V. Nečas - Academia.edu (original) (raw)
Papers by V. Nečas
The European Physical Journal Special Topics
Non-destructive testing is capable of detecting defects of important components, where a failure ... more Non-destructive testing is capable of detecting defects of important components, where a failure could pose a significant hazard and cause severe economic losses. Currently, imaging techniques utilizing gamma or X-ray sources are mainly used and allow to examine of devices ranging in size from units to tens of centimetres, but for industrial-sized large components, these methods usually fail. The utilization of fast neutrons in radiography is a promising alternative, especially for industrial applications. Currently, due to the higher neutron-matter interaction probabilities, most neutron-imaging systems use thermal neutrons originating from research reactors. Due to the intention of on-site inspection of revealing the possible ruptures and failures, this option is not applicable. The paper presents the newly developed neutronic models of printed circuit heat exchangers where the specific ruptures are defined. Next, neutron transport simulations are performed to investigate the diff...
The European Physical Journal Special Topics
This paper describes cooperation of the Slovak University of Technology in Bratislava (STU) and t... more This paper describes cooperation of the Slovak University of Technology in Bratislava (STU) and the Slovak Institute of Metrology (SMU) in the verification of the calibration factors for the solid-state nuclear track detector utilized for radon dosimetry and establishing traceability for the STU laboratory. The SMU operates the Air Radon Standard which consists of a radon chamber and calibrated radon atmosphere monitoring system. A specification of the radon chamber is that the radon concentration changes during the exposure time. The radon atmosphere is created by the insertion of a specified amount of radon gas into the chamber, which subsequently decays over time. The STU laboratory is equipped with the TASLImage™ system for radon dosimetry with corresponding TASTRAK CR-39 type detectors and diffusion chambers. These chambers allow radon to diffuse inside and prevent dust and radon progeny to enter the container. Each diffusion container is characterized by its own diffusion rate...
2020 13th International Conference on Advanced Semiconductor Devices And Microsystems (ASDAM), 2020
We present an anomal current decrease for Mg contact on semi-insulating (SI) GaAs. The explanatio... more We present an anomal current decrease for Mg contact on semi-insulating (SI) GaAs. The explanation of the observed effect is discussed within the light of the observed transport characteristics and XPS analysis.
Journal of Instrumentation, 2018
Applied Surface Science, 2018
Journal of Instrumentation, 2018
Journal of Instrumentation, 2016
In this work we have focused on detection of thermal neutrons generated by 239Pu–Be isotopic neut... more In this work we have focused on detection of thermal neutrons generated by 239Pu–Be isotopic neutron source. A high quality liquid phase epitaxial layer of 4H-SiC was used as a detection region. The thickness of the layer was 70 μ m and the diameter of circular Au/Ni Schottky contact was 4.5 mm. Around the Schottky contact two guard rings were created. The detector structure was first examined as a detector of protons and alpha particles for energy calibration. Monoenergetic protons of energies from 300 keV up to 1.9 MeV were used for detector energy calibration and a good linearity was observed. The energy resolution of 35 keV was obtained for 1.9 MeV protons. The 6LiF conversion layer was applied on the detector Schottky contact. In the experiment we used different thicknesses of conversion layers from 5 μ m up to 35 μ m. Measured detected spectra show two parts corresponding to alpha particles detection in lower energy channels and 3H in higher energy channels. We have also performed simulations of thermal neutron detection using MCNPX (Monte Carlo N-particle eXtended) code. The detection efficiency and the detector response to thermal neutrons was calculated with respect to the 6LiF layer thickness. The detection efficiency calculation is found to be in good agreement with the experiment.
Journal of Instrumentation, 2020
2020 13th International Conference on Advanced Semiconductor Devices And Microsystems (ASDAM), 2020
In this paper we investigate the spectrometric deterioration of semi-insulating GaAs detectors de... more In this paper we investigate the spectrometric deterioration of semi-insulating GaAs detectors degraded by high doses of 5 MeV electrons, beyond the previously set limit of their operation ability of 1 MGy. The spectrometric characterization is made by measuring the 59.5 keV gamma\gammagamma-rays from 241 Am, evaluating their charge collection efficiency (CCE). The accumulative dose has deteriorated the CCE of detectors, at 200 V reverse biasing, from initial 74% down to 15% at 2 MGy, independently of the dose rate in the range from 20 to 80 MGy/h. The 75% of tested detectors were able to operate with 24% CCE after 1 kGy degradation. On the other hand, only 33% of them sustain operable after next dose step of 1.25 MGy. The highest applied degradation-dose withstand operable only one detector of all twelve samples, demanding the noise subtraction to achieve the observable signal from registered photons.
2016 11th International Conference on Advanced Semiconductor Devices & Microsystems (ASDAM), 2016
We have studied the semi-insulating (SI) GaAs Schottky diode with 6LiF conversion layer as a dete... more We have studied the semi-insulating (SI) GaAs Schottky diode with 6LiF conversion layer as a detector of thermal neutrons. Two parameters of detector were optimized: the active detector volume modified by the reverse voltage applied on barrier and the thickness of conversion layer. Increasing the reverse voltage, the active volume of detector spreads into depth of GaAs substrate. Optimal reverse voltage was determined to be 50 V, as the responsible active detector thickness effectively absorbs the neutron products but the accompanying gamma rays from neutron source are absorbed with very low probability. The optimal thickness of 6LiF conversion layer was set to 26 μη according to our experiments, when the layer reached the maximum conversion efficiency of neutrons.
1 Institute of Electrical Engineering, SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovakia, 2 Ins... more 1 Institute of Electrical Engineering, SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovakia, 2 Institute of Physics v.v.i., Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Praha 6, Czech Republic, 3 IMEM-CNR Institute, Parco Area delle Scienze 37/A, I-43010 Fontanini, Parma, Italy, 4 Faculty of Electrical Engineering and Information Technology, SUT, Ilkovičova 3, 812 19 Bratislava, Slovakia, 5 Department of Electronics, Academy of Armed Forces, Demänová 393, 031 06 Liptovský Mikuláš, Slovakia E-mail: elekfdub@savba.sk
Journal of Instrumentation, 2018
This work focused on a Timepix chip [1] coupled with a bulk semi-insulating GaAs sensor. The sens... more 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.
Journal of Instrumentation, 2018
In this work, we have focused on Timepix detectors coupled with the semi-insulating GaAs material... more 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 241Am and 133Ba 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.
Journal of Instrumentation, 2020
A radiation hardness study of developed Schottky barrier semi-insulating (SI) GaAs detectors agai... more A radiation hardness study of developed Schottky barrier semi-insulating (SI) GaAs detectors against 5 MeV electrons is described in this paper. The influence of cumulative dose (up to 1000 kGy) and the applied dose rate (20, 40 or 80 kGy/h) on spectrometric and electrical properties of detectors is presented. The spectrometric properties were evaluated from measured 241Am gamma spectra and the electrical properties were obtained measuring the current-voltage characteristics. The cumulative dose has negatively affected all spectrometric properties. The detector CCE (Charge Collection Efficiency) exponentially decreased with dose from initial 74% down to 24% at maximum applied dose of 1000 kGy. The relative energy resolution was increasing from 17% before irradiation up to almost 50% at maximum dose applied. The peak-to-valley ratio firstly increased from 10 to 17 at 24 kGy and then rapidly decreased below 2 at 200 kGy. The electrical properties were obtained from measured current-voltage characteristics of detectors. The breakdown voltage of detectors firstly decreased to 60% of its original value and at few hundred kGy it started to increase exceeding the initial value twice. The reverse current exhibited similar behaviour decreasing up to 200 kGy and then increasing. Concerning obtained results we can determine the limit of sufficient functionality of developed SI GaAs detectors against 5 MeV electrons to be 1000 kGy for 59.5 keV particles. The second investigated parameter of irradiation, the dose rate in chosen ranges, did not greatly alter the spectrometric properties of studied detectors.
Journal of Instrumentation, 2019
Nowadays, pixelated detectors based on Timepix readout chip play important role in various fields... more 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.
Nuclear Instruments and Methods in Physics Research, 1984
Journal of Instrumentation, 2016
The radiation hardness of Semi-Insulating (SI) GaAs detectors against 5 MeV electrons is investig... more 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.
Applied Surface Science, 2017
The European Physical Journal Special Topics
Non-destructive testing is capable of detecting defects of important components, where a failure ... more Non-destructive testing is capable of detecting defects of important components, where a failure could pose a significant hazard and cause severe economic losses. Currently, imaging techniques utilizing gamma or X-ray sources are mainly used and allow to examine of devices ranging in size from units to tens of centimetres, but for industrial-sized large components, these methods usually fail. The utilization of fast neutrons in radiography is a promising alternative, especially for industrial applications. Currently, due to the higher neutron-matter interaction probabilities, most neutron-imaging systems use thermal neutrons originating from research reactors. Due to the intention of on-site inspection of revealing the possible ruptures and failures, this option is not applicable. The paper presents the newly developed neutronic models of printed circuit heat exchangers where the specific ruptures are defined. Next, neutron transport simulations are performed to investigate the diff...
The European Physical Journal Special Topics
This paper describes cooperation of the Slovak University of Technology in Bratislava (STU) and t... more This paper describes cooperation of the Slovak University of Technology in Bratislava (STU) and the Slovak Institute of Metrology (SMU) in the verification of the calibration factors for the solid-state nuclear track detector utilized for radon dosimetry and establishing traceability for the STU laboratory. The SMU operates the Air Radon Standard which consists of a radon chamber and calibrated radon atmosphere monitoring system. A specification of the radon chamber is that the radon concentration changes during the exposure time. The radon atmosphere is created by the insertion of a specified amount of radon gas into the chamber, which subsequently decays over time. The STU laboratory is equipped with the TASLImage™ system for radon dosimetry with corresponding TASTRAK CR-39 type detectors and diffusion chambers. These chambers allow radon to diffuse inside and prevent dust and radon progeny to enter the container. Each diffusion container is characterized by its own diffusion rate...
2020 13th International Conference on Advanced Semiconductor Devices And Microsystems (ASDAM), 2020
We present an anomal current decrease for Mg contact on semi-insulating (SI) GaAs. The explanatio... more We present an anomal current decrease for Mg contact on semi-insulating (SI) GaAs. The explanation of the observed effect is discussed within the light of the observed transport characteristics and XPS analysis.
Journal of Instrumentation, 2018
Applied Surface Science, 2018
Journal of Instrumentation, 2018
Journal of Instrumentation, 2016
In this work we have focused on detection of thermal neutrons generated by 239Pu–Be isotopic neut... more In this work we have focused on detection of thermal neutrons generated by 239Pu–Be isotopic neutron source. A high quality liquid phase epitaxial layer of 4H-SiC was used as a detection region. The thickness of the layer was 70 μ m and the diameter of circular Au/Ni Schottky contact was 4.5 mm. Around the Schottky contact two guard rings were created. The detector structure was first examined as a detector of protons and alpha particles for energy calibration. Monoenergetic protons of energies from 300 keV up to 1.9 MeV were used for detector energy calibration and a good linearity was observed. The energy resolution of 35 keV was obtained for 1.9 MeV protons. The 6LiF conversion layer was applied on the detector Schottky contact. In the experiment we used different thicknesses of conversion layers from 5 μ m up to 35 μ m. Measured detected spectra show two parts corresponding to alpha particles detection in lower energy channels and 3H in higher energy channels. We have also performed simulations of thermal neutron detection using MCNPX (Monte Carlo N-particle eXtended) code. The detection efficiency and the detector response to thermal neutrons was calculated with respect to the 6LiF layer thickness. The detection efficiency calculation is found to be in good agreement with the experiment.
Journal of Instrumentation, 2020
2020 13th International Conference on Advanced Semiconductor Devices And Microsystems (ASDAM), 2020
In this paper we investigate the spectrometric deterioration of semi-insulating GaAs detectors de... more In this paper we investigate the spectrometric deterioration of semi-insulating GaAs detectors degraded by high doses of 5 MeV electrons, beyond the previously set limit of their operation ability of 1 MGy. The spectrometric characterization is made by measuring the 59.5 keV gamma\gammagamma-rays from 241 Am, evaluating their charge collection efficiency (CCE). The accumulative dose has deteriorated the CCE of detectors, at 200 V reverse biasing, from initial 74% down to 15% at 2 MGy, independently of the dose rate in the range from 20 to 80 MGy/h. The 75% of tested detectors were able to operate with 24% CCE after 1 kGy degradation. On the other hand, only 33% of them sustain operable after next dose step of 1.25 MGy. The highest applied degradation-dose withstand operable only one detector of all twelve samples, demanding the noise subtraction to achieve the observable signal from registered photons.
2016 11th International Conference on Advanced Semiconductor Devices & Microsystems (ASDAM), 2016
We have studied the semi-insulating (SI) GaAs Schottky diode with 6LiF conversion layer as a dete... more We have studied the semi-insulating (SI) GaAs Schottky diode with 6LiF conversion layer as a detector of thermal neutrons. Two parameters of detector were optimized: the active detector volume modified by the reverse voltage applied on barrier and the thickness of conversion layer. Increasing the reverse voltage, the active volume of detector spreads into depth of GaAs substrate. Optimal reverse voltage was determined to be 50 V, as the responsible active detector thickness effectively absorbs the neutron products but the accompanying gamma rays from neutron source are absorbed with very low probability. The optimal thickness of 6LiF conversion layer was set to 26 μη according to our experiments, when the layer reached the maximum conversion efficiency of neutrons.
1 Institute of Electrical Engineering, SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovakia, 2 Ins... more 1 Institute of Electrical Engineering, SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovakia, 2 Institute of Physics v.v.i., Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Praha 6, Czech Republic, 3 IMEM-CNR Institute, Parco Area delle Scienze 37/A, I-43010 Fontanini, Parma, Italy, 4 Faculty of Electrical Engineering and Information Technology, SUT, Ilkovičova 3, 812 19 Bratislava, Slovakia, 5 Department of Electronics, Academy of Armed Forces, Demänová 393, 031 06 Liptovský Mikuláš, Slovakia E-mail: elekfdub@savba.sk
Journal of Instrumentation, 2018
This work focused on a Timepix chip [1] coupled with a bulk semi-insulating GaAs sensor. The sens... more 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.
Journal of Instrumentation, 2018
In this work, we have focused on Timepix detectors coupled with the semi-insulating GaAs material... more 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 241Am and 133Ba 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.
Journal of Instrumentation, 2020
A radiation hardness study of developed Schottky barrier semi-insulating (SI) GaAs detectors agai... more A radiation hardness study of developed Schottky barrier semi-insulating (SI) GaAs detectors against 5 MeV electrons is described in this paper. The influence of cumulative dose (up to 1000 kGy) and the applied dose rate (20, 40 or 80 kGy/h) on spectrometric and electrical properties of detectors is presented. The spectrometric properties were evaluated from measured 241Am gamma spectra and the electrical properties were obtained measuring the current-voltage characteristics. The cumulative dose has negatively affected all spectrometric properties. The detector CCE (Charge Collection Efficiency) exponentially decreased with dose from initial 74% down to 24% at maximum applied dose of 1000 kGy. The relative energy resolution was increasing from 17% before irradiation up to almost 50% at maximum dose applied. The peak-to-valley ratio firstly increased from 10 to 17 at 24 kGy and then rapidly decreased below 2 at 200 kGy. The electrical properties were obtained from measured current-voltage characteristics of detectors. The breakdown voltage of detectors firstly decreased to 60% of its original value and at few hundred kGy it started to increase exceeding the initial value twice. The reverse current exhibited similar behaviour decreasing up to 200 kGy and then increasing. Concerning obtained results we can determine the limit of sufficient functionality of developed SI GaAs detectors against 5 MeV electrons to be 1000 kGy for 59.5 keV particles. The second investigated parameter of irradiation, the dose rate in chosen ranges, did not greatly alter the spectrometric properties of studied detectors.
Journal of Instrumentation, 2019
Nowadays, pixelated detectors based on Timepix readout chip play important role in various fields... more 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.
Nuclear Instruments and Methods in Physics Research, 1984
Journal of Instrumentation, 2016
The radiation hardness of Semi-Insulating (SI) GaAs detectors against 5 MeV electrons is investig... more 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.
Applied Surface Science, 2017