Fluke Biomedical 6000-528 - Manual (Page 11)

Operation
Theory of Operation
3
3-1
Section 3
Operation
3.1 Theory of Operation
An ionization chamber consists of a defined volume of air in which ions produced by radiation passing
through the chamber can be collected and measured. The Model 6000-528 is a parallel plate ion
chamber, consisting of a guarded center electrode placed between two outer plates, which also serve as
windows. A potential difference in the range of approximately 200 - 300 volts is placed across the plates
of the ion chamber. When ionizing radiation passes through the chamber, ion pairs are produced, each
pair consisting of one positive and one negative ion. Under the influence of the electric field produced by
the potential on the plates, the ions move toward their oppositely charged plate. Upon arrival, they are
neutralized by the free charges on the plates, taking an electron from the negative plate and adding an
electron to the positive plate. This causes a current to flow through the external electronics connected to
the plates, the magnitude of which is proportional to the rate of exposure to radiation.
The sensitivity of an ion chamber depends on the number of air molecules in the chamber; in fact these
quantities are directly proportional. The number of molecules is a function of volume, temperature, and
pressure. The volume of air in the chamber is fixed, but since it communicates with the atmosphere,
temperature and pressure will vary. A correction factor should be applied to the reading given by the ion
chamber, based on the ambient temperature and barometric pressure at the time the measurement is
made. For diagnostic x-ray use, this is usually unnecessary since the errors are on the order of 0.3% per
degree Celsius and 0.1% per mmHg. In any event, the correction factor is calculated by the following
expression:
cf = P
0
x T + 273.16
P T
0
+273.16
Where T is the temperature in degrees Celsius and P is the pressure in mmHg. T
0
and P
0
are the
temperature and pressure, respectively, at which the chamber was calibrated.