Fluke Biomedical 6000-529 - Manual (Page 9)

Theory of Operation
Theory of Operation
2
2-1
Section 2
Theory of Operation
2.1 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 6000-529 is a parallel plate ion
chamber, consisting of a guarded center electrode placed near a conductive plate that serves as a
window as well as an electrode. A potential difference in the range of approximately 200 - 300 volts is
placed across the plates of the ion chamber (the high voltage applied to the ion chamber will depend on
the host instrument used). 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 between 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. The chambers are calibrated at Fluke Biomedical at a temperature of
22°C and a pressure of 760 mmHg. 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. However, if you are using the 6000-529 at an elevation greatly
different than sea level, the error could be significant. The correction factor is calculated by the following
expression:
760 T +
273.16 cf =
P
x
295.16
Where T is the temperature in degrees Celsius and P is the pressure in mmHg.
2.2 Sample Applications
Beam Quality Measurements
Beam quality is an indication of the penetrating ability of diagnostic x-rays. The quantity being measured
is the half-value layer (HVL) and is reported is units of millimeters of aluminum. The HVL is defined as the
thickness of an absorbing material (aluminum in this case) required to reduce the intensity of the x-ray
beam to one half its unattenuated intensity. Also, by definition of the HVL, radiation scatted by the
absorber must not be included in the measurement, thereby necessitating care to be taken in setting up
the measurement geometry.
To perform a beam quality measurement, one makes several exposure (or air kerma) measurements with
the Model 6000-529 ionization chamber, while increasing the thickness of an absorbing material from
zero to a sufficient quantity to reduce the reading to less than one half the reading measured with no
absorber. A logarithmic interpolation formula is then used to interpolate between the measurement made