Fluke Biomedical 440RF - Manual (Page 16)

Victoreen 440RF/D
Operators Manual
2-8
Small Cross-Section Beam
As described earlier (refer to Section 2.6, Proximate Source), constant radiation intensity throughout the
volume, or a correction factor, is required. This is the sole practical manner in which chamber calibration
can be performed.
If a small cross-sectional beam ionized only a fraction of the chamber volume, as shown in Figure 2-6, the
resulting indication will reflect an overall average value instead of the beam cross-sectional value. In
order to correct for this type of measurement condition, the actual cross-sectional area of the beam must
be known.
The Model 440RF/D cannot be used to determine beam cross-sectional areas. To determine such an
area, one possibility includes the use of photographic sheet film. If the cross-sectional area is well
defined (rather than diffuse) on the film, the cross-sectional area can be scaled off. This will allow for the
computation of a rough correction factor. For example, the Small Cross Section Beam Correction Factor
is equal to:
Cross-Sectional Area of Chamber
=
10 cm 2
Total Cross Sectional Area of Beam X cm 2
Most beams found associated with color television receivers are on the order of 1 cm
2 cross-sectional
area, or larger.
Skew Beams
Just as the cross-sectional areas of X-ray beams vary, so does the three-dimensional angle, at which the
beam leaves the apparatus where it is generated.
Normally, the Exposure Rate Measuring System should be positioned so that its magnesium window is
parallel to the external surface of the apparatus being surveyed. This positions the ion chamber axis
parallel to any beams that leave the apparatus normal, or at right angles, to the surface. It is likely that
some beams will not follow this pattern. If the angle is small with respect to the normal, little or no error
occurs. In addition, if the beam has a large cross-section (compared to the chamber), the error is slight.
However, for small cross-sectional beams at significant angles from normal, the error will be large (see
Figure 2-7). The Model 440RF/D can be tipped off its normal axis in the direction and angle which yields
the maximum indication. This will reduce the error associated with the small cross-sectional area at
significant angles.
Plane Source
A plane source of radiation presents a unique measurement situation. A color television picture tube face
plate can be treated for all practical purposes as a plane source due to the speed of scan and frame rate.
The bulk of the X-ray photons are generated when the electron beam strikes the shadow mask. This
thick glass face plate functions in a manner similar to a low energy filter or absorber. In Figure 2-8, the
geometry of the tube and measurement system is shown.
The most important characteristic of a plane source is that it attempts to illuminate the chamber volume
from a large oblique angle. Therefore, instrument centering becomes meaningless.
At higher energies, the case and chamber wall are almost as transparent to photons as the magnesium
and mylar windows. However, in the energy range expected for a color television receiver, this is not the
situation; and some oblique angle contribution is lost in the case wall. Matters are moderately improved
due to the increased absorption for oblique angle photons which have a longer travel path in the glass
faceplate.