Description
Pixelated hybrid detectors, such as Timepix3, enable spatially and temporally resolved measurements of photons and are highly valuable for diagnosing high-energy electron populations. In particular, they provide a compact tool for studying runaway electrons in tokamaks via detection of bremsstrahlung radiation.
This contribution summarizes progress in the development and experimental validation of a simulation framework for photon detection with Timepix3 hybrid pixel detectors, as well as in the preparation of validation measurements in complex radiation fields. The main focus is on tuning a physically realistic digitization model, performing benchmark experiments, and building supporting data and software infrastructure.
As part of benchmark experiments for gamma radiation detection, the Allpix2 simulation chain for the Timepix3 detector with a 300~\textmu m thick silicon sensor was validated. The result is quantitative agreement between simulated and measured spectra. The validation was supported by laboratory measurements of radionuclides (e.g., $^{241}$Am, $^{137}$Cs) in several irradiation geometries and measurements of XRF spectra, including analysis of energy spectra and cluster size distribution.
To extend the simulation chain towards tokamak-relevant conditions, we are preparing source models based on bremsstrahlung photon spectra generated from different runaway-electron distribution functions. These spectra will be used as input sources in Allpix2 to propagate photons through realistic detector geometry and shielding configurations and to quantify the impact of electron distribution assumptions on the detector response (energy spectra, rate, and cluster observables). This enables end-to-end forward modeling from candidate runaway-electron populations to measurable quantities and provides a controlled path towards validation in mixed and time-dependent radiation fields.
At the same time, measurements were carried out on the GOLEM tokamak with the aim of preparing the groundwork for future validation of simulations in complex mixed radiation fields. A permanent installation of the Timepix3 detector was put into operation on the GOLEM tokamak, enabling continuous monitoring of discharges with fully automated data collection and processing. The first processed results from the 1000~\textmu m thick Si sensor show the time evolution of the detected energy during the discharge and the corresponding energy spectra, which will be used for the subsequent validation phase of the simulation chain.