X-Ray Technology for Industrial Used

X-Ray Technology for Industrial Used

X-Ray Technology for Industrial Used

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.

  1. X-rays are electromagnetic radiations, similar to visible light, with higher energy.
  2. They can pass through matter and get absorbed / scattered in the process.
  3. They can affect X-ray / photographic films.
  4. They can excite and ionize atoms of the medium, through which they pass.
  5. 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.

  1. 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.
  2. They have small focal spot size, which helps to obtain sharper images.
  3. Use of X-rays results in better image contrast, as X-rays have continuous spectrum.
  4. X-ray units ensure complete radiation safety, when they are switched 'OFF'.

b. Disadvantages of Industrial Radiography.

  1. X-ray units are bulky', for use at intricate & inaccessible locations.
  2. They require electric power for operation.
  3. 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.

X-Ray Technology for Industrial Used

 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.

X-Ray Technology for Industrial Used

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.

  1. a low voltage, to heat the filament,
  2. a high voltage, to accelerate the electrons.
The filament of an X-ray tube is normally operated at 6-12 volts with 5-6 amps of current. This is derived from the mains line using a step down transformer. The high voltage is usually obtained from a step-up transformer.

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


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