Fluke Biomedical 440RF - Manual (Page 14)

Victoreen 440RF/D
Operators Manual
2-6
2.4 Chamber Location
Some information regarding the actual location of the ion chamber within the case, in terms of outside
markings and dimensions, will aid in using the instrument for precision or special geometry applications.
The ion chamber is in the form of a right cylinder. It is located directly below the panel meter end of the
case with the axis parallel to the case length. Refer to Figure 2-3 for exact dimensions. Three plastic
bumpers are arranged in a triangle on the outside mounting ring of the magnesium window. Their tops
define a plane. This plane will include any flat surface that all three bumpers contact. The midplane of the
ion chamber is exactly 5 cm deep from the bumper plane.
The ion chamber midplane is defined as; that
plane, at right angles to the axis, which separates
the total effective volume of the chamber into two
equal volumes.
A black ring (1-½ inch diameter) is located on the outside surface of the magnesium window. It serves as
a guide to ion chamber size and location.
The ion chamber end window is aluminized mylar. It is parallel to, congruent with, and directly behind the
ring on the magnesium end window.
2.5 Measurement of Exposure Rate
Unit of Measurement
The Model 440RF/D is used to measure ionizing radiation in terms of exposure rate, whose unit is
Roentgen per unit time.
An exposure rate of one Roentgen per unit time is that intensity of X- or gamma radiation which produces
ion pairs amounting to one electrostatic unit of charge per unit time in each cubic centimeter of air at
standard conditions.
Exposure rate is not really defined for beta
particles. Any indications should be indications of
relative beta particle intensity and not indications of
exposure rate (or milliroentgen per hour).
This instrument indicates in thousandths of a Roentgen per hour or milliroentgen per hour (mR/h).
The following characteristics are implied when any ionization chamber is used to determine exposure rate
as defined above.
1. Total collection of all ion pairs created,
2. Relative independence of ion pair generation on the energy of photons.
In many real measurement situations, some of the above conditions will be difficult to meet. In order to
approach a true value, a correction factor is necessary.
NOTE
NOTE