Unit 3
Radiographic Methods
Electromagnetic waves are coupled electric and magnetic oscillations that move with the speed of light and exhibit typical wave behaviour.
The following properties are associated with electromagnetic waves traveling through free space:
- Electromagnetic travels with the speed of light
- Electromagnetic waves are the transverse wave
- Electromagnetic waves carry both energy and momentum, which can be delivered to a surface.
- The ratio of electricity to the magnetic field in an electromagnetic wave equals the speed of light.
Key Takeaway points:
It is the coupled electric and magnetic oscillations that move with the speed of light and exhibit typical wave behaviour.
The disintegration of nuclei of high atomic number into a lower atomic number is known as radioactivity. It is found that in elements of an atomic number higher than 82, repulsive forces are very high and such elements are no longer stable.
These elements start disintegrating to form stable elements of lower atomic numbers. During the process of disintegration, alpha particles, beta particles, and gamma rays are emitted.
Radioactive disintegration occurring per unit time is proportional to the total number of radioactive atoms present
.
=λN, where λ=Constant of proportionality
N= Number of radioactive atoms present
The negative sign indicates a decrease in number with time.
Key Takeaway points:
The disintegration of nuclei of high atomic number, owing to repulsive Coulomb force is called radioactivity.
There are five types of decay, all are mentioned here:
- Alpha Decay:
The emission of alpha particles reduces the size of the nucleus. The main reason for the instability is the larger size of the nucleus.
2. Beta Decay:
The emission of an electron by a neutron in the nucleus changes the neutron to a proton. The main reason behind the instability is the nucleus has too many neutrons relative to the numbers of protons.
3. Positron Emission:
Emission of positron by a proton in the nucleus changes the proton to a neutron. The reason behind instability is the nucleus has too many protons relative to some neutrons.
4. Electron Capture:
The capture of an electron by a proton in the nucleus changes the proton to a neutron. The reason behind the instability is the nucleus has too many protons relative to some neutrons.
5. Gamma Decay:
The emission of gamma rays reduces the energy of the nucleus. The reason behind instability is nucleus has excess energy.
The attenuation of radiation is also known as the total absorption of radiation.
The total absorption/attenuation is a combination of three absorption processes: Photoelectric effect, Crompton effect, and pair productions.
The rest two: Thompson and photodisintegration are considered only under special circumstances else their effect is considered to be negligible.
In all three absorption process, the energy of X-rays lowers and it gets scattered in different directions with different wavelengths.
The contribution of these effects to the total linear attenuations coefficient (µ) is plotted for the steel against radiation energy.
Fig 1: Attenuation of radiation
Key Takeaway points:
Attenuation is a combination of three absorption processes, photoelectric effect, Compton Effect, and pair production.
When a photon of X-ray of relatively low energy passes through a material and gets collided with an atom of the material, it kicks off an electron from the material surface and these electrons can be detected as a change in the electric charge of the metal or as an electric current.
This ejection of an electron from the shell of the atom ionizes the atom and this ionized atom returns to the equilibrium or neutral state with the emission of X-ray characteristics of the atom.
This phenomenon is called a photoelectric effect and occurs in the object, in the film, and in any filters used.
It is also called Thompson scattering or coherent scattering. When a photon gets scattered with no change in internal energy of the scattering atom that is this phenomenon occurs without loss of energy.
When photons of X-ray with higher energies (100 KeV to 10MeV) get interacted with an electron, causes a part of the energy of photons to be transferred to the electrons, and the electron gets ejected from its orbital position.
At the same time, photons continue to travel through the material along the deflected path from the initial angle of incidence and scattered X-ray photons emerges as radiation of reduced energy, Scattered in all direction.
Since scattered X-ray photon has less energy, it will have a longer wavelength and less intensity of penetration than the incident photon. The change in wavelength of the scattered X-ray photon is given by,
Where λ’-λ= (h/Cme)(1-cosθ)
λ’=Wavelength of incident X-ray photon
λ=Wavelength of scattered X-ray photon
h=Planck’s constant, C= speed of light,
me=mass of the electron, θ=angle at which scattering takes places
When high energy photon (about 1 MeV) interacts with the nucleus of the atom, the energy of photons get transferred and results in the production of pairs of irons, electron (e-), and positrons (e+).
Positrons (e+) have small life and they disappear with the formation of two photons of 0.5 MeV energy.
Pair production takes place when high energy photon interacts through a high atomic number material.
1. Beam Geometry:
X-ray and gamma rays both obey the common law of light. The formation of the shadow of any object is also a property of light. Naturally, the shadow cast by the object will show some enlargement and this enlargement of the shadow is calculated as degree or enlargement which is mathematically expressed as,
(S0/Ss) = (D0/Ds) where, S0 = Size of the object, Ss=Size of the shadow, D0=Distance from the source of radiation, and Ds= distance from the source of radiation to the recording surface.
Similarly, the degree of sharpness of any shadow depends on the size of the source of light and the position of the object.
Radiations from X-ray tubes and radioactive source always produce radiographic images with a certain amount of blurring, which is called unsharpness (Ug)
(Ug)=(S.DD)/(DF-DD) where, (Ug)= geometric unsharpness, S=Size of source and DD=Distance between defect and film
The maximum value of Ug can be calculated through expression,
(Ug)max=(S.t)/(DF-t), where, t=thickness of the object
From the above equation, it is very much clear that geometric unsharpness can be reduced to any limit between source and film.
2. Scattering factor:
When an X-ray or gamma-ray passes through any object, it gets attenuated due to absorption and scattering of radiation.
The attenuation of radiation depends upon the uniformity of the object. If the object is uniform then the attenuation will be uniform and the radiation image will be evenly exposed. When the radiation beam is focused on any object, some part of the radiation is transmitted through the object and forms the image. Some get scattered contributing to the non-forming of image.
Thus the total radiation is the sum of transmitted radiation and scattered radiation.
Total radiation=Rt+Rs
The ratio of transmitted radiation and scattered radiation is called the scattered radiation factor or scattering factor.
Scattering factor=( Rt/Rs)
Key Takeaway points:
This enlargement of the shadow is calculated as degree or enlargement which is mathematically expressed as,
(S0/Ss) = (D0/Ds)
X-rays are the electromagnetic radiations that can be produced by the Bremsstrahlung radiation method and K-shell emission method.
- Principle:
X-rays are produced when a high-speed moving electron get collided with the nucleus of an atom, x-rays are produced in an X-ray tube which consists of a glass or ceramic envelope containing an anode, a positively charged electrode, evacuated to an ultra-high vacuum of 109 hectopascals.
Cathode acts s a producer of electrons, when electrical tension is set up between the cathode and anode, these electrons are attracted by the anode and get accelerated up.
Fig.2: X-ray radiography
These electrons from the cathode are connected into a beam by a focusing cup and when these electrons get collided with the anode, a part of their energy is transformed into X-radiations called X-rays.
2. Equipment and methodology:
The main components of X-ray radiography are the tube, the high voltage generator, the control console, and the cooling system.
X-rays are generated by directing steam of high-speed electrons at a target material such as tungsten, which has the highest atomic number. When the electrons are slowed or stopped by the interaction with the atomic particles of the target, X-radiation is produced.
In X-ray equipment, there is a glassed enclosed tube known as an X-ray tube, consists of a glass bulb under vacuum, a negatively charged electrode known as the cathode, and a positive electrode known as the anode. The cathode is in form of a filament.
When the suitable current of few amperes is applied to the filament, it emits electrons which are then attracted by the anode. The stream of electrons is concentrated in the form of a beam of high-speed particles.
As the beam high-speed electrons strike the targeted it produces X-rays.
Electrons striking with more energy results in X-rays with higher penetrating power. The area struck by the high-speed electron beam is called the target of the focal spot.
The projection of the focal spot on the surface perpendicular to the axis of a beam is known as optical focus. A pinhole is made in thick material; it is aligned parallel to the tube axis and perpendicular to the beam of X-rays.
Because the defect possesses a lesser density than the parent metal, they transmit the X-rays better than the parent metal. Thus a dark radiographic image of defects obtained.
3. Applications:
- X-ray radiography is used in the fields of testing of aircraft.
- This technique is used for detecting flaws in the pressure vessels and boilers.
- This technique is used for the detection of cracks and other flaws in marine vehicles like ship structure etc.
There are three types of radiations are there mentioned below:
- X-ray:
These rays are produced by an X-ray generator and gamma radiation is the product of radioactive atoms. X-rays are an extremely short wavelength, a form of electromagnetic radiation that is capable of penetrating many materials that reflect or absorbs visible light.
X-rays are generated from X-ray tube equipment. They are operating at 400 kV, can inspect steel objects having a thickness up to 60 mm.
2. Gamma-ray:
These rays are emitted during the disintegration of radioactive nuclei. It is also electromagnetic radiation of short wavelength.
It has high penetrating power than X-rays so that it is generally used in industrial applications.
3. Neutron Beam:
Various radioactive isotopes are used as the radiation source. Neutron beams are generally derived from nuclear reactors, radioisotopes, or nuclear accelerators.
For most applications, it is necessary to moderate the energy and collimate the beam before use.
4. Limitations:
- It cannot detect small weld defects.
- Radiation precaution is necessary due to high energy radiation
- As it can’t be operated electrically, it can’t be switched off.
Gamma rays are also electromagnetic radiations like X-rays with a very short wavelength of less than one-tenth of a nanometre and are distinguished only by their source of origin rather than their nature.
- Principle:
Gamma rays in the range of 102-104 GeV are sometimes termed as very high gamma rays, while those above 1014 eV are termed as ultra high energy gamma rays.
It is different from X-ray radiography. It is a product of radioactive atoms that depends on the ratio of neutrons to protons within its nucleus, an isotope of a particular element may be stable or unstable.
2. Equipment used:
- In the case of gamma rays radiography, a device called a camera is used to store, transport, and expose the object containing material.
- Shielding material is used to prevent radiation exposure.
- A device called a radiographer is used for the detection of defects.
Fig.3: Gamma-ray radiography
3. Source of radioactive materials and technique:
These rays are emitted during the disintegration of radioactive nuclei. It is also electromagnetic radiation of short wavelength.
It has high penetrating power than X-rays so that it is generally used in industrial applications.
Gamma radiations are electromagnetic radiations like x-rays which are generated by a Gamma-ray source pencil.
There is a device attached to it called a camera which is used to store, transport, and expose the object containing radioactive material.
A radiographer is used to emit the gamma rays and a shielding material is present there which is responsible for preventing the direct exposure of radiation.
The radiographer uses a drive cable to force the radioactive material into the guide tube, where gamma rays will pass through the specimen, the defect can be detected easily.
Advantages are mentioned below:
- 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 easily available in nature.
- After a certain distance of penetration, gamma rays decay themselves by losing all their energy.
- Equipment of gamma rays is portable and safe.
1. Under the aspect of precautions against radiation hazards certain rules are necessary:
- All workers should be monitored regularly.
- The dose received by the person should be well within prescribed limits.
- Causes of excessive exposure should be detected with minimum delay and the suitable corrective measure taken to avoid future excessive exposure.
- The cumulative records of the individual worker should be maintained for the entire period during which they work with the radiation source.
- Radiation protection is the prevention of illness or injury from over-exposure to X-ray and nuclear radiation.
- Radiation is considered hazardous when a person is exposed to it beyond a certain limit.
- The human body is exposed to background radiation from naturally occurring radio-isotopes and cosmic rays.
- Radiation protection activity consists of:
- Measurement and evaluation of pressure level.
- Introducing measures to minimize exposure and eliminate needless exposure.
3.15 Case study- casting and forging:
Gas porosity appears as round or elongated smooth dark spots, occurring individually throughout the casting. This is caused by gas formation during solidification by the evaporation of moisture from the mould surface.
Gas holes appear as dark circular images, isolated or in clusters. These are caused by gas entrapment in molten metal. If the molten metal solidifies before all gases escape, the gas is entrapped in the casting resulting in gas holes.
Shrinkage appears as dendrite, filamentary or jagged darkened areas. These are caused due to contraction of metal while the casting solidifies.
Cold shut appears as a dark line of variable length with a definite, smooth outline. Cold shuts are formed when two streams of molten metal flowing from different directions fail to unite. The formation of a cold shut is due to interrupted pouring the metal at too low temperature.
Misruns appears as a prominent, darkened area of varying dimensions with a definite, smooth outline. These are produced by the failure of the molten metal to fill a section of casting.
References:
- Dr. D Vijay Kumar, A textbook on Non- destructive testing.
- Google websites.
- Palash Awasthi, notes on NDT
