Charge collection efficiency, underlying recombination mechanisms, and the role of electrode distance of vented ionization chambers under ultra-high dose-per-pulse conditions.

Autor: Kranzer R; PTW-Freiburg (R&D), Freiburg 79115, Germany; University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, 26121, Germany., Schüller A; Physikalisch-Technische Bundesanstalt, Braunschweig 38116, Germany., Gómez Rodríguez F; Departamento de Fisica de Particulas, Universidad de Santiago, Santiago de Compostela, Spain; Laboratorio de Radiofisica, Universidad de Santiago, Santiago de Compostela, Spain., Weidner J; PTW-Freiburg (R&D), Freiburg 79115, Germany., Paz-Martín J; Departamento de Fisica de Particulas, Universidad de Santiago, Santiago de Compostela, Spain., Looe HK; University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, 26121, Germany., Poppe B; University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, 26121, Germany.
Jazyk: angličtina
Zdroj: Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB) [Phys Med] 2022 Dec; Vol. 104, pp. 10-17. Date of Electronic Publication: 2022 Nov 07.
DOI: 10.1016/j.ejmp.2022.10.021
Abstrakt: Purpose: Investigating and understanding of the underlying mechanisms affecting the charge collection efficiency (CCE) of vented ionization chambers under ultra-high dose rate pulsed electron radiation. This is an important step towards real-time dosimetry with ionization chambers in FLASH radiotherapy.
Methods: Parallel-plate ionization chambers (PPIC) with three different electrode distances were build and investigated with electron beams with ultra-high dose-per-pulse (DPP) up to 5.4 Gy. The measurements were compared with simulations. The experimental determination of the CCE was done by comparison against the reference dose based on alanine dosimetry. The numerical solution of a system of partial differential equations taking into account charge creations by the radiation, their transport and reaction in an applied electric field was used for the simulations of the CCE and the underlying effects.
Results: A good agreement between the experimental results and the simulated CCE could be achieved. The recombination losses found under ultra-high DPP could be attributed to a temporal and spatial charge carrier imbalance and the associated electric field distortion. With ultra-thin electrode distances down to 0.25 mm and a suitable chamber voltage, a CCE greater than 99 % could be achieved under the ultra-high DPP conditions investigated.
Conclusions: Well-guarded ultra-thin PPIC are suited for real-time dosimetry under ultra-high DPP conditions. This allows dosimetry also for FLASH RT according to common codes of practice, traceable to primary standards. The numerical approach used allows the determination of appropriate correction factors beyond the DPP ranges where established theories are applicable to account for remaining recombination losses.
Competing Interests: Declaration of Competing Interest Rafael Kranzer and Jan Weidner are PTW employees.
(Copyright © 2022 Associazione Italiana di Fisica Medica e Sanitaria. Published by Elsevier Ltd. All rights reserved.)
Databáze: MEDLINE
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