Development of an ultra‐thin parallel plate ionization chamber for dosimetry in FLASH radiotherapy

Abstract

Background: Conventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra-high dose per pulse as used for FLASH radiotherapy. Purpose: We present the design, development, and characterization of an ultrathin parallel plate IC that can be used in ultra-high dose rate (UHDR) deliveries with minimal recombination. Methods: The charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 μ s pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra-high pulse dose rate electron beams. Results: It has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra-high dose per pulse range, at standard voltage of clinical grade electrometers,well into several Gy per pulse.The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so-called ultrathin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 μ s pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 μ s pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch-Technische Bundesanstalt, PTB). Conclusions: This work demonstrates the ability to extend the dose rate operating range of ICs to ultra-high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams.

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

“…Reducing the electrode distance has been shown to be more effective than increasing the chamber voltage. Based on theoretical considerations and experiments, it has been shown that a reduction of the electrode distance down to 0.25 mm can be well suited [19] . In this work, well-guarded vented parallel-plate ionization chambers (PPIC) with three different electrode distances are investigated experimentally and by means of simulations under ultra-high DPP conditions.…”

A new solution for UHDP and UHDR (Flash) measurements: Theory and conceptual design of ALLS chamber

“…Until early 2021, "active" dosimeters were not available to measure such beams properly [6][7][8][9][10][11][12]. Then, two promising new devices were presented and tested: the ultra-thin ionization chamber, mainly studied and developed by F. Gomez Rodriguez et al [10][11][12][13][14], and the Flash diamond, developed by M. Marinelli and G. Verona Rinati et al [15,16].…”

“…In this work a new theoretical approach to Ultra High Dose per Pulse (UHDP) and Ultra High Dose Rate (UHDR) measurements by means of a gas chamber is considered and discussed. In general, the analytical description of ion collection under such conditions is not feasible [13,14,17]. However, the general equation can be simplified by choosing some parameters properly so that an analytical description can be derived even in UHDP/UHDR conditions.…”

“…1. eliminate recombination issues (within 0.1 % [14], provided that the electric field is always and everywhere greater than zero; 2. allow the analytical description of the electric field during the charges collection process.…”

“…Therefore, there is no generation of negative ions and the only possible recombination scenario remains the direct recombination between positive ions and electrons. Such recombination can be considered negligible if the electric field is always and everywhere greater than zero (within 0.1 % [14]). Electrons have a mobility at least three orders of magnitude higher than positive ions [25].…”