T.O. 33B-1-1
6-36
Table 6-11. Sample Result.
Source Power
Relative Exposure Times for Equivalent
Density Under Standard Conditions
Without Screens
With Lead Screens
120 kVp
150 kVp
200 kVp
Iridium 192
Cobalt 60
1.0
1.0
1.0
1.0
1.0
1.0
0.7
0.5
0.25
0.50
NOTE: The results were obtained with a 0.004-inch front screen and a 0.006-inch back screen.
6.4.3.2.1.1
Lead screens should be used whenever they improve radiographic quality. Because of the resulting improvement in
radiographic quality, lead screens are generally preferred to calcium tungstate screens. Whenever there is a need to
perform a radiographic inspection using a combination of screens and film, they SHALL be of the same plane
dimensions and in close contact with each other during exposure.
6.4.3.2.2
Effects. Lead screens in direct contact with film have two effects:
a. They intensify incident radiation. Incident radiation with energies above 88 keV eject photoelectrons
from the atoms of the lead. These photoelectrons act on the emulsion in the same way as the primary
radiation beam.
b. They improve clarity by absorbing scattered radiation of longer wavelengths.
6.4.3.2.3
Selection of Screen Relative to Radiation Energy.
Lead screens are available in various thicknesses and should be chosen according to the radiation energy being used.
At energies less than 125 keV, very thin front lead screens are used to filter scattered radiation. With X-ray energies
ranging from 125 to about 250 keV, it is common to use a front lead screen of 0.005-inch thickness and a back lead
screen of 0.010-inch thickness. In some instances where scattered radiation is very high, such as in the radiography of
thick sections of graphite, a 0.010 lead front screen is used to increase the absorption of the scatter. When the X-ray
energies exceed 250 keV, thicker lead screens are appropriate. Usually 0.010 front and back screens are used at 250 to
400 keV for energies in the MeV ranges lead screens on the order of 0.050 are often used. There is no fixed rule for the
lead screen thickness appropriate for a particular energy level, but generally speaking, the lead thickness of the screen
is increased as the radiation energy increases.
6.4.3.2.4
Care.
CAUTION
Hydrogen peroxide or other common cleaning agents should never be used for this
purpose because their chemical composition will cause fogging of the sensitive film
emulsions.
Do not touch sensitive surfaces of the screens because fingerprints and dust particles may show in the radiograph and
interfere with accurate interpretation. X-ray screens are given a special waterproof protection coating to both sides. If
the surfaces accidentally acquire markings, wash with mild soap and water, and dry thoroughly with a soft cloth. Since
the intensifying action of a lead foil screen is caused by the electrons emitted under X-ray or gamma ray excitation, and
