Operating Manual

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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 radio-
graphs 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.
Fluorescent screens
The term fluorescence (often mistaken for phosphorescence) is used to indicate the cha-
racteristic of a substance to instantly instantly emit light under the influence of electro-
magnetic radiation. The moment radiation stops, so does the lighting effect. This pheno-
menon 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, see section 16.4.
Fluorescent salt screens
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 expo-
sure 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.
Fluorometallic screens
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 types F6 or F8,
see section 8.1.
The degree of intensification achieved largely depends on the spectral sensitivity of the
X-ray film for the light emitted by the screens. Due to the considerable exposure time reduc-
tion the application is attractive for work on lay barges and in refineries.
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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 sur-
face 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 bet-
ween 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
Fig. 4a-6. Radiograph of a casting without lead
intensifying screens
Fig. 4b-6. Radiograph of a casting with lead intensi-
fying screens