B. Zat'ko

GaAs detectors irradiated by low doses of electrons

Semi-insulating (SI) GaAs detectors were irradiated by 5 MeV electrons up to a dose of 69 kGy, in order to test their radiation hardness. The electric and spectrometric stability of detectors was examined as a function of the absorbed dose. Investigated detectors showed a very good detector radiation resistance within a dose up to 40 kGy followed by deterioration of some spectrometric and electric properties. However, the reverse current and the detector charge collection efficiency showed minimum changes with the overall applied doses. The obtained results will be used as a preliminary study for further radiation-hardness investigations of GaAs detectors against high energy electrons. This will complete our previous studies of GaAs detector radiation hardness against fast neutrons and γ-rays.

High resolution alpha particle detectors based on 4H-SiC epitaxial layer

We fabricated and characterized 4H-SiC Schottky diodes as a spectrometric detector of alpha particles. A thin blocking contact of Ni/Au (15 nm) was used to minimize the influence on alpha particles energy. Current-voltage characteristics of the detector were measured and a low current density below 0.3 nAcm−2 was observed at room temperature. 239Pu241Am244Cm was used as a source of alpha particles within the energy range between 5.1 MeV and 5.8 MeV for detector testing. The charge collection efficiency close to 100 % at reverse bias exceeding 50 V was determined. The best spectrometric performance shows a pulse height spectrum at a reverse bias of 200 V giving an energy resolution of 0.25 % in the full width and half maximum for 5.486 MeV of 241Am.

Characterization of epitaxial 4H-SiC for photon detectors

High purity epitaxial 4H-SiC became a serious candidate for the fabrication of spectrometric radiation detectors with a high resistance to neutrons and gamma rays damage and suitable for applications in hot plasma diagnostics. The present work reports on i) the characterization of high purity epitaxial 4H-SiC grown by liquid phase epitaxy on SiC substrates and ii) the performances of metal/4H-SiC detectors fabricated on the same material. X-ray diffraction and topography as well as I-V, C-V and DLTS measurements are used for the evaluation of the material properties and device characteristics. The UV and X-ray detection abilities are evaluated by photocurrent measurements in the 3-6 eV region and pulse-height spectra measurements of the 241Am, respectively. Preliminary results of the detector hardness to fast neutron and gamma ray radiations are also reported.

MCNPX Monte Carlo simulations of particle transport in SiC semiconductor detectors of fast neutrons

The aim of this paper was to investigate particle transport properties of a fast neutron detector based on silicon carbide. MCNPX (Monte Carlo N-Particle eXtended) code was used in our study because it allows seamless particle transport, thus not only interacting neutrons can be inspected but also secondary particles can be banked for subsequent transport. Modelling of the fast-neutron response of a SiC detector was carried out for fast neutrons produced by 239Pu-Be source with the mean energy of about 4.3 MeV. Using the MCNPX code, the following quantities have been calculated: secondary particle flux densities, reaction rates of elastic/inelastic scattering and other nuclear reactions, distribution of residual ions, deposited energy and energy distribution of pulses. The values of reaction rates calculated for different types of reactions and resulting energy deposition values showed that the incident neutrons transfer part of the carried energy predominantly via elastic scattering on silicon and carbon atoms. Other fast-neutron induced reactions include inelastic scattering and nuclear reactions followed by production of α-particles and protons. Silicon and carbon recoil atoms, α-particles and protons are charged particles which contribute to the detector response. It was demonstrated that although the bare SiC material can register fast neutrons directly, its detection efficiency can be enlarged if it is covered by an appropriate conversion layer. Comparison of the simulation results with experimental data was successfully accomplished.

Semi-insulating GaAs detectors optimized for fast neutron detection

Semi-insulating (SI) GaAs detectors with a HDPE (High Density PolyEthylene) conversion layer were optimized for detection of fast neutrons (from 0.5 MeV to 12 MeV). Based on previous simulations of neutron transport in HDPE and SI GaAs carried out by the Monte Carlo radiation transport computer code MCNPX (Monte Carlo N-Particle eXtended, version 2.5.0) we used a SI GaAs wafer with a larger thickness of 270 μm. The area of a single AuZn Schottky contact was enlarged from 6.25 mm2 to 7.36 mm2. Thanks to the pixel structure of the new metallization the breakdown voltage increased from 60 V to 280 V as deduced from the measured I−V characteristics. Various thicknesses of HDPE layers in the range from 100 μm to 2000 μm were used with SI GaAs detectors for neutron conversion. The measured relative detection efficiency for fast neutrons using SI GaAs detectors with various HDPE thicknesses varied from 0.07 to 0.12% at lower applied voltages, with a maximum for a 500 μm thick conversion layer.

Detection of fast neutrons using detectors based on semi-insulating GaAs

Detectors with AuZn square Schottky contact of the area of 2.5 ? 2.5 mm2 were fabricated. On the back side, the whole area AuGeNi eutectic ohmic contact was evaporated. The thickness of the base material (semi-insulating GaAs) was 220 ?m. The connection of 4 detectors in parallel was tested to get the detection area of 25 mm2. The 239Pu-Be fast neutron source with energies between 0.5 and 12 MeV was used in experimental measurements. We have investigated the optimal thickness of HDPE (high-density polyethylene) conversion layer for fast neutron detection. The spectra of the neutrons were measured by detectors covered by HDPE converter of different thicknesses. The fast neutron detection efficiency proved experimentally was compared with results from simulations performed by MCNPX (Monte Carlo N-Particle eXtended) code.

GaAs detectors irradiated by electrons at different dose rates

The radiation hardness of Semi-Insulating (SI) GaAs detectors against high-energy electrons was investigated. The detectors were irradiated by 5 MeV electrons. The influence of two irradiation parameters, the total absorbed dose (up to 24 kGy) and the applied dose rate (20, 40 and 80 kGy/h), on their spectrometric properties was studied. An 241Am gamma-ray source was used to evaluate the spectrometric properties. The applied dose has negatively affected the detector CCE (Charge Collection Efficiency) and has influenced also the energy resolution. Nevertheless, a global increase of detection efficiency with the dose was observed. Three different dose rates used during irradiation did not affect the CCE, but in the range of doses from 4 to 16 kGy an influence of the applied dose rate upon two other parameters was observed. With higher dose rates, a steeper increase in the detection efficiency and significant worsening of energy resolution were achieved.

Semiconductor detector based on 4H-SiC and analysis of its active region thickness

The 4H polytype silicon carbide is a promising material for radiation-resistant sensors of ionizing particles. The wide band gap of 3.26 eV offers operation at increased temperatures up to several hundred degrees of Celsius. In this work we focused on the analysis of active region thickness of detectors based on 4H-SiC. The detectors investigated are fabricated from a 105 ?m epitaxial layer grown on 350 ?m 4H-SiC substrate. The circular Schottky contacts with diameter of 1.4 mm using an Au/Ni double layer were evaporated onto both sides of the detector material. Three methods for determination of the active thickness were used. The capacitance-voltage measurements allowed us to estimate free carrier concentration profile. The second method was based on detection of ?-particles generated by an 241Am source. Using SRIM calculations and known Bragg curve absorbed energy in detector volume was estimated. The last method consists of measuring detection efficiency of ?-rays at reverse bias voltages up to 500 V.

Detector of fast neutrons based on silicon carbide epitaxial layers

High quality liquid phase epitaxial layer of 4H-SiC with a thickness of 105 μm was used for fabrication of detector with the circular Schottky contact formed by using Au/Ni double layer contact metallization. The detector structure was characterized by current-voltage measurements showing low reverse current density at room temperature. Following the detection of fast neutrons was studied using the 239Pu-Be source with the mean neutron energy of about 4 MeV. Detected spectrum revealed silicon and carbon-recoil ion continuum. The simulation of detection efficiency vs. thickness of the detector active layer shows linear dependence.

First results observed with test X-CT system using GaAs radiation detector working in single photon counting regime

The aim of present work is application of novel single photon counting system for detection of gamma-rays in CT using semiconductor radiation detector based on semi-insulating (SI) GaAs compound. A simple positioning test system consisting of two stepper motors with 241 Am gamma source of 60 keV photons and single SI GaAs radiation detector operated in single photon counting mode is used for taking CT projections. First, rather preliminary results of CT reconstruction applied to imaging of inner structure of a small phantom using developed reconstruction algorithms is demonstrated