Determining charge collection efficiency in parallel-plate liquid ionization chambers

Abstract

An accurate computation of the collection efficiency due to recombination has a capital importance in order to perform high-precision dose measurements with ionization chambers. The two-voltage method, developed to compute charge collection efficiency in gas chambers, cannot be directly applied to liquid-filled ionization chambers (LICs) due to the ionized charge strong dependence on the collection electric field, which is caused by initial recombination. It is shown that in order to apply the two-voltage method to parallel-plate LICs it is necessary to introduce explicitly the slope of the ionization charge yield with respect to the polarization voltage applied in the chamber. A three-voltage method is also proposed to avoid the introduction of the slope parameter. That method allows decoupling initial and general recombination. It leads to a nonlinear system of equations which can be solved using numerical methods.

Modeling ion recombination in liquid ionization chambers — Improvement and analysis of the two‐dose‐rate method

“…, cannot be applied directly to ionized liquids without a robust theoretical description of initial recombination. An experimental solution to this problem for continuous beams has been found by employing a two‐dose‐rate method, based on the general collection efficiency according to Greening, to disassociate initial and general recombination in LICs . This method is straightforward, involving charge measurements ( ) with a reference detector with a signal proportional to dose, here charge readings with an air‐filled ionization chamber () corrected according to the two‐voltage method, and a LIC at two different dose rates ( and ) to experimentally determine the parameter , as described in Eq.…”

“…Consequently, loss of measurement signal due to both initial and general recombination can be severe in LICs, limiting the usability of these instruments for both quality assurance and radiation dosimetry. In recent years, two methods have been proposed for the purpose of general recombination correction for LICs, the three‐voltage method and the two‐dose‐rate method . A comparative investigation of these methods has been done, where it could be concluded that the two‐dose‐rate method is more robust than the three‐voltage method .…”

“…As a solution to the problem of correcting LIC measurement readings for general recombination (general collection efficiency), Andersson and Tölli proposed a two‐dose‐rate method that employed a description of general recombination in continuous beams proposed by Greening . Experimental works have indicated that application of Greening's description of general recombination for liquids should be valid for general collection efficiencies over 90% . However, Andersson et al found different valid ranges of general collection efficiencies for annihilation photons (> 70%), as compared to the original investigation that involved 120 kVp X‐rays (> 90%) …”

Characterization of tetramethylsilane for liquid-filled ionization dosimeters: Ion mobilities, free-ion yield and general recombination

“…General collection efficiencies, f, in LICs irradiated with pulsed radiation beams are well described in the near saturation range f 4 0:9 by the Boag equation [14][15][16]8]:…”

On the property of measurements with the PTW microLion chamber in continuous beams

“…Much effort has therefore been given in finding experimental 75 methods that accurately correct for general recombination losses. [10][11][12][13][14] …”