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Ion recombination and polarity effect of ionization chambers in kilovoltage x‐ray exposure measurements
Author(s) -
Das Indra J.,
Akber Syed F.
Publication year - 1998
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598360
Subject(s) - electrometer , ionization chamber , ionization , polarity (international relations) , volume (thermodynamics) , ion , beam (structure) , polarity symbols , materials science , x ray , nuclear medicine , atomic physics , optics , voltage , physics , analytical chemistry (journal) , chemistry , breakdown voltage , medicine , quantum mechanics , biochemistry , chromatography , cell
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 ionand k polat 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 poland 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 ionis linearly dependent on kVp for large volume (>150 cm 3 ) chambers and independent for small volume (⩽150 cm 3 ) chambers. In general, P ionis higher at higher exposures (increasing kVp, mAs, and decreasing distance); however, k polis independent of exposure rate and kVp, but strongly depends on the sensitive volume of an ion chamber. P ionand k polvary between 1–48% and 1–16%, respectively, among various chambers and exposure conditions. Chambers with larger volumes have higher values of P ionand 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.