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Timepix detectors have a great advantage in the capability of spectral particle track imaging in um scale, reflecting the pixel size. This requires precise per pixel energetic calibration. The Timepix detector energetic calibration is achieved usually by photons with energies up to 100keV that create a charge in a single pixel, so-called low-energy calibration. High-energy heavy charged particles generate tracks across multiple pixels and the per pixel energy deposited into some pixels is often outside the low-energy calibration range leading to an underestimation of the measured particle energy. The principle of high-energy per pixel calibration of the Timepix2 detector with a Si 500 um sensor and Katherine readout was developed by the authors in [1] using MeV protons and Am-241 alpha particles and biasing the detector to 150V – 300V. However, the voltage applied to the Timepix detector affects the collecting electric field distribution and charge collection efficiency, which might affect the per pixel calibration.
This work focuses on the experimental investigation of charge collection by detecting protons of energies in the range of 0.1 - 1.6 MeV with Timepix2 detector with 500 um thick Si sensor and MiniPIX readout [2] depending on the applied bias in the range of 20-200 V. The proton energy spectra were compared with measurements of a single pad Si detector for precise calibration purposes. The measurements were performed at the Van de Graaff accelerator at IEAP CTU in Prague and supplemented with alpha particle measurements from Am-241 and U-233 Pu-238 Pu-239 triple source with energies from 4.8 to 5.5 MeV. The results have shown that the biasing of MiniPIX Timepix2 detector under 40 V causes incomplete charge collection and is inapplicable for spectrometry. With increasing biasing, the charge collection is complete, and spectral tracking of heavy charged particles can be done correctly with low-energy calibration up to certain particle energy. Further increase in bias causes signal distortion depending on particle energy and high-energy calibration must be used. The obtained results will help to avoid the need for complicated high-energy calibration by tuning appropriate bias in spectrometry of heavy charged particles of high energies. This will also be applicable for detecting fast neutrons, which produce heavy charged particles in the sensor when interacting.
[1] B. Bergmann et al., 2022 JINST 17 C01025
[2] C. Granja et al., 2022 JINST 17 C11014
Acknowledgement: This work was partially supported by grants APVV-22-0382 and DS-FR-24-0020 of the Slovak Research and Development Agency and funded by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project No. 09I05-03-V02-00073. Carlos Granja was affiliated at Advacam, Prague when the experimental measurements were performed.