Radiation measurements using Timepix3 with silicon sensor and bare chip in proton beams for FLASH radiotherapy

Abstract This study investigates the response of Timepix3 semiconductor pixel detectors in proton beams of varying intensities, with a focus on FLASH proton therapy. Using the Timepix3 application-specific integrated circuit (ASIC) chip, we measured the spatial and spectral characteristics of 220 MeV proton beams delivered in short pulses. The experimental setup involved Minipix readout electronics integrated with a Timepix3 chipboard in a flexible architecture, and an Advapix Timepix3 with a silicon sensor. Measurements were carried out with Timepix3 detectors equipped with experimental gallium arsenide (GaAs) and silicon (Si) sensors. We also investigated the response of a bare Timepix3 ASIC chip (without sensor). The detectors were placed within a waterproof holder attached to the positioning system of the IBA Blue water phantom, with additional measurements performed in air behind a 2 cm-thick polymethyl methacrylate (PMMA) phantom. The results demonstrated the capability of the Timepix3 detectors to measure time-over-threshold (ToT, deposited energy) and event counts (number of events in a pixel) in both conventional and ultra-high-dose-rates (UHDR) proton beams. The bare ASIC chip configuration sustained up to a dose rate (DR) of 270 Gy/s, the maximum tested intensity, although it exhibited limited spatial resolution due to low detection efficiency. In contrast, Minipix Timepix3 with experimental GaAs sensors showed saturation at low DR∼5 Gy/s. Furthermore, the Advapix Timepix3 detector was used in both standard and customized configurations. In the standard configuration (Ikrum = 5), the detector showed saturation at DR∼5 Gy/s. But, in the customized configuration when the per-pixel discharging signal (called “Ikrum”) was increased (Ikrum = 80), the detector demonstrated enhanced performance by reducing the duration of the ToT signal, allowing beam spot imaging up to DR=∼28 Gy/s in the plateau region of the Bragg curve. For such DR or higher, the frame acquisition time was reduced to the order of microseconds, meaning only a fraction of the pulse (with pulse lengths on the order of milliseconds) was captured.