Exposure measurements with ionization chambers are dependent on the correction factors related to the beam energy (k(e)), temperature and pressure (k(tp)), ionization recombination (P(ion)), and polarity (k(pol)) effects. In this work, six different chambers commonly used in diagnostic radiology were investigated for the P(ion) and k(pol) at various exposure rates by changing the tube voltage, beam current, exposure time, and distance. A special triaxial connector was used to connect chambers to an electrometer capable of measuring positive and negative polarity and 150 V and 300 V electrode potentials to measure the k(pol) and P(ion), respectively. A mammography unit (24-35 kVp) and a diagnostic x-ray unit (60- 125 kVp) were used. Results indicate that the magnitude of the P(ion) is linearly dependent on kVp for large volume (>150 cm3) chambers and independent for small volume (≤ 150 cm3) chambers. In general, P(ion) is higher at higher exposures (increasing kVp, mAs, and decreasing distance); however, k(pol) is independent of exposure rate and kVp, but strongly depends on the sensitive volume of an ion chamber. P(ion) and k(pol) vary between 1- 48% and 1-16%, respectively, among various chambers and exposure conditions. Chambers with larger volumes have higher values of P(ion) and k(pol). The desired accuracy of ±5% in exposure measurements might not be feasible unless both the polarity and recombination effects are known and accounted accurately.
- Ion chambers
- Ion recombination
- Polarity effects
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging