X-Ray Technology for Industrial Used
1. Introduction:
X-rays were discovered by a German scientist, Prof. Wilhelm Conrad Roentgen in 1895. Some of the properties of X-rays are given below.
- X-rays are electromagnetic radiations, similar to visible light, with higher energy.
- They can pass through matter and get absorbed / scattered in the process.
- They can affect X-ray / photographic films.
- They can excite and ionize atoms of the medium, through which they pass.
- They can cause injury to biological systems.
2. X- Ray And Gamma Rays:
X and gamma rays have similar
properties. Gamma rays are emitted by the nucleus, whereas X-rays are generated
outside the nucleus when high speed electrons interact with atoms. Gamma rays
have definite, discrete energies, whereas, X-rays have continuous energies. The
maximum energy of X-rays depends on the incident electron energy (Fig. 2. l).
2.1: Advantages and Disadvantages of X-ray Equipment for
Radiography
a. Advantages of Industrial Radiography.
- X-ray machines have higher radiation output (about 45 R/min at 50 cm from a 200 kV, 15mA X-ray unit, compared to 40 R/h at 50 cm from a 20 Ci iridium-192 source), enabling larger turnover of workload.
- They have small focal spot size, which helps to obtain sharper images.
- Use of X-rays results in better image contrast, as X-rays have continuous spectrum.
- X-ray units ensure complete radiation safety, when they are switched 'OFF'.
b. Disadvantages of Industrial Radiography.
- X-ray units are bulky', for use at intricate & inaccessible locations.
- They require electric power for operation.
- They require high capital investment.
3: Production of X-Rays
X-rays are produced when a beam of high energy electrons
collides with any material (target).
X-ray production increases with increase in atomic number of the target atom and with increase in the incident electron energy. In an X-ray machine, less than one per cent of the electrical power supplied is converted to X-rays and the remaining power appears as heat. If this heat is not removed efficiently, the target material may melt.
Although X-ray intensity is different for different target
material, the distribution of X-ray energies for all targets is similar, the
maximum energy being the energy of the incident electrons. In an X-ray machine,
if the potential difference between the filament and the target or the applied
kilo-voltage is 200 kV, then the energy of the electrons hitting the target is
200 keV and the maximum energy of the X-rays would be 200 keV. A typical X-ray
spectrum is shown in Fig. 2.1. The continuous X-ray spectrum will also contain
one or more sharp peaks. These peaks are of definite energies, dependent on the
target element, hence are called characteristic X-rays.
The quality of an X-ray beam can be described by its Half
Value Thickness (HVT). The HVT is a function of the effective energy of the
X-ray beam, which is approximately 1/3rd of the applied kilo-voltage. It also
depends upon the nature of the power supply and the added filtration.
Fig. 2.2 : Hooded Anode X-ray Tube.
The essential requirements for the production of X-rays are
I. a source of electrons (heated tungsten filament),
II. high voltage supply to accelerate the electrons,
III. a target, usually tungsten, to stop the electrons and to convert their energy to X-rays
The cross-section of a typical X-ray unit is shown in Fig.
2.2.
The target is usually of small dimension, say 2-3 mm. When
the electrons hit the target, much of the energy appears in the form of heat
and it has to be rapidly removed. Copper, is used for the purpose of heat
removal. Certain anodes are hollow in construction, so that primary coolants
can be circulated through the same to remove the generated heat. Mineral oil is
also sometimes used around the X-ray unit, to remove heat and serve as
electrical insulator.
The penetration of X-ray beam depends on the applied
kilo-voltage, whereas, the intensity is decided by the current flowing through
the filament (mill-amperage).
Selection of the target material is based on the following
properties:
I. The target material should have a high melting point.
II. It should possess a high atomic number.
III. It should possess high thermal conductivity (to dissipate
the heat quickly).
IV. It should have low vapor pressure at high temperatures
(to prevent evaporation of the target material and its deposition on the walls
of the X-ray tube, as this would cause
absorption of X-rays and disturbance in the insulation properties of the tube).
Tungsten, having an atomic number 74, and melting point 3400
0 C is the most preferred target material.
X-ray tube is contained in a suitably shaped steel shell for ruggedness. The power ratings, viz., kilo-voltage (kVp), tube current (mA), besides the cooling pattern, decides the structure of an X-ray unit.
4. Requirements Of An Industrial X-Ray Tube:
1. An industrial X-ray tube must be capable of operating
continuously for indefinite periods at maximum loading.
2. It should be able to pass appreciable current over the
lowest range of operating voltages. This is to permit such radiographs to be
taken, which necessitate low voltage techniques, within reasonable exposure
periods.
3. It should possess the smallest possible focal area. For
maximum radiographic definition, a point source of radiation is one of the
requirements. Modern X-ray tubes have very small focal areas.
4. The design safety should include sufficient shielding
material (say lead or equivalent steel) so that the leakage radiation level at
every rating combination (kV, mA) does not at I meter from the target.
5. Special Industrial X-Ray Tubes:
Industrial radiography involves inspection of objects of
various materials and in many shapes and sizes. For objects containing organic
compounds, eg. food stuff, plastic insulating materials, etc., the required
voltage is in the range of 50-100 kV. The examination of light metal and steel
castings, welds in pipelines, pressure vessels, ships and bridges and weapons
of war, requires kilo-voltage in the range 150 k V - 2 M V. Most frequently
used voltage is between 150 kV and 400 kV. Portable X-ray units, in the voltage
range 150 kV to 250 kV are used for field radiography. X-ray units of higher
voltage are generally stationary ones, for use in enclosed installations.
X-ray units can also be used as cabinet installations with
incorporated lead shielding and safety interlocks (eg., the unit becomes
operable, by actuation of certain microswitches, only when the object occupies
a preset position in front of the beam port).
5.1 : Fluoroscopy:
The fluoroscopy technique is used for
continuous production line scanning of die castings, in food processing
industry, etc. A fluoroscopy unit consists of X-ray source, fluorescent screen
(zinc cadmium supplied) and leaded glass barrier. The equipment is normally
supplied in shielded enclosures. The object to be examined is placed in between
X-ray beam and fluorescent screen. A Shadow image is produced on the screen and
it is viewed through television monitor system or by the use of image
intensifiers.
5.2: Fine Focus Tube:
The use of fluorescent screens, for
examination of castings and assemblies at considerable magnification, is made
possible by using a tube with a very fine focus of about 0.2 mm in diameter.
The small size of the spot reduces geometric unsharpness and also produces
image magnification.
Fig. 2.3: Rod Anode System
5.3: Rod Anode Tube:
The examination of confined
spaces, like the pipes of a steam boiler or the cylinder heads of internal
combustion engine, has given rise to an X-ray equipment with the target at the
end a long tube. The target and therefore, the whole anode is earthed, so that
the source of radiation can be pushed into the cavities mentioned above. In
X-ray units used for circumferential radiography, the target is placed at right
angle to the tube axis and as a result, e radiation emerges all round in the
form of a disc. For unidirectional beam, the target is 45 0 inclination (Fig.
2.3).
5.4 : Crawler X-ray Units:
These units are useful for
cross-country pipe line inspection, with automatic movement from joint to joint
and are becoming increasingly popular. The power input is obtained from diesel
generators.
6. X-Ray Generator Circuits:
The power supply required for the operation of an X-ray tube are.
- a low voltage, to heat the filament,
- a high voltage, to accelerate the electrons.
To maintain the target at positive potential with respect to the filament, different types of rectification circuits are used, viz., half-wave rectification, full-wave rectification and constant potential units. There are various advantages of using a constant potential X-ray unit. It yields better X-ray output than that produced by a pulsating potential having the same peak voltages. It gives a more penetrating beam, as required in industrial radiography.
7. Linear Accelerator:
To obtain high energy X-rays in the MeV range, for
inspection of very thick objects, linear accelerators are used. In these, the
X-ray intensity can be of the order of few hundred Roentgen per minute at one
meter.
8. X-RAY Control Panel User Notice.
I. Make sure the Connection between Machine and Ground.
II. Aging the Machine Manually or Automatically strictly according to OPERATION INSTRUCTIONS before running.
III. The Machine can assign work time and break time as 1:1 automatically. If exposure time exceeds 5 minutes the machine will first work for 5 minutes then break for another 5 minutes, then finish the extra work.
IV. After finishing the work, atleast waiting for 5 minutes before cut off the power supply.
V. Do not used the Machine when the gas pressure in X-RAY generator lower then 0.35MPa.
VI. Display Instructions.
dd - Preparing (Setting)
AA - Ready
Fd - KV pre-setting exceeding lower
FF - KV pre-setting exceeding higher
6b - Lower supply voltage
dA - Stop aging
OA - Non current or Low current
FA - Over current
Ob - Non-output voltage
Fb - Over output voltage
FC - X-RAY generator over temprature