Speaker
Description
X-and γ-rays are a unique window into the most extreme environments in the Universe such as accretion discs around neutron stars or black holes. The measurement of the polarization of the detected photons adds parameter space further allowing for advancing our current understanding of astrophysical γ-ray sources and environmental conditions by probing physical anisotropies, for example magnetic fields, aspheric matter distributions or black hole spin [1]. Even though providing these fundamental pieces of information, to date only a few measurements of linear polarization of cosmic X- and γ-ray sources exist, in particular in the energy range from 0.1 to 10 MeV (“MeV gap”).
Considering the already well-established use as a single-layer Compton Camera [2] and the space heritage of Timepix-type detectors [3], we evaluate Timepix3 [4] for use as a polarimeter in future space missions. Timepix3 offers a fine grid of 256 x 256 pixels with 55 um pitch and a per-pixel 1.56 ns time-stamping precision. The latter provides means for 3D reconstruction of photon interactions [5], which allows for Compton Camera imaging and simultaneous Compton Scatter Polarimetry through detection of Compton electron’s and scattered photon’s energy and 3D location within a “thick” semiconductor sensor. In mixed radiation environments, requiring least two coincident interactions and the inherent track classification provided by Timepix3 provides sufficient background suppresion.
In the present work, we evaluate the performance of a single-layer Timepix3 for polarimetry in a simple laboratory experiment and by complementary simulation. For this purpose, we performed a laboratory experiments creating polarized photons by irradiating a plastic target of 2 x 2 x 2 cm3 by X-rays from a Hamamatsu microfocus tube at a voltage of 75 kV and 90 kV. We then detect the polarization with a Timepix3 detector with 1 mm thick silicon sensor (Figure 1) at different angles with respect to the tube-to-target axis. We found a modulation µ of up to 80% (Figure 2) at scattering angle 90 degrees, which reduces at lower scattering angles. The results are complemented with simulations in Allpix2 [6] determining the modulation to a 100% polarized beam µ100 as a function of photon energy. In the best case a modulation of µ100 = 96 % could be achieved.
We will further demonstrate the Compton imaging performance using the above acquired experimental data and implementing the origin ensemble with the resolution recovery [7] method. The standard deviation of the source image was around 15 degrees due to measurement uncertainties (Figure 3).
We will outline the consequences of the presented findings for possible usage of Timepix3 or the newer Timepix4 detectors in X-ray and gamma-ray astronomy.
[1] Weisskopf, M.C. “An Overview of X-Ray Polarimetry of Astronomical Sources.“ Galaxies, 2018, 6, 33.
[2] Turecek, D. et al. “Single layer Compton camera based on Timepix3 technology.” JINST, 2020
[3] Bergmann, B. et al. " Results and Perspectives of Timepix Detectors in Space—From Radiation Monitoring in Low Earth Orbit to Astroparticle Physics." Instruments, 2024
[4] Poikela, T. et al. “Timepix3: a 65K channel hybrid pixel readout chip with simultaneous ToA/ToT and sparse readout.” JINST, 2014
[5] Bergmann, B. et al. “ 3D track reconstruction capability of a silicon hybrid active pixel detector.”, The Eur. Phys. Jour. C, 2017
[6] Spannagel, S. et al. " Allpix2: A modular simulation framework for silicon detectors." Nucl. Ist. and Meth. in Phys. Res. A, 2018
[7] Andreyev, A. et al. “Resolution recovery for Compton camera using origin ensemble algorithm.” Medical Physics, 2016
