What types of material are used for radiographic intensifying screens?
What types of material are used for radiographic intensifying screens?
The radiographic image is formed by only approximately 1 % of the amount of radiation energy exposed at the film. The rest passes through the film and is consequently not used. To utilise more of the available radiation energy, the film is sandwiched between two intensifying screens. Different types of material are being used for this purpose.
Under the impact of X-rays and gamma-rays, lead screens emit electrons to which the film is sensitive. In industrial radiography this effect is made use of: the film is placed between two layers of lead to achieve the intensifying effect and intensity improvement of approximately factor 4 can be realised. This method of intensification is used within the energy range of 80 keV to 420 keV, and applies equally to X-ray or gamma-radiation, such as produced by Iridium192.
Intensifying screens are made up of two homogeneous sheets of lead foil (stuck on to a thin base such as a sheet of paper or cardboard) between which the film is placed: the so called front and back screens.
The thickness of the front screen (source side) must match the hardness of the radiation being used, so that it will pass the primary radiation while stopping as much as possible of the secondary radiation (which has a longer wavelength and is consequently less penetrating).
The lead foil of the front screen is usually 0.02 to 0.15 mm thick. The front screen acts not only as an intensifier of the primary radiation, but also as an absorbing filter of the softer scatter, which enters in part at an oblique angle, see figure 2-6. The thickness of the back screen is not critical and is usually approx. 0.25 mm.
The surface of lead screens is polished to allow as close a contact as possible with the surface of the film. Flaws such as scratches or cracks on the surface of the metal will be visible in the radiograph and must, therefore, be avoided. There are also X-ray film cassettes on the market with built-in lead-screens and vacuum packed to ensure perfect contact between emulsion and lead foil surface.
Figure 4a-6 and figure 4b-6 clearly show the positive effect of the use of lead screens.
Summarizing, the effects of the use of lead screens are :
improvement in contrast and image detail as a result of reduced scatter
decrease in exposure time
A total processing cycle of a few minutes is possible with the use of an automatic film processor which makes it a very attractive system to deploy offshore (on lay barges) where weld examination has to be done at a very fast rate and few concessions are made towards image quality. Fig. 5-6 shows that a time saving at 10(3.7-2.8) or 100.9 works out at approximately a factor 8. The actual time saving is often closer to factor 10.
These RCF screens are also used for “on-stream” examination, whereby long exposure times and mostly hard (gamma) radiation are applied because of the penetrating power required. However, the relatively long exposure time (causing reciprocity) and hard radiation (Cobalt60) together considerably reduce the light emission effect, as tables 1-6 and 2-6 show.
On balance, the relative time saving is much smaller; usually no more than a factor 2 for an F6-film (at Ir192 and Co60) instead of 10 in the D7 lead screen technique. See the bold figures (2.5 and 1.7) in table 2-6.
Figure 6-6 gives an overview of graphs from which the relative exposure times can be deduced when using different films and screens at 200 kV, (for film-density 2). The graph shows that an F8-film with RCF screen (point C) is approximately 8 times faster than a D8-film with lead (point B) and approximately 15 times faster than a D7-film with lead (point A). Since on-stream examination as well as examination of concrete, and also flash radiography allow concessions to image quality, a special fluorometallic screen (NDT1200) has been developed with extremely high light emission. In combination with an F8-film it may result in a reduction in exposure time at a factor 100 at 200 kV, against a D7-film with lead (point D as opposed to point A in figure 6-6), or even a factor 140 to 165, depending on source selection, see table 2-6. The intensification factor of the NDT1200 screens increases significantly at lower temperatures.
Table 2-6 shows the effect of radiation hardness on relative exposure times for the various film/screen combinations compared with D7 film with lead screen. Noticeably, for the NDT1200 screen and F-8 film the factor increases with the increase in energy, but for the F6 film the factor decreases at energy levels exceeding 300 keV.es at energy levels exceeding 300 keV.
It is clear from the above tables and graphs that there are many ways to reduce the exposure time or radiation dose needed. The required image quality is decisive (a higher exposure rate automatically means reduced image quality), and next the economic factors, for example the cost of the screens against time saved need to be weighed.
Steel and copper screens
For high-energy radiation, lead is not the best material for intensifying screens. With Cobalt60 gamma-rays, copper or steel have been shown to produce better quality radiographs than lead screens. With megavoltage X-rays in the energy range 5-8 MeV (linac) thick copper screens produce better radiographs than lead screens of any thickness.
The term fluorescence (often mistaken for phosphorescence) is used to indicate the characteristic of a substance to instantly instantly emit light under the influence of electromagnetic radiation. The moment radiation stops, so does the lighting effect. This phenomenon is made good use of in film based radiography. Certain substances emit so much light when subjected to ionising radiation, that they have considerably more effect on the light sensitive film than the direct ionising radiation itself..
The term phosphorescence is used to describe the same luminescent phenomenon, but once the electromagnetic radiation ceases, light fades slowly (so called after-glow).
NDT additionally uses the “memory effect” of some phosphorous compounds to store a latent radiographic image in order to develop it later into a visible image with the aid of laser stimulation. The image quality is mediocre because relatively coarse phosphorous crystals are used. The possibility of producing memory phosphors with smaller crystals is studied.
Fluorescent screens consist of a thin, flexible base coated with a fluorescent layer made up from micro-crystals of a suitable metallic salt (rare earth; usually calcium tungstate) which fluoresce when subjected to radiation. The radiation makes the screen light up. The light intensity is in direct proportion to the radiation intensity. With these screens a very high intensification factor of 50 can be achieved, which means a significant reduction in exposure time. The image quality, however, is poor because of increased image unsharpness. Fluorescent screens are only used in industrial radiography when a drastic reduction of exposure time, in combination with the detection of large defects, is required.
Apart from fluorescent and lead intensifying screens, there are fluorometallic screens which to a certain extent combine the advantages of both. These screens are provided with a lead foil between the film base and the fluorescent layer. This type of screen is intended to be used in combination with so-called RCF-film (Rapid Cycle Film) of the type Structurix F6 or F8.
The degree of intensification achieved largely depends on the spectral sensitivity of the X-ray film for the light emitted by the screens.
To achieve satisfactory radiographs with fluorometallic screens, they should be used in combination with the appropriate F-film type.
When used correctly and under favourable conditions, exposure time can be reduced by a factor 5 to 10, compared with D7 film in combination with lead screens. This is not a constant factor because the energy level applied (radiation hardness) and ambient temperature also affects the extent of fluorescence. For example, at 200 kV a factor 10 can be achieved, but with Iridium192 (nominal value 450 kV) it will only be a factor 5 compared to D7 film. Table 1-6 shows the relative exposure factors for the RCF-technique.