The radiation hardness of Semi-Insulating (SI) GaAs detectors against 5 MeV electrons is investigated in this paper. The influence of two parameters, the accumulative absorbed dose (from 1 to 120 kGy) and the applied dose rate (20, 40 or 80 kGy/h), on detector spectrometric properties was studied. The electron irradiation has negatively affected the detector CCE (Charge Collection Efficiency). Un-irradiated detectors exhibited the CCE of 79% at maximum operating reverse voltage of 300 V and reached the maximum CCE of 51% at 200 V after irradiation by a dose of 120 kGy. Relative energy resolution was also affected by electron irradiation. Its global degradation was observed in the range of doses from 24 up to 120 kGy, where an increase from 19% up to 39% at 200 V reverse voltage was noticed. On the other hand, a global increase of detection efficiency with dose, by about 30% at 120 kGy, was observed with all samples. We did not observe any significant influence of chosen dose rates applied during irradiation on investigated spectrometric properties of detectors.
A: Nowadays, pixelated detectors based on Timepix readout chip play important role in various fields of science and research like particle physics, advanced spectrometry and especially X-ray imaging for medical and material purposes. Present work is focused on the evaluation of the 350 µm thick prototype (semi-insulating) SI GaAs sensor performance and its comparison to 300 µm thick Si sensor that was chosen as a reference. Both sensors consist of 256 × 256 pixels matrix with pixel pitch of 55 µm. We performed flat field illumination to test the pixel homogeneity and found out that the response of GaAs sensor is less homogeneous in terms of number of counts across the sensor compared to Si sensor. The imaging performance was tested using two samples of different absorption strength, specifically one less-absorbing sample and one more-absorbing sample. We found out that our fabricated SI GaAs sensor shows better imaging performance than Si sensor in case of imaging more-absorbing sample and vice versa. This is due to different prevalent detection efficiency regions. The spatial resolution was determined to be 8.73 lp/mm and 8.25 lp/mm for GaAs and Si sensor, respectively. Also, spectrometric measurements were carried out and results show that Si sensor has about 2 times higher energy resolution in comparison with SI GaAs sensor.
K: X-ray detectors; Inspection with x-rays; X-ray radiography and digital radiography (DR)
We have examined semi-insulating (SI) GaAs detectors with high density polyethylene (HDPE) conversion layer by a mono-energetic neutrons with kinetic energy of 16.755 MeV generated by a deuterium -tritium nuclear reaction. First, the influence of HDPE layer thickness on the relative detection efficiency of fast neutrons was studied. The MCNPX (Monte Carlo N-particle eXtended) code has been used to support the analysis of the experiment. The theoretical optimum thickness of the conversion layer was determined to 1.9 mm using the MCNPX code. The HDPE conversion layers of various thicknesses, in the range from 50 µm to 3200 µm, were glued on the top Schottky contact of SI GaAs detector in the experiment. The neutron detection efficiency was evaluated from measured spectra and compared to results from simulations. The experimental data showed very good agreement with simulation results. Then the effect of active detector thickness modified by detector reverse bias on neutron detection efficiency was studied. Finally, the effect of the angle of irradiation on neutron detection efficiency was evaluated exhibiting decreasing tendency with increasing deviation from perpendicular direction of impinging neutrons.
In this work, we have focused on Timepix detectors coupled with the semi-insulating GaAs material sensor. We used undoped bulk GaAs material with the thickness of 350 µm. We prepared and tested four pixelated detectors with 165 µm and 220 µm pixel size with two versions of technology preparation, without and with wet chemically etched trenches around each pixel. We have carried out adjustment of GaAs Timepix detectors to optimize their performance. The energy calibration of one GaAs Timepix detector in Time-over-threshold mode was performed with the use of 241 Am and 133 Ba radioisotopes. We were able to detect γ-photons with the energy up to 160 keV. The X-ray imaging quality of GaAs Timepix detector was tested with X-ray source using various samples. After flat field we obtained very promising imaging performance of tested GaAs Timepix detectors.
A: Single absorption grating based X-ray phase contrast imaging (XPCI) with a hybrid pixel detector Timepix is a promising technique in many application fields (material sciences, biology, medicine etc.). Conventional coded aperture approach with microfocus source uses two absorption gratings where one of them is moved in steps (so-called phase stepping). In contrast to that, the proposed XPCI approach provides the absorption, phase contrast and dark field images from a single acquisition. This leads to higher mechanical stability of the system and decrease of the delivered dose. Moreover, faster data acquisition increases the usability of the system for CT measurements and dynamic phase radiography.Custom-built laboratory X-ray imaging setup equipped with Timepix detector and nano-focus X-ray tube was utilized for the measurements. The maximal source to detector distance (SDD) available in the setup is roughly 90 cm. Such system can provide long enough phase propagation distance while using the absorption grating with 60 µm strip period, 55 µm large detector pixels and three pixel illumination. The system enables insertion and positioning of both 1D and 2D absorption gratings producing micro-line or micro-pencil beams, respectively.
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