Characteristics of electrometers on ionization chamber dosimetry in ultra-high dose-rate proton beam irradiation

Purpose This study investigated the influence of differences in electrometers on the dosimetry of UHDR scanning proton beam irradiation. Method A measurement system that combined ionization measurements using an ionization chamber and electrometers with luminescence observations of an intensifying screen was used. Proton beams were delivered to this system at various doses and dose rates, and dose-to-luminescence ratios were calculated. Performance was compared among three different electrometers: RAMTEC DUO (RAMTEC), Sakura Proof (Sakura), which are charge accumulation-type, and UNIDOS webline (UNIDOS), which is current integration-type. Results The differences between the maximum and minimum dose-to-luminescence values were 0.851 % (4 Gy) and 1.543 % (20 Gy) for the RAMTEC, 0.913 % (4 Gy) and 1.626 % (20 Gy) for the Sakura, and 1.885 % (4 Gy) and 1.672 % (20 Gy) for the UNIDOS. A comparison of the variances showed that the UNIDOS exhibited significantly larger variances at 20.0, 50.0, and 100.0 Gy/s for 4 Gy and at 124.0 and 167.0 Gy/s for 20 Gy. Regarding dose rate correlations, at 4 Gy, RAMTEC and Sakura showed a positive correlation at dose rates of 10 Gy/s or less. At 20 Gy, no difference was observed among the three electrometers for dose rates up to 50 Gy/s. However, only the UNIDOS showed a negative correlation at dose rates of 100 Gy/s or higher. Conclusion Our findings suggest that current integration-type electrometers perform better with conventional dose rates, whereas charge accumulation-type electrometers allow for more stable measurements in UHDR environments.