Fenwick, John D;
Kumar, Sudhir;
Pardo-Montero, Juan;
(2024)
Collection efficiencies of cylindrical and plane parallel ionization chambers: analytical and numerical results and implications for experimentally determined correction factors.
Physics in Medicine & Biology
, 69
(15)
, Article 155023. 10.1088/1361-6560/ad63ed.
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Abstract
Objectives. To derive a collection efficiency formula, f Gauss , for cylindrical ionization chambers in pulsed radiation beams from a volume recombination model of Boag et al (1996 Phys. Med. Biol. 41 885-97) including free electrons. To validate f Gauss and a parallel plate chamber formula f exp using an ion transport code and calculate changes in collection efficiencies caused by electric field charge screening at 0.1-100 mGy doses-per-pulse. And to determine collection efficiencies C E ∞ predicted at infinite voltage in the absence of avalanche effects by fitting scaled formulae to efficiencies computed for 100-400 V chamber voltages and 10 and 100 mGy doses-per-pulse. Approach. Calculations were performed for an idealized parallel plate chamber with 2 mm electrode separation d , and for an idealized cylindrical chamber with 0.5 and 2.333 mm inner and electrode radii r in and r out . Main results. f Gauss and f exp predict the same collection efficiencies for cylindrical and parallel plate chambers satisfying d 2 = ( r out 2 − r in 2 ) ln ( r out / r in ) / 2 , an equivalence condition met by the chambers studied. Without charge screening, efficiencies computed using the code equalled f Gauss and f exp . With screening, efficiencies changed by ⩽0.03%, ⩽1.1% and ⩽21.3% at 1, 10 and 100 mGy doses-per-pulse, and differed between the chambers by ⩽0.9% and ⩽19.6% at ⩽10 and 100 mGy dose-per-pulse. For fits of f exp and f Gauss , C E ∞ values were ⩽1.2% and ⩽17.6% from unity at 10 and 100 mGy per pulse respectively, closer than for other formulae tested. Significance. Allowing for screening, f Gauss and f exp described computed collection efficiencies to within 0.03%, 1.1% and 21.3% at doses-per-pulse ⩽1, 10 and 100 mGy. Equivalence of the two chambers broke down at 100 mGy per pulse. Departures of C E ∞ values from unity suggest that collection efficiencies determined experimentally by fitting f Gauss or f exp to readings made at multiple voltages will be accurate to within 1.2% and 17.6% at 10 and 100 mGy per pulse respectively.
| Type: | Article |
|---|---|
| Title: | Collection efficiencies of cylindrical and plane parallel ionization chambers: analytical and numerical results and implications for experimentally determined correction factors |
| Location: | England |
| Open access status: | An open access version is available from UCL Discovery |
| DOI: | 10.1088/1361-6560/ad63ed |
| Publisher version: | http://dx.doi.org/10.1088/1361-6560/ad63ed |
| Language: | English |
| Additional information: | Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 license, https://creativecommons.org/licenses/by/4.0/. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. |
| Keywords: | Ionization chamber, ion recombination, dose-per-pulse, collection efficiency, charge screening |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Med Phys and Biomedical Eng |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10195941 |
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