First experimental characterization of a low-pressure nitrogen filled parallel-plate ionization chamber for UHDP...

Ultra-high dose-per-pulse (UHDP) dosimetry remains a key challenge in FLASH radiotherapy. Conventional ionization chambers (ICs) experience severe electric field perturbations under UHDP conditions due to high charge densities, leading to severe recombination. A novel IC design, the ALLS chamber, has been proposed to overcome these limitations by using a low-pressure noble gas, eliminating ion recombination, and enabling an analytical description of charge collection up to 40 Gy/pulse with argon at 1 hPa pressure as the active medium. However, designing such an IC requires meeting both dosimetric and mechanical constraints for low-pressure operation. Since the actual requirements for FLASH dosimetry involve DPP up to 10 Gy, less extreme de-pressures in the range of 50-1000 hPa could be applied, even though such a scenario cannot be described analytically. Numerical simulations and experimental measurements are essential to explore new gas and pressure configurations.

This work presents the first experimental proof-of-concept of an ALLS-based low-pressure parallel-plate IC (PPIC) for UHDP electron beam dosimetry.

In order to experimentally investigate the response of the chamber to variations in the filling gas and its pressure, a custom PPIC prototype was developed in a sealed Polymethyl Methacrylate (PMMA) Di vessel with controlled depressurization. Numerical simulations of the charge transport in noble or inert gases were used to predict the chamber response. Experimental measurements were performed with UHDP electron beams. The prototype was tested in air and in nitrogen at pressures in the range of 50-1000 hPa, varying the dose per pulse (DPP) up to 9.88 Gy.

Measurements in air showed expected saturation behavior and good agreement with commercial parallel plate chambers with similar geometry, validating the basic functionality of the prototype. In nitrogen, experimental data demonstrated good agreement with simulated charge collection predictions across all tested pressures, with residuals generally within $\pm 5\%$ . The response of the system was shown to be linear with the DPP up to 1.21 Gy at 500 hPa, 4.48 Gy at 100 hPa, and 9.88 Gy at 50 hPa.

The results validated the theoretical approach and demonstrated that a low pressure nitrogen-filled chamber allows a linear response with charge collection efficiency close to one, far exceeding the limits of conventional ionization chambers. These findings offer a promising path toward developing clinically applicable, real-time dosimeters for FLASH radiotherapy.