ION RECOMBINATION AND POLARITY EFFECT OF IONIZATION CHAMBERS IN KILOVOLTAGE X-RAY-EXPOSURE MEASUREMENTS

Exposure measurements with ionization chambers are dependent on the co rrection factors related to the beam energy (k(e)), temperature and pr essure (k(tp)), ionization recombination (P-ion), and polarity (k(pol) ) effects. In this work, six different chambers commonly used in diagn ostic radiology were investigated for the P-ion and k(pol) at various exposure rates by changing the tube voltage, beam current, exposure ti me, and distance. A special triaxial connector was used to connect cha mbers to an electrometer capable of measuring positive and negative po larity 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 diagnost ic x-ray unit (60-125 kVp) were used. Results indicate that the magnit ude of the Pi,, is linearly dependent on kVp for large volume (>150 cm (3)) chambers and independent for small volume (less than or equal to 150 cm(3)) chambers. In general, Pi,, is higher at higher exposures (i ncreasing kVp, mAs, and decreasing distance); however, kpol is indepen dent of exposure rate and kVp, but strongly depends on the sensitive v olume of an ion chamber. Pi,, and kpol vary between 1-48% and 1-16%, r espectively, among various chambers and exposure conditions. Chambers with larger volumes have higher values of P-ion and k(pol). The desire d accuracy of +/-5% in exposure measurements might not be feasible unl ess both the polarity and recombination effects are known and accounte d accurately. (C) 1998 American Association of Physicists in Medicine.