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Title: SU-C-201-03: Ionization Chamber Collection Efficiency in Pulsed Radiation Fields of High Pulse Dose

Abstract

Purpose: To investigate the reduction of collection efficiency of ionization chambers (IC) by volume recombination and its correction in pulsed fields of very high pulse dose. Methods: Measurements of the collection efficiency of a plane-parallel advanced Markus IC (PTW 34045, 1mm electrode spacing, 300V nominal voltage) were obtained for collection voltages of 100V and 300V by irradiation with a pulsed electron beam (20MeV) of varied pulse dose up to approximately 600mGy (0.8nC liberated charge). A reference measurement was performed with a Faraday cup behind the chamber. It was calibrated for the liberated charge in the IC by a linear fit of IC measurement to reference measurement at low pulse doses. The results were compared to the commonly used two voltage approximation (TVA) and to established theories for volume recombination, with and without considering a fraction of free electrons. In addition, an equation system describing the charge transport and reactions in the chamber was solved numerically. Results: At 100V collection voltage and moderate pulse doses the established theories accurately predict the observed collection efficiency, but at extreme pulse doses a fraction of free electrons needs to be considered. At 300V the observed collection efficiency deviates distinctly from that predicted by anymore » of the established theories, even at low pulse doses. However, the numeric solution of the equation system is able to reproduce the measured collection efficiency across the entire dose range of both voltages with a single set of parameters. Conclusion: At high electric fields (3000V/cm here) the existing theoretical descriptions of collection efficiency, including the TVA, are inadequate to predict pulse dose dependency. Even at low pulse doses they might underestimate collection efficiency. The presented, more accurate numeric solution, which considers additional effects like electric shielding by the charges, might provide a valuable tool for future investigations. This project was funded by the German ministry of research and education (BMBF) under grant number: 03Z1N511 and by the state of Saxony under grant number: B 209.« less

Authors:
;  [1];  [1];  [2]
  1. Oncoray - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universitaet Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden (Germany)
  2. (Germany)
Publication Date:
OSTI Identifier:
22624308
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; APPROXIMATIONS; EFFICIENCY; ELECTRIC POTENTIAL; ELECTRON BEAMS; FARADAY CUPS; FINANCING; IONIZATION CHAMBERS; RECOMBINATION

Citation Formats

Gotz, M, Karsch, L, Pawelke, J, and Helmholtz-Zentrum Dresden - Rossendorf, Dresden. SU-C-201-03: Ionization Chamber Collection Efficiency in Pulsed Radiation Fields of High Pulse Dose. United States: N. p., 2016. Web. doi:10.1118/1.4955543.
Gotz, M, Karsch, L, Pawelke, J, & Helmholtz-Zentrum Dresden - Rossendorf, Dresden. SU-C-201-03: Ionization Chamber Collection Efficiency in Pulsed Radiation Fields of High Pulse Dose. United States. doi:10.1118/1.4955543.
Gotz, M, Karsch, L, Pawelke, J, and Helmholtz-Zentrum Dresden - Rossendorf, Dresden. 2016. "SU-C-201-03: Ionization Chamber Collection Efficiency in Pulsed Radiation Fields of High Pulse Dose". United States. doi:10.1118/1.4955543.
@article{osti_22624308,
title = {SU-C-201-03: Ionization Chamber Collection Efficiency in Pulsed Radiation Fields of High Pulse Dose},
author = {Gotz, M and Karsch, L and Pawelke, J and Helmholtz-Zentrum Dresden - Rossendorf, Dresden},
abstractNote = {Purpose: To investigate the reduction of collection efficiency of ionization chambers (IC) by volume recombination and its correction in pulsed fields of very high pulse dose. Methods: Measurements of the collection efficiency of a plane-parallel advanced Markus IC (PTW 34045, 1mm electrode spacing, 300V nominal voltage) were obtained for collection voltages of 100V and 300V by irradiation with a pulsed electron beam (20MeV) of varied pulse dose up to approximately 600mGy (0.8nC liberated charge). A reference measurement was performed with a Faraday cup behind the chamber. It was calibrated for the liberated charge in the IC by a linear fit of IC measurement to reference measurement at low pulse doses. The results were compared to the commonly used two voltage approximation (TVA) and to established theories for volume recombination, with and without considering a fraction of free electrons. In addition, an equation system describing the charge transport and reactions in the chamber was solved numerically. Results: At 100V collection voltage and moderate pulse doses the established theories accurately predict the observed collection efficiency, but at extreme pulse doses a fraction of free electrons needs to be considered. At 300V the observed collection efficiency deviates distinctly from that predicted by any of the established theories, even at low pulse doses. However, the numeric solution of the equation system is able to reproduce the measured collection efficiency across the entire dose range of both voltages with a single set of parameters. Conclusion: At high electric fields (3000V/cm here) the existing theoretical descriptions of collection efficiency, including the TVA, are inadequate to predict pulse dose dependency. Even at low pulse doses they might underestimate collection efficiency. The presented, more accurate numeric solution, which considers additional effects like electric shielding by the charges, might provide a valuable tool for future investigations. This project was funded by the German ministry of research and education (BMBF) under grant number: 03Z1N511 and by the state of Saxony under grant number: B 209.},
doi = {10.1118/1.4955543},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • A recently developed method for measuring dose and quality parameters in mixed neutron-gamma radiation fields is extended for neutron energy regions relevant to current neutron therapy. It is theoretically shown that if a polyethylene high-pressure ionization chamber filled with a gas mixture of 95% ethylene and 5% carbon dioxide is used at suitable gas pressures, it may overcome the limitations found for a similar tissue-equivalent chamber and measure both the gamma ray absorbed dose fraction and the mean incident neutron energy for neutrons between 0.1 and at least 30 MeV, if certain conditions exist. The simultaneous use of the polyethylenemore » high-pressure ionization chamber as a conventional neutron dosimeter also allows determination of both the neutron and gamma ray absorbed doses. Estimated accuracies are 15% for the mean incident neutron energy and 15%--30% for the doses determination. The present method may complement currently available techniques as it presents certain advantages in situations where the neutron spectrum is poorly known. If coupled with other dosimeters, the accuracy of the method can be improved.« less
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