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Several types of scatter cause problems in radiography, these being side scatter, back scatter and internal scatter (self scatter). The angle formed between the direction of travel of the primary beam and the direction of travel of the scattered radiation (reaching the film) is called the scattering angle or angle of scatter. Side scatter and internal scatter have an angle of scatter which is less than or equal to 90° while for back scatter the angle exceeds 90°.

9.2.2.1 SIDE SCATTER

Radiation may be scattered by parts of the object that are not within the diagnostic area of the radiograph or by the walls of the exposure room. This is termed ‘side scatter’.

This type of scatter can be reduced by collimating the beam such that only the area to be examined is subjected to the primary beam and by the use of lead masking, diaphragms or grids. In x-radiography the use of a filter may help to reduce side scatter.

Side scatter causes ‘undercutting’ of the radiographic image around the edges of a component where these can be seen on the radiograph or at any site where there is a large change in section thickness (e.g. a bolt hole). Undercutting causes a lack of sharpness and may mask possible defect indications.

9.2.2.2 BACK SCATTER

Back scatter is caused by the primary beam striking an object behind the film and scattering back.

This type of scatter can easily be reduced by shielding the back of the film cassette with a sheet of lead. A sheet of lead approximately 2 mm thick is adequate for most

applications. In x-radiography the use of a filter may help to reduce back scatter.

The presence of excessive back scatter may be detected by placing a lead letter ‘B’

on the back surface of the cassette (i.e. the cassette surface furthest from the source of radiation). If there is excessive back scatter then a light image of the letter ‘B’ will be seen on the developed film. The use of a lead letter ‘B’ is mandatory when working in accordance with the ASME code and is required for each new technique by BS EN 1435 (i.e. not for production radiography). In accordance with BS EN 1435 the lead letter ‘B’ shall be a minimum of 10 mm high and 1.5 mm thick.

Note: If a dark image of the letter ‘B’ appears this is not an indication of excessive back scatter. It merely indicates scatter caused by the letter ‘B’ itself.

Should back scatter be detected then the thickness of the lead sheet shielding the back of the film cassette must be increased.

9.2.2.3 SELF-SCATTER

Self-scatter is scattered radiation originating from within the test component itself.

The detrimental effect of self-scatter on film quality can be reduced by the use of lead intensifying screens placed in contact with the film and, in x-radiography, by the use of filters.

If the radiation source is an x-ray tube then the use of a copper filter can help to reduce the effects of this type of scatter. A copper filter significantly reduces the proportion of low energy radiation within the primary beam. Since it is the low energy radiation which is chiefly responsible for scatter the use of such a filter can reduce the overall amount of scatter occurring and in this way improve image quality. Filters made from lead, steel or other metals may be used in a similar way.

Metallic foil intensifying screens made from lead or other metals reduce the effects of self scatter for both x-ray and gamma ray radiography. They filter out the low energy scattered radiation and prevent it from reaching the film.

The principle of collimation is simply that if there is less radiation then there will be proportionally less scatter.

9.2.3.2 DIAPHRAGMS

Diaphragms take collimation a step further. They consist of a sheet of lead which has a hole cut in it the same shape as the object which is being radiographed. Using a diaphragm the radiographer is attempting to shield out all unwanted radiation, the set up for radiography must however, be extremely accurate if the use of a diaphragm is to be

successful. Diaphragms are therefore more likely to be seen where a fully automated technique is in use that allows for a very high degree of repeatability in the set up accuracy.

9.2.3.3 MASKING OR BLOCKING

Masking or blocking consists of placing sheets of lead, bags of lead shot or barium putty or any other radiation absorbing material around the object which is being radiographed in order to reduce the undercutting effect of side scatter. Figure 44 below attempts to show the benefits of blocking.

Figure 43. Using a diaphragm

Figure 44. Radiographs produced with & without blocking 9.2.3.4 GRIDS

The use of a grid is generally limited to medical radiography. A grid consists of a matrix of parallel metal bars which is set in oscillation during exposure such that the grid itself does not produce a radiographic image. The use grid is a very effective method of reducing the effects of side scatter, but grids are very rarely a practical option for industrial situations. In order to be effective the grid must be placed as close as possible to the film. In microfocus x-radiography it may be placed between the film and the object.

9.2.3.5 FILTERS

Figure 42 above shows how the percentage of scattered radiation is high when the radiation energy is low. Placing a thin sheet (typically 1 to 2 mm) of copper or other metal in the primary beam, close to the source of radiation, greatly reduces the amount of low energy radiation while permitting most of the higher energy radiation to pass through. If there is less low energy radiation there will be less scatter, although it is possible that film contrast will be reduced. The use of a filter to reduce scatter is limited to x-radiography because gamma ray sources do not produce long wavelength low energy radiation.

Figure 46. The effect of filtration on a typical x-ray spectrum 9.2.3.6 METALLIC FOIL SCREENS

Lead screens, or screens made from other metals such as steel or copper are a very effective means of reducing scatter, for both x and gamma radiography, particularly when the energy of the primary beam exceeds 120 keV. Such screens are placed in contact with the film. The front screen works like a filter, greatly reducing the proportion of low energy radiation which reaches the film. Scattered radiation is always lower energy than the primary beam, so the scatter is more affected by the filtration effect than is the primary beam. The back screen reduces back scattered radiation which reaches the film.

In addition to this both screens intensify the effect of radiation, the energy of which, exceeds 120 keV. The screens do this by producing secondary electrons to which the film emulsion is sensitive. Most of the radiation exceeding 120 keV will be part of the primary beam. Thus the effect of the primary beam is amplified at the expense of the unwanted scattered radiation.

Figure 47. The effect of lead screens

9.2.3.7 HIGHER RADIATION ENERGY

Up to a radiation energy of around 1.5 MeV increasing the maximum radiation energy of the primary beam will reduce scatter (in proportion to the primary beam) and improve image quality. At higher radiation energy there is a continued decrease in the proportion of scattered radiation, but pair production within the film emulsion begins to have an increasingly detrimental effect on film unsharpness. See figure 42 above.

9.2.3.8 CHANGE FROM X-RAY TO GAMMA RAY RADIOGRAPHY

The absence of low energy components in radiation obtained from radioactive materials such as Iridium 192 or Cobalt 60 is the reason why gamma ray radiography is much less affected by scatter than is x-ray radiography.

9.2.3.9 REDUCING THE FOCUS OR SOURCE TO FILM DISTANCE

Reducing focus or source to film distance reduces the amount of matter through which the penetrating radiation is passing, in so doing reducing the amount of scatter. The use of a source to film distance that will produce a value of geometric unsharpness which is less than the inherent unsharpness for the film – screen combination in use is therefore not recommended.