Ion recombination and polarity effect of ionization chambers in kilovoltage x‐ray exposure measurements

Abstract

Exposure measurements with ionization chambers are dependent on the correction factors related to the beam energy (ke), temperature and pressure (ktp), ionization recombination (Pion), and polarity (kpol) effects. In this work, six different chambers commonly used in diagnostic radiology were investigated for the Pion and kpol 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 kpol and Pion, 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 Pion is linearly dependent on kVp for large volume (> 150 cm3) chambers and independent for small volume (< or = 150 cm3) chambers. In general, Pion 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. Pion and kpol vary between 1-48% and 1-16%, respectively, among various chambers and exposure conditions. Chambers with larger volumes have higher values of Pion and kpol. The desired accuracy of +/- 5% in exposure measurements might not be feasible unless both the polarity and recombination effects are known and accounted accurately.

Percent depth‐dose distribution discrepancies from very small volume ion chambers

“…Later, the polarity effect was studied by Ramsey et al (22) for electron beams, but its magnitude was less than 1% at for plane‐parallel chambers and no polarity effect was observed for cylindrical chambers. Another study found strong dependency of the polarity effect on the sensitive volume of an ion chamber for kilovoltage beams (23) . However, we believe that our observed discrepancy is not related to the polarity effect, as defined in the literature, because the discrepancy disappears when measured using an external electrometer system.…”

Câmara de ionização de placas paralelas para radiação-X de radiografia convencional e mamografia

“…Tests were performed with the ionization chamber coupled with a PTW Unidos electrometer. The polarization voltage was +400 V. The efficiency of the ions collection is > 99% at +400 V for both faces of the chamber (27)(28)(29) . The leakage current is < 2 fA and the accuracy is > ±0.05% for both faces of the chamber.…”

Desenvolvimento de câmaras de ionização especiais para controle de qualidade em mamografia

“…A câmara de ionização de placas paralelas é basicamente formada por dois eletrodos planos e paralelos, opostos, separados por alguns milímetros e sujeitos a um campo elétrico (SHANI, 2001 A recombinação inicial tem influência desprezível em aplicações clínicas (SHANI, 2001), contudo, a recombinação geral apresenta influência na coleta de carga de câmaras de ionização usadas em radiodiagnóstico (GELEIJNS et al, 1995). A medição do efeito de recombinação deve ser realizada para cada câmara de ionização para ser obtido o fator de correção para este efeito (GELEIJNS et al, 1995;DAS & AKBER, 1998).…”