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arXiv · 2512.14525

Charge collection efficiency of thimble ionization chambers exposed to ultra-high dose per pulse

José Paz-Martín·Andreas Schüller·Marvin Apel·Araceli Gago-Arias·Juan Pardo-Montero·Faustino Gómez
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Abstract

Background: Commercially available ionization chambers (ICs) exposed to ultra-high dose per pulse (UHDP) exhibit deviations from a linear dose response due to volume recombination. Simulation models have been developed to describe the charge collection efficiency (CCE) but focused on parallel-plate ICs. This study aims to measure and simulate the CCE and polarity effect of thimble ICs in UHDP. Methods: The response of two PinPoint3D T31022 (PP3D) and two PinPoint T31023 (PP) ICs was investigated experimentally at the national metrology institute of Germany (PTB). The ICs were irradiated using the UHDP reference electron beam with dose per pulse up to 9.3 Gy for different voltages. A novel finite element code capable of simulating 1D and 2D geometries was developed. Results: Thimble ICs exhibit a pronounced polarization effect when irradiated with UHDP. When the sign of the collected charge is negative, the CCE is larger than when it is positive. The major contribution to the polarity effect can be attributed to the polarity-dependent charge transport and recombination. Experimental and simulated CCE (polarity effect correction factor) agrees within 1.4 % (7.0 %) and 1.6 % (3.2 %) for the PP3D and PP ICs, respectively. The CCE of parallel-plate and thimble ICs is related through a geometrical rule. Conclusions: The PP IC shows greater CCE due to its smaller external radius. The numerical model is able to satisfactory reproduce the actual CCE and polarity effect for these two chambers. At UHDP, thimble ICs should be used with caution due to their large polarity effect.

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José Paz-Martín, Andreas Schüller, Marvin Apel, Araceli Gago-Arias, Juan Pardo-Montero, Faustino Gómez. 2025-12-16. Charge collection efficiency of thimble ionization chambers exposed to ultra-high dose per pulse. https://arxiv.org/abs/2512.14525

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