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- INTENSITY-DISTANCE INVERSE SQUARE LAW
Applet> Intensity-Distance Law
The intensity-distance inverse square law is given by
where I1 and I2 are the intensities at 2 points and d1 and d2 are their respective distances from the source. The law assumes a fixed source.
- SFD-EXPOSURE FORMULA
Applet> SFD-Exposure Formula
The sfd-exposure formula is given by
where E1 and E2 are the respective exposures required to produce the same quality of image when the film is at distances SFD1 and SFD2 from the source. An exposure can be changed in 3 ways:
1) Varying the source intensity, Is, keeping the exposure time, t, fixed.
2) Varying the exposure time, t, keeping the source intensity, Is, fixed.
3) Varying both the source intensity, Is, and the exposure time, t.
In the Java simulation, a constant source intensity(Is1 = Is2) is assumed and since
The respective exposure times, t1 and t2 , required at
the source-to-film distances, SFD1 and SFD2,
are given by
the formula,
2) Theory
- A Conventional X-Ray Unit
Figure 1 Conventional design of glass envelope X-ray
tube
X-rays are a form of electromagnetic radiation, of the same physical nature as visible
light, radiowaves, etc. However, they have a wavelength which allows them to
penetrate most materials with partial absorption during transmission. Their
wavelength varies from about 10 nm for low energy radiation to about
10-4 nm for high energy x-rays which will penetrate up to 500 mm in
steel.
If a film is exposed at a distance "D" from the x-ray tube and a certain density produced, then to produce the same density on a similar film at a distance of "2D", a greater exposure is required. The exposure required to produce the same quality of image is related to the source-to-film distance(sfd) by,
An exposure can be changed in 3 ways:
1) Varying the source intensity, Is, keeping the exposure time, t, fixed.
2) Varying the exposure time, t, keeping the source intensity, Is, fixed.
3) Varying both the source intensity, Is, and the exposure time, t.
In the Java simulation, a constant source intensity(Is1 = Is2) is assumed and since
X-rays are generated when a beam of high
energy electrons is stopped suddenly by a metal target. The essentials of an
X-ray tube are shown in Figure 1.
To produce x-rays, the filament is heated
with a current until it is hot enough to emit electrons. The tungsten target or
anode is charged positively with respect to the filament and therefore the
negatively charged electrons are attracted to the anode. At the anode, the
electrons are stopped and they then produce x-rays which are directed towards
the object to be radiographed.
Three variables which can be adjusted using
the instrument panel connected to the x-ray tube are energy (in kV),
intensity (in mA) and the exposure time (in min) which the x-rays
are left on for during an x-ray shot. These variables need to be adjusted
accurately in order to produce a satisfactory radiograph.
The high voltage difference between the filament and the target is termed the kV used for the radiograph. Typical values in industrial radiography are 50 to 300 kV although higher and lower values are used. If the kV is higher, the x-rays produced have more energy and therefore they can penetrate a thicker component. The general practice is to increase the kV used as the thickness of the component to be radiographed increases. The kV can be considered as the quality of the x-rays.
The high voltage difference between the filament and the target is termed the kV used for the radiograph. Typical values in industrial radiography are 50 to 300 kV although higher and lower values are used. If the kV is higher, the x-rays produced have more energy and therefore they can penetrate a thicker component. The general practice is to increase the kV used as the thickness of the component to be radiographed increases. The kV can be considered as the quality of the x-rays.
The mA is a measure of the electron flow
striking the target. The focusing cup can control the mA and also focus the
electrons onto the target. The greater the mA value, the greater the quantity of
x-rays produced at the target. Therefore the mA is a measure of the amount of
x-rays leaving the target. This is also called the intensity.
The Exposure (in mA.min), E, given
to a film is equal to the product of the intensity, I, and the exposure time, t,
i.e., E = I x t.
At the anode, only a small proportion
(1-10%) of the energy of the electrons is converted to x-rays and most becomes
heat energy. The tungsten target therefore needs to be air, water or oil
cooled.
The effective width of the source of the x-rays is considerably smaller than the area of the target on which the electrons are incident. Effective source widths vary up to 5 mm in diameter.
The effective width of the source of the x-rays is considerably smaller than the area of the target on which the electrons are incident. Effective source widths vary up to 5 mm in diameter.
- Production of A Radiograph
Figure 2 Arrangement for film
radiography
A radiograph is a photographic image
produced by a beam of penetrating ionised radiation after passing through a
specimen and radiography is the production of radiographs.
The usual arrangement for producing a radiograph is shown in Figure 2, using a small diameter source G, and a sheet of film as a detector.
The usual arrangement for producing a radiograph is shown in Figure 2, using a small diameter source G, and a sheet of film as a detector.
The cavity in the specimen, as shown at B,
causes a lower absorption along the path GBF. More radiation reaches the film at
point F, compared with, say, point N. Therefore an x-ray "image" of the
cavity is produced.
To produce a radiograph, the x-rays are
allowed to reach the film for an appropriate exposure time, which depends on the
intensity of the x-rays, the
thickness of the specimen, and
the characteristics of the
film. The film is then processed (developed, fixed, washed and dried) and the
defects can be seen as blackened areas. The film is then placed on an
illuminated screen so that the image can be examined and
interpreted.
- Intensity-Distance Law
Figure 3 Intensity varies inversely to the distance from
source
As ionised radiation travels in straight
lines outward from a source, i.e., it is not focused, the simple "inverse square law" applies since
when the distance between the film and the source is increased, the
radiation has to cover a larger area and so is reduced in intensity as shown in
Figure 3. Assuming a fixed source, intensities
I1 and I2
at two points which are at distances d1 and
d2 from the source, are related
as,
Therefore,
- SFD-Exposure Formula
If a film is exposed at a distance "D" from the x-ray tube and a certain density produced, then to produce the same density on a similar film at a distance of "2D", a greater exposure is required. The exposure required to produce the same quality of image is related to the source-to-film distance(sfd) by,
where E1 and E2 are the respective
exposures required when the film is at distances SFD1 and SFD2
from the source.
1) Varying the source intensity, Is, keeping the exposure time, t, fixed.
2) Varying the exposure time, t, keeping the source intensity, Is, fixed.
3) Varying both the source intensity, Is, and the exposure time, t.
In the Java simulation, a constant source intensity(Is1 = Is2) is assumed and since
The respective exposure times, t1 and
t2 , required at the source-to-film distances, SFD1 and
SFD2, are given by the formula,
Therefore,
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