Unit-5 RADIOGRAPHY

Introduction

 The historic discovery of X-rays by W.C. roentgen in 1895 and radioactivity by

Becquerel in 1896 and their subsequent and logical application to the examination of
materials object provided the starting point for the development and advancement of
industrial radiography.

 This technique is one of the most widely used NDT methods for the detection of internal
defect such as porosity and voids.

 With proper orientation of the X-ray beam, planar defects can also be detected with
radiography.

 It is also suitable for detecting changes in material composition, thickness measurement

and locating unwanted or defective components hidden from view in an assembled part.

The primary advantages and disadvantages in comparison to other NDT methods are:
Advantages

 Both surface and internal discontinuities can be detected

 Can be used for inspecting hidden areas

 Very minimal or no part preparation is required

 Permanent test record is obtained

 Good portability especially for gamma rays.

Disadvantage

 Hazardous to operator and other nearby personnel.

 High degree of skill and experience is required for exposure and interpretation

 The equipment is relatively expensive.

 The process is generally slow.

 Depth of discontinuity is not indicated.

 It requires a two sided access to the component.

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Principle

 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 denser areas will stop more of
the radiation.

 The radiation that passes through the part will expose the film and forms a shadow graph
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 (higher 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.

Fig 5.1 Principle of radiographic examination

Production of X-rays

In the widely used conventional X-radiography the source of radiation is an X-ray tube. The X-
ray tube consists of a glass bulb under vacuum enclosing a positive electrode or anode and a
negative electrode or cathode. The cathode comprises a filament which when brought to

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incandescence by a current of a few amperes emit electrons. Under the effect of electrical tension
set up between the anode and cathode these electron from the cathode are attached to the anode.
The stream of electron is concentrated in a beam by a cylinder or a focusing cup. The anticathode
is a slip of metal with high melting point recessed into the anode at the place where it is struck by
the beam of electrons. It is by impinging on the anticathode that fast moving electrons give rise
to X-rays. Fig 5.2 shows the layout of a typical X-ray tube build by collidge in 1913 which also
used a heated filament to produce electrons.

Fig. 5.2 Schematic set up of X-ray tube and circuit

The replacement of the glass tube by metal ceramic ones has led to an extended tube life. X-ray
machines are characterized by the operating voltage and current which determine the
penetrability and intensity of the radiation produced. Modern X-ray generators are available upto
450 kV and 15 mA. X-ray equipment with dual focal spot sizes and ultra-small local spot and
portable (15 Kg) equipment with an output voltage of 200 jkV and 3 mA current are also
available. Highly automated self propelled X-ray mini crawlers which travel within pipelines are
used to take radiograph of pipeline/welds from inside.
The area of anticathode which is struck by the electron flux is called the focal spot or TARGET.
It is essential that this area should be sufficiently large, in order to avoid local overheating which
might damage the anticathode and to allow rapid dissipation of heat. The projection of the focal
spot on a surface perpendicular to the axis of the beam of X-rays is termed as the optical focus or
focus. This focus has to be as small as possible in order to achieve maximum sharpness in the
radiographic image.

X-rays are produced when fast moving electrons are suddenly brought to rest by colliding with
matter. Electron may also lose energy by ionization and excitation of the target atoms. However
these do not result in X-ray production. The accelerated electrons therefore lose their kinetic
energy very rapidly at the surface of the metal plate and energy conversion consequently occurs.
The kinetic energy of the accelerated electrons can be converted in three different ways.
(i) A very small fraction ie less than 1% is converted into X-radiation. The conversion factor
f can be estimated by an approximate empirical relation
F= 1.1×10-9 ZV

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 Where Z is the atomic number of the target and V the energy of electron n volts. For
tungsten (Z=74) target, the fraction of X-ray energy converted into X-rays are 120 kV is
0.98.
(ii) Approximately 99% of energy of electron is converted into heat by increasing the thermal
vibration of the atoms of the target, the temperature of which may consequently rise
considerably.
(iii) Some of the electron has sufficient energy to eject orbital electrons from the
atoms of the target material which are ionized. The secondary electron produced in this
way may escape from the surface of the target and subsequently be recaptured by it
producing further heat or secondary radiation.
High energy X-ray source
Examination of thicker section is carried out using high energy X-rays whose energy value is 1
Mev or more. Using high energy X-rays, possibilities of large distance to thickness ratio with
correspondingly low geometrical distortion, short exposure times and high production rate can be
achieved. A number of machines such as synchrotron, betatron and Van De Graff type
electrostatic generators are available of which electron Linear Accelerator (Linac) is the most
popular.
Gamma Ray Sources
In contra distinction to X-ray machines which emit a broad band of wavelengths gamma ray
sources emit one or few discrete wavelengths. Radiography with gamma rays has the advantages
of simplicity of the apparatus used, compactness of radiation source and independence from
outside power. This facilitates the examination of pipe, pressure vessels and other assemblies in
which the access to interior is difficult. Gamma rays are electromagnetic radiation emitted from
an unstable nucleus. Each isotope with unstable nucleus will have characteristic nuclear energy
and intensities for the emitted radiation. The gamma ray energy level remains constant for a
particular isotope but the intensity decays with time as indicated by the half life.
Where a variety of radioisotopes are produced in a nuclear reactor only a select few have been
utilized for the purposes of radiography. The rest of the other isotopes produced have been found
to be unsuitable for a variety of reasons such as shorter half life, low intensity and high cost of
production. The most popular radiographic sources are cobalt 60 (co-60) Iridium 192 (Ir-192)
Cesium 137 (Cs-137) and Thulium 170 (Th-170). Cobalt 60 and Iridium 192 are available in
high specific activities and thus tiny sources of these radioisotopes giving intense radiation have
found popular use. Specific activity is defined as activity in curies per gram of material.
Interaction of X-rays and Gamma rays with Matter
Penetrating radiation like X-rays or gamma rays passing through a material medium interact with
matter in a complex manner. The effect of interaction is attenuation of incident radiation.
Attenuation take place in two ways absorption and scattering Fig 5.3 shows complex interaction

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 Fig 5.3 Interaction of radiation with matter

The important modes of this absorption are 1. Photoelectric effect 2. Rayleigh scattering
3. Compton scattering 4. Pair production. The radiation attenuation i n the specimen of thickness
x can be calculated using the following expression.
I=I0Be-μx
Where I= Intensity of radiation emerging out of the specimen.
I0=intensity of radiation when value of x=0, μ=linear absorption coefficient per mm
thickness, B=build up factor.

Properties of X- and Gamma rays

X-rays and gamma rays are electromagnetic radiations similar to light waves except that their
wavelength is much shorter. Some of their properties are given below.
1. They move in straight line and at the speed of light.
2. They cannot be deflected by means of lens or prism although their path can be bent
(diffracted) by a crystalline grid.
3. They pass through matter. The degree of penetration depends on the kind of matter and
the energy of radiation
4. They are ionizing radiation that is to say they liberate electrons in matter.
5. They can repair and destroy living cells.
6. Many substance fluoresce whey they absorb X-radiation notable among them are calcium
tungstate, zinc sulphide, lead barium sulphate and some cadmium compounds.

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Effect of radiation on film
The radiation source required to produce a certain blackening on the film depends on the energy
of radiation. Low energy radiation needs smaller dose to ensure a certain film density as
compared with hard radiation. Various isotopic sources with different radiation energy will cause
different film ionization. For example Cobat-60=1; Iridium-192=2.35 Caesium-137=5.5;
Thulium-170=4-4.5

Inherent sharpness
The inherent sharpness is the result of the interaction of high energy radiation emulsion on the
film. During the interaction, the electrons are dislodged from the silver halide emulsion after
gaining excess energy in the form of kinetic energy. The electrons with high kinetic energy tend
to fly off causing ionization in the adjacent silver halide grains. Thus the boundaries of the
exposed areas will show an unsharpness of the image which is called inherent unsharpness or
film unsharpness. Inherent unsharpness value is about 0.1mm without lead screen and it is 0.2
mm when a film is used with a lead screen.

FACTORS AFFECTING IMAGE IN A RADIOGRAPH

The appearance of distinguishable image in a radiograph depend on several factor

1. Geometrical factor
2. Radiographic film
3. Intensifying screen
4. Film density
5. Radiographic sensitivity
6. Penetrameter
7. Determining radiographic exposure
1. Geometrical factor
True focal spot for sources used in conventional radiography is not point source but rather is of
few millimeters in size. Due to finite source size, the image projected on to the film is enlarged
leading to geometric unsharpness.
Geometric unsharpness is given by
Ug=Ft/L0
F= Focal spot size, t= distance from the object to the receiving plane, L 0=distance from
the source to the object.

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 Fig. 5. Geometric unsharpness in radiographic examination
2. Radiographic film
Radiographic film consists of central carrier called the film base that is made up of thin sheets of
polyester type material. Polyester material serve as carrier for chemically reactive material that
from the emulsion. Emulsion consists of silver halide recording medium with a binder is applied
on both side of the base. Additionally a protective layer may be applied over the emulsion. Silver
halide is a granular material and its grain size has a significant effect on the exposure as well as
resolution ability of the film for defect detail. When radiation strikes the emulsion a change take
place in the emulsion. The change is referred to as latent image. Upon processing the grains that
have been exposed will be darkened. The silver halide is removed from the unexposed grain
during film processing leaving a transparent area.
Film speed is another important film parameter. A film is called high speed film when it grains
would begin reacting to the exposure considerable sooner than other film. Realizing that
exposure is the product of time and intensity the effect of film speed is rather significant. It is to
be noted that faster speed films have larger grains and therefore may not be able to produce
minute detail. Low speed films have extra fine grain or fine grain and give better quality even
though the exposure time is longer.
Film Characteristic Curve
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 the control of the number of photons
reaching the film is exposure.
Different types of radiographic films 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. The relationship commonly varies over a range

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of film densities, so the data is presented in the form of a curve such as the one for Kodak
AA400 shown in Fig.

This plot is usually called a film characteristic curve or density curve. A log scale is sometimes
used for the x-axis or it is more common that the values are reported in log unit on a linear scale
as seen in the figure. Also relative exposure values (unitless) are often used. Relative exposure is
the ratio of two exposures. For example, if one film is exposed at 100kV for 6 mA.min and a
second film is exposed at the same energy for 3 mA.min, then the relative exposure would be 2.
The location of the characteristic curves of different films along the x-axis relates to the speed of
the film. The faster to the right that a curve is on the chart, the slower the film speed (Film A has
the highest speed while film C has the lowest speed). The shape of the characteristic curve is
largely independent of the wavelength of the X-ray or gamma ray, but the location of the curve
along the x-axis, with respect to the curve of another film, does depend on radiation quality.

3 Intensifying screens
Use of thin screen foils made out of heavier metals has been found to produce intensification
when exposed them along with films to X or gamma radiation. The screen helps to cut down the
exposure time by utilizing more effectively the radiation reaching the film. The intensification
effect is due to liberation of photoelectrons from the screen/foil. The intensification factor with
the above screen is generally 2 to 2.5. Fluorescent screen though give higher intensification
factor (app 5 to 6) are not recommended for use in radiography because of high screen sharpness.

4. Film density

All radiographs must have a readable density (blackening of the film). This is one of the first
checks made in a radiograph before attempting to interpret it. The radiograph when exposed will
have various densities depending upon how much exposure it received. The variable that affect
density in a radiograph are kV, milliampere/source strength, distances, development procedure,
film speed and time. A measure of the amount of exposure seen by the developed film is the light
transmission density or optical density or film density D which is given by
D= log10I0/It
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 I0=light intensity
It=transmitted light intensity

5. Radiographic sensitivity
Radiographic sensitivity is the ability of the technique to reveal the smallest discontinuity on the
radiograph. The term sensitivity is used in the sense that smaller the value better is the detection
capability. The sensitivity can be expressed either in absolute value or in %. It is mathematically
expressed as
Radiographic sensitivity = (∆t/t)×100
Where
∆t= smallest size of an artificial defect
T= thickness of the object
Contrast sensitivity
It is the ease with which an image can be seen against the radiograph background. Without
contrast the defect in the radiograph cannot be seen

Detail sensitivity
Detail sensitivity is needed to identify the various type of defect. In radiographic inspection
detail sensitivity is determined by the sharpness with which image detail of the penetrameter is
shown.

Various types of unsharpness that contributes to the definition of the image are
1. Geometric unsharpness
2. Movement unsharpness
3. Inherent unsharpness
4. Scattering unsharpness
5. Film and processing factor
Geometric unsharpness is a major factor which can be controlled under a certain
exposure set up. Movement unsharpness can be considered negligible when source object
and the film are stationery during the exposure. Inherent unsharpness is fixed once the
radiation energy is selected for the test. Scattered radiation produced within the object
reduces contrast whereas scatter radiation generated from the edge of the specimen
lowers the specimen. Total unsharpness can be calculated using the following relationship
U=√U2g+ U2i + U2s

6. Penetrameter
A Penetrameter also known as Image quality indicator is a gauge used to establish radiographic
technique or quality level. To accomplish this IQI must be made of material radiographically
similar to the material being radiographed. The identifying numbers in the Penetrameter are in
thousand of an inch. The Penetrameter letter/symbol on the radiograph indicates what type of
material the Penetrameter is made of. The entire outer edge or outline of the Penetrameter must
be visible on the radiograph if it is not, the radiograph does not have contrast sensitivity. The
proper hole must be visible if not the radiograph does not have detail sensitivity.

Hole-Type IQIs

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ASTM standard E1025 gives detailed requirements for the design and material group
classification of hole-type image quality indicators. Hole type IQIs are classified in eight groups
based on their radiation absorption characteristics. A notching system is used to indicate the IQI
material. The number on the IQI indicates the sample thickness that IQI would typically be
placed on. Also, holes of different sizes are present where these holes should be visible on the
radiograph. It should be noted however that the IQI is used to indicate the quality of radiographic
technique and not intended to be used as a measure of the size of a cavity that can be located on
the radiograph.

Wire IQIs
ASTM standard E747 covers the radiographic examination of materials using wire IQIs to
control image quality. Wire IQIs consist of a set of six wires arranged in order of increasing
diameter and encapsulated between two sheets of clear plastic. Wire IQIs are grouped in four sets
each having different range of wire diameters. The set letter (A, B, C or D) is shown in the lower
right corner of the IQI. The number in the lower left corner indicates the material group.

7. Determining Radiographic exposure

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 Exposure time can be calculated using the following expression

Exposure time in minutes = fd22(x/HvL)/c(RHM)1002

F= film factor
x=thickness of specimen
HVL= half value layer
C=source strength in curies
RHM= Radiation output in roentgen per hour by one curie at 1m.
Filter
Metallic sheets of high atomic number are used as filters to absorb soft radiation ( long
wavelength) emanating from the tube port and allow comparatively hard radiation (short
wavelength) to penetrate the specimen.
The use of filter results in the following advantage
Increased contrast around the specimen edge
Reduced undercut scatter at the edge of thinner sections
Record wide range of specimen thickness
Loss of intensity caused by the addition of filters is compensated either by increasing the time of
exposure or the KV.
Generally filters are made of aluminium copper or lead.
In the million volt range the use of a filter at the tube window does not improve radiographic
quality. However filters between the film and specimen improve image quality for specimens
above 40 mm thickness. Below the thickness filters do not improve image quality. A lead filter of
thickness 3 mm is found useful in the thickness range of 40-100 mm of steel. Above 100 mm
thick steel a lead filter of thickness 6 mm thickness improves image quality.

Inspection Technique

The technique used for various engineering component for radiographic inspection are given
below
Single wall single image technique
This technique is used when both the side of the specimen are accessible. This is used for plates,
cylinders, shells and large diameter pipes. This technique is illustrated in Fig. The source is kept
outside and the film inside or vice versa and the weld is exposed part by part (a smaller length of
weld).

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 Radiographic examination of pipe with single wall penetration

Panoramic technique
In this technique the radiation source is kept in the centre of the pipe and the film is fixed
around the weld on the outer surface of the pipe. The total circumferential weld length is
exposed at a time. This technique reduces the examination time considerably. It can be
effectively employed only when the source to film distance is sufficient enough to ensure
the proper sensitivity. The required IQI, as per the governing code can be placed either on
source or film side as the case may be.

Double penetration technique

The double wall penetration technique can be effectively adopted in three different
methods based on the prevailing pipe diameter and site restrictions. They are: (a) Double
wall single image; (b) Double wall double image and (c) Double wall superimposing
image

The techniques are shown in Fig. These techniques are used where the inside surface of
the pipe is not accessible. The source of radiation and the film are kept outside. The
radiation penetrates both the walls of the pipe.

(a) Double wall single image

The radiation source generally is kept on the pipe or very near to the OD and just near
the weld so that the source side weld is not falling on the film side weld. This
technique is employed for the pipes with diameter more than 90 mm OD. The IQI is
placed on the film side. Here film side weld only can be interpreted. As the
interpretable weld length is being small, this technique requires number of exposures
to cover the entire weld length depending upon the pipe diameter.
(b) Double wall double image
This technique is specially suited for the smaller diameter pipes up to 90 mm OD.
The radiation source is kept at a distance (SFD) with an offset from the axis of the
weld, to avoid the superimposing of the source side weld over the film side weld and
to obtain an elliptical image on the film. The IQI is positioned on the source side. In
this both the source and the film side weld can be interpreted from the image. This
requires minimum of two exposures, perpendicular to each other, to cover the entire
circumference of the weld.

(c) Superimposing technique

This technique is attempted whenever the required offset to obtain double image
could not be possible due to site restriction for the pipes with diameter up to 90 mm
OD. The source is kept at a distance (SFD) without offset. Thereby the source side
weld is superimposed on the film side weld on the film. The IQI is positioned on the
source side. This requires minimum of 3 exposure at 120 each to cover the entire
length of the weld.

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 Radiographic examination of pipe with double wall penetration
Latitude technique
Latitude of a film is closely associated with contrast. It is the range of thickness of a
material that can be recorded on the radiograph within the useful range of film density. A
high contrast film has less latitude and conversely a low contrast film has higher latitude.
There is a limitation on the specimen thickness range that can be inspected satisfactorily
in a single radiograph. One method of extending this thickness range and thereby
reducing the number of exposure required for a particular specimen involves the
simultaneous exposure of two films of different speeds. When two films of different
speeds are used to image the same subject in one exposure, the latitude of the films is
summed to expand the total latitude for the exposure. The technique is called double film
technique.

Double film technique

With proper selection of films and exposure conditions, the thicker sections will be
recorded on the faster film and the thinner sections on the slower film. The double film
technique can be used with or without lead screens. A centre screen, between the two
films may also be used to advantage. Using the above factors manipulation can be done
to get better latitude.

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 Fig. Double film technique

Special techniques
In a complex part, it is often required to consider certain areas individually and prepare a
separate technique for each area. Some pipe lines are designed to have the configurations
of core pipe and an envelope pipe to meet the intended purposes. Double envelope welds
can be tested by using multiwall penetration technique.

Multiwall penetration technique

This technique can be divided into two: (a) Multiwall single image technique and (b)
Multiwall double image technique. In these techniques, the radiation beam penetrates
all the four walls. Due to the geometry of the joint, the interpretable weld length in a
single exposure is much reduced in both the techniques compared to the techniques
normally employed for the same diameter pipe.

Multiwall single image technique

This technique is used for double envelope pipe of more than 90 mm OD and the
interpretable length is ascertained by the radiographic weld density. Hence a number of
exposures are required to cover the entire length of the weld.

Fig. Multiwall single image technique

Multiwall double image technique

This technique is used for double envelope pipe of 90 mm OD or less. Usually four
exposures are taken for each weld joint. Proper care should be taken to keep the film

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 normal to the radiation beam without wrapping the film on the pipe. The annular gap is
estimated from the film by taking care of the enlargement of the image of the core and the
envelope pipes. Minimum of three exposures are to be taken at 120° each to assess the
annular gap in various directions.

Fig. Multiwall double image technique

Application of Radiographic Inspection

1. Radiography can be used to inspect most types of solid materials both ferrous and
nonferrous alloys as well as nonmetallic materials and composites.
2. It can be used to inspect the condition and proper placement of components for liquid
level measurement in sealed components etc.
3. The method is used extensively for casting, weldment and forgings when there is a
critical need to ensure that the object is free from internal flaws.
4. Radiography is well suited to the inspection of semiconductor devices for detection of
cracks, broken wires, unsoldered connections, foreign material and misplaced
component, whereas other methods are limited in ability to inspect semiconductor
devices.
Real Time Radiography
Real time radiography uses X- or gamma radiation, as does conventional radiography to
produce a visible volumetric image of an object. A major difference is in viewing the
image. During film radiography, the image is viewed in a static mode; during real time
radiography the image is interpreted generally at the same time as the radiation passes
through the object. Another difference of real time image is that a positive image is
normally presented whereas the X-ray film gives a negative image.

The term fluoroscopy is synonyms with real time radiography and electronic radiography.
Basic equipment for conventional fluoroscopy consists of a source of radiation, a
fluoroscopic conversion screen, mirror and viewing port. To get a basic real time image ,
an object is placed between the source of radiation and fluoroscopic screen that converts

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 the transmitted radiation to visible light. A specially coated mirror then reflects the visible
image to a viewing port that lets the interpreter view the object. Because low light levels
are produced during conventional direct-viewing fluoroscopy, a device called image
intensifier is used to provide brightness of 100 or more times the intensity of the
fluoroscopic screen.

The image intensifier is a large glass enclosed electron tube. The function of the image
intensifier is to convert radiation to light, light to electron for intensification and electron
back to light for viewing. To make the conversions, the tube contains an input phosphor, a
photocathode, accelerating and focusing electrode and a final output phosphor. Like the
fluoroscopic screen the input phosphor converts the radiation passing through the object
to a light image. Photocathode emits electrons when excited by the input phosphor light
conversion is necessary because waves in the electromagnetic spectrum cannot be
accelerated whereas electron can. The acceleration of the electrons produces a brighter
image when they are converted back to light by the output phosphor.

Real time radiography has the advantages of high speed and low cost of inspection. Real
time radiographic concept can be applied in the case of microfocal radiography. In real
time microfocal radiography, zooming or projection magnification of the object is carried
out by dynamically positioning the object with the manipulators between the X-ray tube
and image receptor. Higher the magnification more the details one can see. Automatic
defect recognition (ADR) is another application of real time radiography. ADR is applied
to parts which can be inspected for the presence or absence of certain
components/materials for or the presence or absence of bonding agents such as solder or
brazing. ADR may also be used at very high speed for objects that can be scanned and
interrogated by intensity statistics, pixel statistics or similar window techniques for voids,
inclusions or other anomalies with good contrast against the surrounding material.
Fluoroscopic units have the disadvantage of lower sensitivity due to higher unsharpness
of the screen.
The use of micro focal units in conjunction with image intensifying system greatly
enhances the versatility and sensitivity of the real time radiographic setup. The inherent
unsharpness of the fluorescent screens would be compensated by the focal spot size (100
µm) of the micro focal units.
It has been reported that real time radiography has been applied to the inspection of laser
welds or electron beam welds in thin pipes having thickness of about 1 mm and porosities
in the range of 0.025-0.1 mm were detected. Approximately 1 second is required to
complete the image.
Microfocal Radiography
In conventional radiography unit the size of the focal spot ranges from 1 to 5mm. In order
to bring the geometric unsharpness (U g) as low as possible the film is placed in intimate
contact with the object and the source to object distance is increased. However the SOD
cannot be increased beyond a limit since this would make the exposure time impractical.
An alternative method is to reduce the focal spot. X-ray equipment in which the size of
the focal spot is between 0.1-1mm is commonly referred to as minifocus unit while X-ray
equipment in which the focal spot size is less than 0.1mm or 100 micrometers is referred
to as microfocus unit. This small focal spot is achieved by focusing the electron beam

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 onto the target. Present day microfocus unit have focal spot in the range of 5-15
micrometers. The fundamental physical processes such as the electron scattering in the
target makes it difficult to achieve focal spot better than 5 micrometers.

Advantage and limitation of microfocal radiography

Once the focal spot size is reduced a number of advantages can be identified. These
include
Projection Magnification
The object need not be in contact with the film during exposure as in conventional
radiography. Thus one can obtain enlarged primary radiographs with magnification
greater than 2X. Magnification reduces the number of features that is masked by the
background image noise thus enhancing the detection sensitivity of micro defects.

Improved radiographic contrast

It is well that in conventional radiography scattered radiation especially generated from
within the object reduces radiographs contrast to the maximum. Once the object is placed
away from the film the amount of scattered radiation reaching it is drastically reduced.
Thus microfocal radiographs have much better contrast as compared to conventional
radiographs.
Possibility of object manipulation
Since the object and the film can be separated without sacrificing image definition real
time radiography of dynamic /temporally changing events is possible. Further the object
can be rotated/translated within the radiation beam making stereo and micro tomography
possible. These techniques allow better detection of planar defects and greater resolution
of detail within the section thickness.
Limitations
Projection magnification has its inherent disadvantages (a) since the object is placed
closed to the source a smaller volume of the object is inspected at any one time. This
means more number of exposures and more number of films (b) since the electrons are
focused on to the target the heat is concentrated in a very small and localized spot. Hence
the target cannot be loaded to a greater extent which limits the tube current. However
both these are not very serious limitations.

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur
 Xero Radiography
Various methods have been introduced for obtaining radiographs among which xero
radiography is a method of imaging which uses the xero radiographic copying process to
record images produced by diagnostic X-rays. It is a method of X ray imaging in which a
visible electrostatic patter is produced on the surface of a photoconductor.
Principle

Procedure
1. The first step in xeroradiographic process is to sensitise the selenium layer by applying a
uniform electrostatic charge to its surface in the dark.

2. It is done by making the plate to move at a uniform rate under the startionary charging
device which may be scorotron or corotron and produces a charge on the surface of
photoconductor.

3. Thus the layer of selenium on one side become charged with a uniformly distributed
positive charge (600-1200 V) which is retained as long as it remain in the dark due to
great resistance of the layer.

4. The charged plate is then enclosed in a cassette which is light tight and is rigid enough to
provide mechanical protection to the delicate plate. The plate cassette combination is
used just as one would use an X-ray film in its cassette.

5. When the charged selenium plate is exposed to X-rays electron hole pairs are formed.

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur
 6. As X-ray photons impinges on this amorphous coat of selenium charges diffuse out in
proportion to energy content of X ray. This occurs as a result of photoconduction.

7. The resultant imprint is made visible (developed) by exposing the surface of the selenium
plate to the fine charged powder particles called toner which usually is a pigmented
thermoplastic material of dark blue colour.

8. The exposed xerographic plate placed on the top of dark box into which an aerosol of
charge toner particles is sprayed through a nozzle the process being called
triboelectrification or contact electrification.

9. The powder picture on the surface of the xeroradiogrphich plate is then transferred to a
special paper and fixed there to form a permanent image.

10. The paper is slightly coated with a slightly doformable layer of plastic such as low
molecular weight polyethylene material.

11. When it is pushed against the powder image under relatively high mechanical pressure
the toner particles become slightly embedded in plastic.

12. The paper is then peeled off the plate and the loosely held powder image is made into
permanent bonded image by heating the paper to a temperature of about 47.5F

13. The heat softens the plastic coating on the paper and allows toner particles to sink into
and become bonded to plastic.

14. The toner particles do not melt or flow. After fixing also called fusing, the imaging
portion of the xeroradiographic process is completed. The completed image is delivered
from the processor ready for viewing.

15. On a xero radiographic image, the areas which receive little X-ray exposure appear light
blue. If a charged plate is inadvertently exposed to room light and then developed the
paper will almost devoid of toner.

16. After transfer of toner to the paper some of it remains on the plate surface. All of the
tonner must be removed before the plate can be reused. This done by exposing the plate
to a light source that reduces the bond holding residual tone to the plate.

17. A pre clean corotron then exposes the plate to an alternating current which serves to
neutralize the electrostatic forces holding the toner to the plate.

18. The residual toner is then brushed off from plate using a clean brush. The plate then can
be reused. It is not charge during storage.

Advantages

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur
 Enhanced visualization of borders between images of different densities, low contrast which
enables differentiation between fat, muscle and bones and wide exposure latitude.
Elimination of accidental film exposure, high resolution, simultaneous evaluation of multiple
tissues, Ease of reviewing, Economic benefit, reduced exposure to radiation hazards.

Disadvantages

1. This technique cannot be used for very thick parts as very high exposure is required.

2. Technical difficulties, fragile selenium coat, transient image retention, slower speed.

3. Technical limitations: Low density of selenium plate which requires increased doses of
X-rays administered make the technique not to be considered as a total substitute for
halide radiograph.

Applications

Xeroradiography has found application in soft tissue imaging in radiographic examination of

the mammary glands, muscles, tendons and ligaments

X-ray computed Tomography

X-ray computed tomography has been applied in recent years to evaluate small metallic and
composite components. In this system a fan shaped X-ray beam is passed through the test object
to get a radiographic image of two dimensional slices of the object with interference from
overlying or underlying areas. Fig below illustrates this. This method is highly sensitive to small
difference (<1%) in material density.

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur
 MCQ
1. X-rays was discovered by (a) Thompson (b) Watson (c) W.C. Roentgen (d)
Becquerel
2. Many industrial X-rays machines contain target made of (a) Beryllium (b)
Magnesium (c) Lead (d) Tungsten
3. The PENTRAMETER is a tool used to check the __________ of a Radiograph (a)
Contrast (b) Definition (c) Sensitivity (d) Emulsion.
4. A DENSIOMETER is a instrument that measures (a) Radiographic contrast
(b)Radiographic sensitivity (c) Radiographic Density (d) Radiographic resolution
5. Most of the energy applied to X-ray tube is converted into (a) X-rays (b) Light (c)
Heat (d) UV radiation.
6. Another name for PENETRAMETER IS (a) Radiographic Shim (b) Image Quality
Indicator (c) Density standard (d) Acceptance standard.
7. An advantage of large grain film is (a) It has higher speed (b) It has better definition
(c) It has better sharpness (d) It has lower speed.
8. Which of the following welding discontinuities considered the most serious (a)
Porosity (b) Incomplete Penetration (c) Crack (d) slag inclusions.
9. The standard unit employed for measuring the strength of radio active substance is
curie which is defined as the quantity of radioactive material giving ________
disintegration per second (a) 2.2 × 108 (b) 3.1 × 108 (c) 1.5 × 1010 (d) 3.7 × 1010
10. Which is the naturally occurring radioactive isotope used in radiography? (a) Iridium-
192 (b) Radium (c) Cobalt-60 (d) Tantalum 192.
11. A term which refers to the sharpness of the radiographic image is (a) Sensitivity (b)
Halo effect (c) shadow effect (d) Definition
12. The film processing step in which the undeveloped silver bromide is removed from
the film emulsion is called (a) Development (b) Stop bath (c) fixing (d) Rinsing.
13. The penetrameter is a tool used to check the ___________ of a radiograph (a)
Contrast (b) Definition (c) Sensitivity (d) Emulsion
14. Which of the following types of radiation is commonly for radiographic testing
(a) Alpha particles (b) Neutrons (c) Gamma rays (d) Beta rays.
15. The ASTM penetrameter for a 25 mm thick test piece contains holes of what size (a)
T, 2T, 3T (b) 2T, 3T, 4T (c) T, 2T, 4T (d) T 3T 4T
16. A thin white line within the film image of a weld crown might be (a) a hair between
the lead screen and the film (b) Incomplete penetration (c) Crack (d) Under cut.
17. Following is not a correct statement about X-rays (a) They are short wavelength
electromagnetic radiation (b) They have greater penetrating power (c) The amount of
X-ray absorption is independent of material thickness not density (d) produced
when fast moving electron are suddenly brought to rest by colliding with matter.

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur
 18. Following is not a property of gamma rays (a) they do not impair and destroy
human cells (b) They are ionizing radiation (c) they pass through matter (d) They
move in straight line.
19. _______________ are electromagnetic radiation emitted from unstable nucleus (a)
ultraviolet rays (b) gamma rays (c) Light rays (d) Microwaves
20. Which of the following is defined as the property of radiation as a result of exposure
of radiation (a) Fluorescence (b) permeability (c) resistivity (d) conductivity
21. Following is the essential requirement of producing a radiograph (a) Source of
radiation (b) Recording medium (c) Processing chemicals (d) Magnetic Particles.
22. Incomplete penetration appears on the radiograph as (a) continuous or intermitted
dark lines of uniform width (b) Dark irregular shapes (c) Isolated white rounded
indications (d) Rounded or elongated smooth dark spots.
23. In industrial radiograph the normal working density varies from (a) 4 to 5 (b) 1.5 to 3
(c) 6 to 7 (d) 8 to 10/
24. Which is not the basic way to control the exposure when working with radiography
sources (a) Time (b) Temperature (c) distance (d) shielding
25. The term that refers to the smallest detail visible in radiogram is (a) Radiograph
sensitivity (b) Radiographic contract (c) subject contrast (d) film contrast.
26. The speed at which X and gamma ray travel is (a) It varies with wavelength (b) the
speed of light (c) the speed of sound (d)Depends on source
27. The process of being radioactive is called (a) Decaying (b) Heating (c)
Bremsstrahlung (d) Rectification.
28. The device which emits radiation of one or few discrete wavelength is (a) X-ray
machine (b) Linear accelerator (c) Gamma source (d) Betatron
29. Which of the most important factor in determining the archival quality of
radiographic film (a) film density (b) Image quality (c) Degree of removal of fixer
residues (d) Degree of removal of developers

4 Marks
1. Enumerate X-ray radiography principle.
2. Write the properties of X-rays and gamma rays.
3. Write short notes on Radiographic sensitivity.
4. Write about the safety aspect in industrial radiography.
5. List out the types of radiation sources.
6. What are the advantages and disadvantages of radiography?
7. Mention the Characteristics of radiographic films.
8. Describe radiographic testing methods.
9. State the applications of radiography.

12 Marks

1. Explain with neat sketch the various inspection techniques followed in radiographic
testing.
2. Explain microfocal radiography with its advantage and limitations.
3. Sketch and explain radiography testing of welded pipes using double wall penetration
method.

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur
 4. Explain with neat diagram the principle of radiography with its limitation and
application.
5. Discuss about the precautions against radiation hazards in radiographic testing.
6. Explain the types of Electromagnetic radiation sources.
7. Explain Xeroradiography with neat sketch .
8. Explain the following terms:
(i) Radiographic film
(ii) Radiographic sensitivity
(iii)Penetrameter
(iv)Radiographic exposure
9. Explain with a neat diagram how X-Rays are produced.
10. (i) Mention the characteristics of radiographic films.
(ii) Write about the safely aspects of Gamma rays cameras.
11. With neat sketch explain multiwall penetration technique in radiography.

Prepared by Dr J.Chandradass, Associate Professor, SRMIST,

Kattankulathur