Imaging performance of a Timepix detector based on semi-insulating GaAs

Abstract

This work focused on a Timepix chip [1] coupled with a bulk semi-insulating GaAs sensor. The sensor consisted of a matrix of 256 × 256 pixels with a pitch of 55 μm bump-bonded to a Timepix ASIC. The sensor was processed on a 350 μm-thick SI GaAs wafer. We carried out detector adjustment to optimize its performance. This included threshold equalization with setting up parameters of the Timepix chip, such as Ikrum, Pream, Vfbk, and so on. The energy calibration of the GaAs Timepix detector was realized using a 241Am radioisotope in two Timepix detector modes: time-over-threshold and threshold scan. An energy resolution of 4.4 keV in FWHM (Full Width at Half Maximum) was observed for 59.5 keV γ-photons using threshold scan mode. The X-ray imaging quality of the GaAs Timepix detector was tested using various samples irradiated by an X-ray source with a focal spot size smaller than 8 μm and accelerating voltage up to 80 kV. A 700 μm × 700 μm gold testing object (X-500-200-16Au with Siemens star) fabricated with high precision was used for the spatial resolution testing at different values of X-ray image magnification (up to 45). The measured spatial resolution of our X-ray imaging system was about 4 μm.

Performance of bulk semi-insulating GaAs-based sensor and its comparison to Si-based sensor for Timepix radiation camera

“…Its electrical resistivity and electron mobility were measured to be 9.1 × 10 7 Ωcm and 6285 cm 2 /Vs at 300 K. On the one side, square Ti/Pt Schottky contacts in shape of the pixels (55 × 55 µm) were formed, whereas on the other side, common electrode was made as an Au/Ge/Ni multilayer. These structures, of a 350 µm thickness, were then wafer solder bonded to the Timepix chip to create hybrid pixel detector [7].…”

“…This is due to irradiation from the back side, through the Timepix chip, as the photons are also attenuated in the aluminium chip holder that is located only at the edges of the active area of the chip, right underneath the Timepix chip which can be seen in figure 6. However, if the bias voltage is set low enough, the inhomogeneities caused by the pixel interferences are absent and the inhomogeneities caused by partial depletion can be suppressed by flat field correction because they are not time dependent [7]. We expect that the homogeneity of the response should significantly improve after the problem with partial depletion in SI GaAs sensors is solved.…”

“…Silicon has been the most used sensor material for hybrid detectors based on Timepix chip [1], in widespread applications [2][3][4][5], due to its well-established fabrication process, which allows creation of high-quality sensors for reasonable price. However, its rather low detection efficiency to X-rays from 15 keV (due to being low Z material) motivates the research of high Z sensor materials, like GaAs [6,7], CdTe [8] (CdZnTe). Semi-insulating (SI) GaAs is one of the most promising candidates due to its favourable properties; in comparison to Si it does have up to 20 times higher linear attenuation coefficient (figure 1) in the important energy range for soft tissue and material imaging (5 to 100 keV) and 3 to 5 times faster charge collection due to about 10 times higher electric field intensity within the sensor.…”

Study of the contrast resolution of Timepix detector with a semi-insulating GaAs sensor

“…The size of one pixel was 55 × 55 µm 2 and the total number of pixels in the detector was 256 × 256. Parameters of the detector were published in detail in our previous work [18]. The SI GaAs Timepix detector showed a very good spectrometric performance that was tested using a monoenergetic radiation of 59.5 keV from 241 Am and 24.1 keV and 27.3 keV Kα and Kβ photons from indium fluorescence [19].…”

“…Figure 1 shows a distribution of the pixel counts of one taken image of an SI GaAs Timepix camera for a uniform illumination from an X-ray tube. The typical background image was shown in our previous works in which we used the same pixel detector [18,19]. We used the counting mode of the imaging Timepix detector for the measurement.…”