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Das IJ , Akber SF
Ion recombination and polarity effect of ionization chambers in kilovoltage x-ray exposure measurements
Medical Physics. 1998 Sep;25(9) :1751-1757
PMID: ISI:000075992200021   
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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 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 (increasing kVp, mAs, and decreasing distance); however, kpol is independent of exposure rate and kVp, but strongly depends on the sensitive volume of an ion chamber. Pi,, and kpol 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. (C) 1998 American Association of Physicists in Medicine.
Times Cited: 0 English Article 121BL MED PHYS