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SU‐FF‐T‐274: Monte Carlo Simulation of Radiation Induced Currents in Parallel Plate Ionization Chambers
Author(s) -
AbdelRahman W,
Seuntjens J,
Verhaegen F,
Podgorsak E
Publication year - 2005
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 - 10.1118/1.1998003
Subject(s) - physics , compton scattering , monte carlo method , electron , photon , imaging phantom , radiation , ionization , electrode , computational physics , atomic physics , extrapolation , optics , nuclear physics , ion , statistics , mathematics , quantum mechanics , mathematical analysis
Purpose: To investigate the polarity effects caused by radiation induced currents, known also as Compton currents, in parallel plate ionization chambers. Compton currents arise as a result of charge imbalance due to charge deposition in electrodes during photon or electron beam irradiations. To allow for theoretical understanding of the effect, a modified user code COMPTON/EGSnrc was developed and used to study the effect in a phantom‐embedded extrapolation chamber (PEEC). Method and Materials: The PEEC has a parallel‐plate geometry with a variable electrode separation. The polarity effect of the PEEC was measured as a function of electrode separation, depth in phantom, and incident field size. Monte Carlo simulations using the COMPTON/EGSnrc user code were used to account for the charge entering into and exiting from the collecting electrode of the PEEC geometry, thereby yielding charge imbalance information. Results: The Compton current in the PEEC has a negligible dependence on electrode separation. In photon beams, the Compton currents with measurement depth exhibit a maximum at the surface and decrease with increasing depth to reach a minimum at the depth of dose maximum. The magnitude of the Compton current decreases with field size. In electron beams, the Compton current has a maximum positive value at the surface; decreases linearly with depth and becomes negative after a depth of about 0.2 I 50 ; then continues to decrease reaching a minimum at about 0.9 I 50 ; then increases rapidly to reach a zero value at RP. Conclusion: Compton currents are the dominant cause of the polarity effect in parallel‐plate ionization chambers and their variation with depth, field size, electrode separation in photon and electron beams can be determined with out modified user code COMPTON/EGSnrc.