Numerical simulations of charge transport in low-pressure noble gases for ultra-high dose per pulse applications

Objective. ultra-high dose-per-pulse (UHDP) dosimetry remains a key challenge in FLASH radiotherapy. Conventional ionization chambers (ICs) experience large general recombination losses under UHDP due to the high charge densities that are enhanced by severe electric field perturbation. 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 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 dose per pulse up to 10 Gy, pressures in range from 1 hPa up to 100 hPa could be applied. Approach. To explore possible configurations in terms of filling gas, pressure and bias electric field to measure a certain dose per pulse, a Python-based numerical simulation was developed to model charge transport in noble gases. The IC response was evaluated in terms of charge collection efficiency (CCE) by varying the dose per pulse, the bias field, the filling gas and its pressure. The aim is to explore suitable experimental conditions in which the response of the IC is stable for a given range of dose per pulse. Main results. Simulations identified helium and nitrogen as best candidates to be used as filling gas of an ALLS-like IC, capable of measuring up to 15 Gy/pulse at 50 and 10 hPa, respectively, while keeping the relative deviations of CCE respect to unity below 1%. Significance. These results support the feasibility of designing ICs for UHDP beams using moderate depressurization, offering a promising path toward the realization of robust, accurate detectors for FLASH reference dosimetry.