51 MeV photons (as a result of annihilation of the electron-
positive pair) of scattered radiation for each high-energy incident
photon that is at or above an energy of 1.02 MeV.
• The two 0.51 MeV photons travel in different directions, causing the
production of electromagnetic radiation through interaction with other
material particles.
• Energies exceeding 1.02 MeV result in additional kinetic energy being
applied to the pair of particles.

2. Radiation Penetrates
• The variables relating to the penetration of the radiation can be expressed with the
term “half-value layer.”
• The half-value layer is defined as the thickness of a specific material that will
reduce the radiation intensity to one-half of that entering the part.
• If the initial radiation intensity is 100 roentgens (100 R), a material that is exactly
one half-value layer will reduce that 100 R to 50 R.
• Another half-value layer thickness will reduce that 50 R to 25 R, and so on. If this
is carried forward, the radiation never reaches zero.
• Another term used is the “tenth-value layer.” The same principle as the half-value
layer applies, except that the thickness is somewhat greater and reduces the initial
radiation to one-tenth on the opposite side.
• The factors involved with these half- and tenth-value layers are:

Conti…
• Energy. The higher the energy, the thicker the half- or tenth-value layers. This is
supported by the fact that the higher energy radiation produces shorter wavelength
radiation, resulting in better penetration. The half- and tenth-value layers are
especially useful for calculating the thickness of shielding when designing an
enclosure or room for radiography. (See Table 6-1 for examples of half and tenth-
value layers).
• Material type. The greater the material density, the thinner the half-value layer.
Material that is low in density, such as aluminum or titanium, will allow more
radiation to pass through, and there will be less absorption and scattering.
Materials with higher density, such as steel and lead, provide a much greater
chance of interaction because of their higher atomic number. With higher-density
materials there will be more absorption and the half-value layer thickness will be
considerably less than with the lowerdensity materials.
• Thickness. As mentioned, the half-value layer thickness is specific for a given
material and energy. As the thickness of the material increases, the amount of
absorption and scatter increases and the amount of radiation that passes through
that thickness

• 3. Radiation Travels in Straight Lines and at the Speed of Light-The speed of light is 186,300
miles per second or 299,800 km per second.
• 4. Radiation Exhibits Energy-On the electromagnetic spectrum, the wavelengths of x- and gamma
rays are much shorter than that of visible light. Recall that as the radiation energy increases, shorter
wavelengths are produced that provide greater penetration through the material being examined.
• 5. Radiation Ionizes Matter- Ionization is a change in the electrical nature or characteristics of
matter. Electrons are displaced or knocked out of orbit, thereby changing the electrical balance.
This, coincidentally, is what causes the greatest concern to humans. When radiation passes through
living tissue, the cells are affected—electrically changed—and damage will result.
• 6. Radiation is Not Particulate- Radiation has no mass, and even though x- and gamma rays
behave like particles, they are actually weightless. To describe the effect of radiation as it interacts
with matter, the radiation is sometimes referred to as photons, which is another way of saying high-
speed energy traveling at the speed of light that behaves like particles; but in fact, there are no
particles in x- or gamma radiation.
• 7. Radiation Has No Electrical Charge- X- and gamma radiation are not affected by either strong
electrical or magnetic fields.
• 8. x- and Gamma Radiation Cannot be Focused- If x- or gamma radiation is directed toward a
glass lens, the radiation would not focus like light does. In fact, the radiation passing through that
lens would be absorbed to a greater extent in the thicker portion of the lens and more radiation
would pass through the thinner regions of the lens.

9. Humans Cannot Sense x- or Gamma Radiation
Radiation cannot be seen, tasted, or felt. It has no odor and if humans are subjected to radiation exposure, the
effects are not realized for a period of time. In other words, the body goes through a latent period before the
harmful effects of the radiation exposure are evident.
10. Radiation Causes Fluorescence in Some Materials
Some minerals and salts will fluoresce when subjected to radiation. Fluoroscopy utilizes a fluorescent screen
that converts some of the radiation to light. When used in radiography, the light that is emitted from the
fluorescent screen creates a much higher exposure effect on the film that is adjacent to it, thereby significantly
reducing exposure.
11. Intensity of Radiation
Radiation is expressed in roentgens per hour (R/hr), roentgens per minute (R/min), or milliroentgens per hour
(mR/hr). The intensity decreases with distance. This is a function of the inverse square law, which states that
the intensity of the radiation varies inversely with the square of the distance. As the distance from the radiation
source is doubled, the intensity
is decreased to one-fourth. This will be discussed in more detail in Section IV, Variables.
12. Radiation Produces Chemical Changes in Exposed Film
The film’s emulsion is chemically changed through ionization as radiation interacts with it. If it weren’t for
this characteristic, it would not be possible to produce a radiographic image on film.

Principles of x-Radiography
• X-rays are produced in a vacuum tube when high-speed electrons,
which are negatively charged, are attracted by a positive potential in
the anode and collide with a target material. The electrons are
produced when a filament, usually tungsten, is heated to
incandescence. The resulting electrons “boil” off and collide with
the target material in the anode.

A higher current applied to the filament will result in a greater number of
electrons and, therefore, higher radiation intensity. The energy of the radiation
is a function of the applied voltage, or kilovoltage to the anode. This causes an
increase in velocity of the electrons and, as higher voltage is applied, higher-
energy radiation results. The energy of the radiation is its single most important
characteristic. It is directly related to the wavelength and to the ability of that
radiation to penetrate. The higher the energy, the shorter the wavelength, and
the greater the penetration. The target in the anode in most x-ray tubes is
typically made of tungsten.

Tungsten has a high atomic number, which makes it an ideal material to interact with the
high-speed electrons. It also has a high melting point, 5880 °F (3249 °C), which is essential
since the target is subject to high temperatures as a result of electron impingement. Since
almost all of the kinetic energy from the electrons is transformed or converted into heat, the
anode must have the capability of dissipating that heat. In fact, 97–99% of the energy
conversion results in the generation of heat and approximately 1–3% is converted to x-rays.
In summary, as the energy is increased, radiation with shorter wavelengths results in greater
penetration. On the cathode side of the tube, which contains the filament and a focusing cup,
the number of electrons increases with an increase of applied milliamperes, which results in
an increase in intensity of the radiation. Figure 6-11a illustrates several typical x-ray tubes
and a cross-sectional sketch of a tube can be found in Figure.

The target size in the anode is very important, since this affects the image sharpness of the
object being radiographed. This is directly related to the definition of the final image. The
smaller the target or radiation source, the sharper the resultant radiographic image. As
mentioned above, the energy of the radiation is expressed in voltage or kilovoltage, with one
electron volt being the amount of voltage that is necessary to energize and displace one
electron. The common expression for x-ray energy is in units of kiloelectron volts (keV), but
this is shorted to the kV. A kilovolt is equal to one thousand volts. The majority of industrial
radiographic techniques use an energy range from about 100 kV to 400 kV. The intensity of
the radiation is expressed in milliamperes (mA) (one milliampere = one one-thousandth of
an ampere). In a general way, the energy of the radiation can also be described as the
“quality” of the radiation and the intensity (in milliamperes) as the “quantity.” Energies
above the 400 kV range are considered to be high-energy x-radiation. Equipment producing
high-energy x-rays include:
X-ray tubes with a resonance-type transformer
Betatrons
Linear accelerators
Van de Graaff generators

The older of these high-energy x-ray units are the resonant transformer x-ray
units, which were developed in the 1940s. Their energy range is from 250kV
to 4 MeV. The x-ray tube is symmetrically located within the transformer.
The electrons are accelerated to very high velocities by electrodes and
focused by a coil as they enter an extension chamber. It is possible to
penetrate up to 8 of steel with this equipment. Betatrons accelerate the
electrons in a circular path by using an alternating magnetic field, and
typically produce energies in the range of 10–30 MeV. The tube is
doughnutshaped and the electrons increase their velocity with each orbit until
they reach a very high energy. They are eventually guided from their circular
path to a target and x-rays are generated.

• Linear accelerators produce high-velocity electrons by the use of radio frequency energy
coupled to a waveguide and have energies from 5 up to about 25 MeV. These systems
contains an electron gun, an accelerator, and a waveguide. The electrons are accelerated
along the waveguide by an electrical field.
• The Van de Graaff generators, also known as electrostatic generators, produce x-ray
energies from about 500 kV to as high as 6 MeV. The systems consist of high-speed belts
that travel about 4000 feet/minute and build up electrical charges that are carried to a high-
voltage terminal.
• This results in a high electrostatic charge that is used to accelerate the electrons through the
tube to the target.

Attenuation of electromagnetic radiations
• The attenuation that results due to 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 some other 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.
• 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. Photoelectric absorption is also dominant for atoms of
high atomic numbers.

Conti..
• Compton scattering (C) (Incoherent scattering)- occurs when the
incident X-ray photon is deflected from its original path by an
interaction 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, therefore,
has a longer wavelength than the incident photon.

Conti…
• Pair production (PP) can occur when the X-ray photon energy is
greater than 1.02 MeV, but really 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 very short lived and disappear (positron annihilation)
with the formation of two photons of 0.51 MeV energy. Pair
production is of particular importance when high-energy photons pass
through materials of a high atomic number.
- Photo disintegration (PD)- When a photon gets interacted with the nucleus of the atom, and its all energy
is transferred to nucleus which ejects an electron from the nucleus. This phenomenon is call photo
disintegration.

Rayleigh’s scattering (coherent scattering)
• Thomson scattering is also known as Rayleigh’s scattering (coherent
scattering). When a photon get interacted with the nucleus of the
atom, photons gets scattered with no change in internal energy of the
scattering atoms i.e. this phenomenon occurs without loss of energy.

Beam geometry and Scattering factor
• This law was first proposed by English
physicist sir William Henry Bragg and his
son Sir William Lawrence Bragg in 1913.
• This law explains why the faces of crystals
reflects the X-rays beams at certain angle of
incidence, intense reflected X-rays are
produced when the wavelengths of the
scattered X-rays interfere constructively.
• This simple observation is commonly
known as X-ray diffraction (XRD) and was
direct evidence for the atomic structure of
crystals

Conti…
• Consider a crystal with lattice planar distance d, where
the difference in path travelled by ray ABC and A’B’C’
is an integer multiple of wavelength.
• Both rays are always in phase and parallel upto the
point at which the top beam ABC strikes the top layer
(x) at B.
• The second Ray A’B’C’ continues to next layer (y),
where it is scattered at point B’.
• Therefore the extra distance travelled by ray-
• A’B’C’= PB’+B’Q
• Where this extra distance must be an integral (n)
multiple of wavelength ().

TYPES OF RADIATION
• Both X-Rays and - Rays are forms of electromagnetic radiation
(EMR) as is high and differs only in their source of origin.
• X-rays are produced by an X-ray generator where gamma rays are
produced by radioactive decay of radioactive elements.
• Both forms of electromagnetic radiations are characterised by a stream
of mass less photons which had frequency , wavelength and velocity,
travelling in a wave like pattern and moving at the speed of light.

X-Radiation
• X-rays are just like any other kind of electromagnetic radiation. They can be
produced in packets of energy called photons, just like light.
• There are two different atomic processes that can produce X-ray photons. One is
called Bremsstrahlung (a German term meaning “braking radiation”) and the other
is called K-shell emission.
• They can both occur in the heavy atoms of tungsten which 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 target
changes.
• The negatively charged electron slows down after swinging around the nucleus of a
positively charged tungsten atom and this energy loss produces X-radiation.
• Electrons are scattered elastically or in elastically by the positively charged nucleus.
• The in elastically scattered electron loses energy, and thus produces X-ray photon,
while the elastically scattered electrons generally change their direction significantly
but without loosing much of their energy.

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 as an emitted X-
ray photon. Meanwhile,higher energy electrons fall into the
vacated energy state in the outer shell, and so on.
• After losing an electron, an atom remains ionized for a very short
time (about 10-14 second) and thus an atom can be repeatedly
ionized by the incident electrons which arrive about every 10-12
second.
• Generally, K-shell emission produces higher-intensity X-rays
than Bremsstrahlung, and the X-ray photon comes out at a single
wavelength.

Gamma Rays
• Gamma rays are also electromagnetic radiation like X-rays with a very
short wavelength of less than one tenth of a nanometer and are
distinguished only by their source of origin rather than their nature.
• It is also one of the three types of natural radioactivity i.e. alpha, beta
and gamma rays.
• Gamma rays is a product of radioactive element’s like Radium (natural
radioactive elements) radioactive decay.
• When any atom of a radioactive element undergoes radioactive
decay, it emits radiations which when interacts with any atom, ionizes
it.

X-ray Radiography Principle
• X-rays are produced when a high speed
moving electron get collided with nucleus of
an atom X-rays are produced in a X-ray tube
which consist of a glass or ceramic envelope
containing anode a positively charged
electrode and cathode a negatively charged
electrode, evacuated to an ultra high vacuum
of 109 hector pascal.
• Cathode act as a producer of electrons, when
electrical tension setup between cathode and
anode, these electrons are attracted by anode
and get accelerated up.
• These electrons from cathode are
concentrated into a beam by a focussing cup
and when these electrons get collided with
anode, a part of their energy is transformed
into X-radiation called as X-rays.

X-Rays, equipment & methodology
Equipment's producing high energy X-ray includes:
• X-ray tubes with a resonance type transformer
• Betatrons
• Linear accelerators
• Van de Graaff generators
• In general, industrial x-ray equipment can be categorized by energy groups. Those x-ray units
that produce energies up to about 125 kV are considered low energy; from 125 up to 400 kV,
medium energy; and those systems that produce radiation energies above 400 kV are considered
high energy. As mentioned above, the majority of examinations performed today are done in the
medium energy range, from 125 kV up to about 400 kV. It is the most widely used for a broad
range of applications and this chapter will focus primarily on that energy range.
• When energy such as 300 kV is mentioned, it usually denotes the maximum energy output for
that particular unit. For any given x-ray unit, the energy can usually be varied from a low energy
up to the maximum energy for which the tube is rated. As discussed, xrays are produced in a
vacuum tube, by heating a filament that boils off negatively charged electrons. They are attracted
to a positively charged anode, where there are very complex collisions and interactions that
produce mainly heat, with only a small amount of the energy converted into x-rays.

Conti…
• Industrial x-ray systems come in different sizes, shapes, and duty cycles.
There are X-ray units in cabinets that are ideal for radiographing smaller
parts made of low-density material or with limited thickness. These
cabinet units typically produce lower energies, usually up to about 150 kV.
• There are x-ray units that are portable, with gas filled tube heads. These
are lightweight and are relatively easy to transport for field radiography.
Many of the stationary units are self-rectified and continuous duty, such as
the constant potential units, which can operate continuously for long
periods of time. The gas filled tubes operate with partial duty cycles, such
as 50%. This means that the x-ray tube can operate for a specific time with
an equal amount of time permitted for cool-down before the next
exposure. Figure illustrates a 300 kV constant potential x-ray system.

Applications
• Although the majority of applications in radiographic testing appear to
involve welds and castings, it has been effectively applied to many other
product forms spanning a wide variety of industries. The major industries
include but are not limited to the following.
• 1. Power Generation. A wide range of weld configurations, thicknesses,
and materials are used at power plants. Radiographic testing requires
radiation sources that are capable of providing energies from the low end of
the spectrum to the highest commercially available. Pressure vessels are
complex structures that require high-quality initial examinations. The
associated piping systems, with their many weld joint geometries, provide
continual challenges for the radiographer.

2. Aerospace. There are countless uses for RT in the aerospace industry. The critical
components in the aircraft engine itself call for thousands of radiographs. The airframe
and other related structures depend greatly on radiography for initial and in-service
inspection. Water ingress in honeycomb cells can cause problems due to freezing when
the aircraft is airborne. Radiographic techniques can effectively detect this condition in
most cases.
3. Petrochemical. Radiography is of great importance in the initial construction of
petrochemical plants. Considering the many service conditions under which this type
of complex operates, establishing a baseline through radiography is of extreme
importance. Just as with power plants, in-service inspection is necessary and RT plays
a key role. The detection of areas of degradation, corrosion, erosion, wear, and other
conditions that develop through the extended operation should be detected and
corrected before an unplanned shutdown becomes necessary. Other than the cost of
initial construction, unplanned outages are the most costly events in this industry. It
should be noted that this also applies to most other major industries.

Conti…
• 4. Nonmetals. This category includes a wide variety of materials that are
inspected by radiography, including plastics, rubber, propellants, ceramics,
graphite, concrete, explosives, and many more. Perhaps the electronic industry,
because there are some metals involved, also belongs in this industry segment.
• 5. Medicine. There are numerous applications in medicine that do not involve
the examination of living creatures. On occasion, foreign objects or
contaminants have inadvertently found their way into various drugs and
medicines during processing. These elements most usually have densities that
are quite different from the products into which they have become included.
• 6. Law Enforcement and Security. Radiography is especially adaptable and
useful for the detection of hidden contraband and weapons and for the
examination of explosive de-vices such as shells and projectiles. Sealed boxes,
envelopes, and other packages have been found to contain devices that were
potentially hazardous if they had not been detected. When passing through the
security area in an airport, carry-on articles are subjected to real-time
radiographic inspection.

7. Food. Various food products are examined with some form of RT for
the purpose of detecting foreign objects that may have been introduced
during processing. In some cases, food items may be inspected for
content, distribution of additives (such as nuts, candy, etc. in ice cream),
or for quantity. Again, the fastest approach would include fluoroscopy or
some other real-time technique.
8. Objects of Art or Historic Value. This industry is perhaps the most
unusual and most interesting for radiographers. Famous paintings have
been radiographed (using low energy, high-contrast techniques) to
disclose hidden paintings or images beneath their outer layer.

Advantages
Radiographic testing has many advantages, some of the most significant of which are
listed below. It:
• 1. Provides an extremely accurate and permanent record
• 2. Is very versatile and can be used to examine many shapes and sizes
• 3. Is quite sensitive, assuming the discontinuity causes a reasonable reduction of
cross section thickness
• 4. Permits discontinuity characterization
• 5. Is widely used and time-proven
• 6. Is a volumetric NDT method

Limitations of radiation
• 1. There are safety hazards with the use of radiation devices
• 2. RT has thickness limitations, based on material density and
energy used
• 3. RT can be time-consuming
• 4. RT is very costly in initial equipment and expendable materials
• 5. It is also very dependent on discontinuity orientation
• 6. RT requires extensive experience and training of the personnel
taking the radiographs and during the interpretation

Procedure
A typical procedure is outlined below to provide guidance, especially when radiography of a specific object is
being attempted for the first time. The recommended steps in a radiographic procedure are:
• 1. Understand the codes, specifications, and customer requirements thoroughly
• 2. Develop a technique based on the thickness and type of material
• 3. Prepare a shooting sketch
• 4. In the darkroom, carefully place the radiographic film in the cassette with the proper lead screens
• 5. Place the film under the area of interest
• 6. Ensure that the correct source to film distance is being employed
• 7. Place the appropriate station markers and identification numbers in the area of interest to assure easy
correlation with a discontinuity if one is detected.
• 8. Set up the exposure parameters
• 9. Make the exposure
• 10. In the darkroom, unload and process the film
• 11. Evaluate the film for artifacts
• 12. Evaluate the film for compliance
• 13. Complete a report and store the film

γ-ray radiography – principle
There are certain characteristics regarding radioactive isotopes that should be understood.
The first is energy. Energy of a radioactive isotope is unique to that particular, specific
radioactive material. Some of these radioactive isotopes actually give off a spectrum, or a
number of unique energy bundles. For sources, energy never changes for a given radioactive
isotope.
Radioactive isotopes decay with time. With the decay comes reduction in activity or
quantity. The radioactive half-life is the amount of time that it takes for a nspecific
radioactive isotope to decay or be reduced to one half of its original activity. For examples
of isotope half-lives. Recall that with an x-ray source, the intensity or quantity of radiation
is a function of the applied milli amperes. With a radioactive isotope, the activity, or the
amount of radiation that is given off by a source, is expressed in curies or becquerels. One
curie is equal to 3.7 × 1010 disintegrations per second and one becquerel is equal to one
disintegration per second.

One way to understand disintegration is to visualize the atom of a radioactive
isotope as having one neutron too many in the nucleus. In time, the nucleus
literally breaks apart or disintegrates. In that act of disintegration, a bundle of
radiation with a unique energy is given off. In addition, other by-product
materials are created. When there are 37 billion of these disintegrations occurring
in a unit time of one second, there is one curie of that radioactive isotope.
Again, the half-life is the time it takes for the source activity to be reduced to one
half of what it was originally. So, if an iridium 192 source is used when new and
the activity is 100 curies, at the end of a half-life, which for iridium is
approximately 75 days, the activity would be reduced to one-half, or 50 curies

. If the source goes through another half-life, it will reduce to 25 curies; another
half-life to 12½ curies; and so on.
In other words, it does not matter how many half-lives the source goes through; it
never reaches absolute zero. For cobalt 60, the half-life is 5.3 years. Its energy is
higher than iridium. For iridium 192, there are seven principal energy packets
produced during the disintegration process ranging from just over 0.2 to about 0.6
MeV (approximately 200 to 600 kV).
With cobalt 60, there are two discrete gamma energies (sometimes expressed as
photon energies), 1.17 and 1.33 MeV. With this higher energy, it is possible to
examine materials that are somewhat thicker or denser. Another factor to consider
when using radioactive isotopes is their specific activity. The specific activity of a
source is expressed in curies per gram.
The greater the number of curies for a given physical size, the higher the specific
activity. The major benefit of a high specific activity source is the improved
definition that will result from the high activity (curies) and small physical size.

Gamma Rays Sources
• Natural radio active sources- Two elements which have been widely used in industry
radiography are radium and mesothorium.
• The only disadvantages with these two elements after high cost is dimensional flexibility.
• Man made Radio active sources- Man made radioactive sources are produced by introducing
an extra neutron to atoms of the source material.
• As the material gets rid of the neutron, energy is released in the form of gamma rays. Two of
the most common industrial gamma-ray sources for industrial radiography are Iridium-192
and Cobalt-60.
• In comparison to an X-ray generator, Cobalt-60 produces energies comparable to a 1.25 MV
X-ray system and Iridium-192 to a 460 kV X-ray system.
• These high energies make it possible to penetrate thick materials with a relatively short
exposure time.
• This and the fact that sources are very portable are the main reasons that 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.

Advantages and Limitations of Man made
radioactive sources-
• Advantages
• Can work without electrical supply.
• Dimensional Flexibility i.e. available in various shape and size.
• Available with wide variety of Radiation hardness.
• Higher radiation hardness than X-rays.
• Limitations
• As it is not operated electrically, it can’t be switched off.
• Energy level can’t be controlled.
• Radiation intensity can’t be controlled.
• Limited service life.
• Produces less contrast.

γ-ray radiography –equipment
• The radium. Figure 6-14 illustrates the “fish pole” technique in which
the radium was transported inside a capsule on the end of a line that
was attached to a fishing pole. The radiographer would place the
“hook” on the end of the line through an eyelet on the capsule, then
remove it from the lead storage pig and carry it to the area where the
object to be radiographed was located, as illustrated.
• The major component in a gamma ray system is the exposure device,
such as the one depicted in Figure. These exposure devices have
shields that contain an S-tube in which the iridium source is
maintained inside a stainless steel capsule and is securely locked into
place when not in use. The shield material is usually made from
depleted uranium, unlike earlier gamma cameras that used shielding
made of cast lead. Depleted uranium is slightly heavier than lead and
has unusually good radiation shielding capabilities.
• A gamma ray set-up is illustrated in Figure, showing the camera with
the source drive control, the source tube that is positioned in a source
stand just above the pipe weld that will be radiographed. The
radiographer “cranks” out the source to the extended position.
• The predetermined exposure time is allowed to elapse and then the
source is cranked back into the camera.

Conti…
Accessories
• There are many different accessories that are used in conjunction with taking radiographs.
• These accessories include, but are not limited to:
• Film and cassettes
• Lead screens
• Lead numbers and letters
• Penetrameters (image quality indicators)
• Densitometers
• High-intensity film illuminators
• Processing equipment (manual or automatic)
• Film hangers (for manual processing)
• Collimators (especially for use with radioactive sources)

advantages of γ-ray radiography over X-ray
Radiography
• X-rays have lower penetrating intensity than Gamma rays.
• Scattering in gamma rays is very less in comparison to X-rays.
• Gamma rays are preferred in industrial radiography where X-
ray is preferred in medical radiography.
• In gamma rays, radiographic sensitivity is very high.
• Sources of gamma rays are cheap and are available in nature.
• After certain distance of penetration, Gamma rays decay it self
by loosing it all energies.
• Equipment of gamma rays are portable and safe.

Precautions against radiation hazards
As mentioned previously, the health risks associated with the radiation is
considered to be one the major disadvantages of radiogaphy. The amount of
risk depends on the amount of radiation dose received, the time over which the
dose is received, and the body parts exposed. The fact that X-ray and gamma-
ray radiation are not detectable by the human senses complicates matters
further. However, the risks can be minimized and controlled when the
radiation is handled and managed properly in accordance to the radiation
safety rules. The active laws all over the world require that individuals
working in the field of radiography receive training on the safe handling and
use of radioactive materials and radiation producing devices.

The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of
numerous factors including:
Type of radiation involved. All kinds of ionizing radiation can produce health effects. The main difference in the
ability of alpha and beta particles and gamma and X-rays to cause health effects is the amount of energy they
have. Their energy determines how far they can penetrate into tissue and how much energy they are able to
transmit directly or indirectly to tissues.
Size of dose received. The higher the dose of radiation received, the higher the likelihood of health effects.
Rate at which the dose is received. Tissue can receive larger dosages over a period of time. If the dosage occurs
over a number of days or weeks, the results are often not as serious if a similar dose was received in a matter of
minutes.
Part of the body exposed. Extremities such as the hands or feet are able to receive a greater amount of radiation
with less resulting damage than blood forming organs housed in the upper body.
The age of the individual. As a person ages, cell division slows and the body is less sensitive to the effects of
ionizing radiation. Once cell division has slowed, the effects of radiation are somewhat less damaging than when
cells were rapidly dividing.
Biological differences. Some individuals are more sensitive to radiation than others. Studies have not been able
to conclusively determine the cause of such differences.

Safety Considerations
The age-old rules of protection against radiation exposure involve time, distance, and
shielding.
• Time. The shorter the exposure, the better. Every effort should be made to keep
personnel exposure time as close to zero as possible. When performing radiography in
the field, zero exposure is virtually impossible. So every attempt should be made to
minimize the time of exposure to the body to as short a time as possible.
• Distance. The further an individual is from a radiation source, the less the exposure.
Recall the inverse square law and how radiation intensity varies inversely proportional
to the distance. If there is an intensity of 200 mR/hour at a certain distance from a
radiation source, simply doubling the distance will decrease the intensity by a factor of
one-fourth, or 50 mR. Doubling the distance again will reduce the intensity to 12.5
mR, and so on. Using the inverse square law will permit the radiographer to calculate
the approximate exposure at any distance.
• Shielding. The greater the thickness and density of the shielding material, the less the
radiation exposure. This is a function of the half- and tenth-value layers, as discussed
in Section II. Lead, of course, makes an excellent shielding material because of its high
density.

Unit III radiographic methods

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Unit III radiographic methods