Vladimír Nečas

Investigation of the electronic energy loss of hydrogen ions in H2O: influence of the state of aggregation

Abstract The gas-solid difference (phase effect) of the electronic stopping cross section (SCS) is investigated for hydrogen projectiles in H2O at energies near the stopping maximum. We find experimentally, that at low ion velocities the SCS of the gas is considerably higher than that of the solid (up to 15%). This result is discussed in terms of differences between the valence states of H2O vapor and of ice and in terms of the projectile inelastic processes.

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.

The impact of radioactive steel recycling on the public and professionals

The decommissioning of nuclear power plants represents a complex process resulting in the generation of large amounts of waste materials, e.g. steel scrap containing various concentrations of radionuclides. Recycling some of these materials is highly desirable due to numerous reasons. Herein presented scenarios of recycling of radioactive steel within the nuclear as well as civil engineering industry are analyzed from the radiation protection point of view. An approach based on the dose constraints principle is chosen. The aim of the study is to derive conditional clearance levels (maximal specific mass activity of material allowing its recycling/clearance) for analyzed radionuclides ensuring that the detrimental impact on human health is kept on a negligible level. Determined conditional clearance levels, as the result of performed software calculations, are valid for the reuse of radioactive steel in four selected scenarios. Calculation results indicate that the increase of the amount of recyclable radioactive steel due to its reuse in specific applications may be feasible considering the radiation impact on the public and professionals. However, issues connected with public acceptance, technical difficulties and financing of potential realization are still open and they have to be examined in more detail.

Femtosecond and highly sensitive GaAs metal–semiconductor–metal photodetectors grown on aluminum mirrors/pseudo-substrates,

In this study, ultrafast GaAs metal–semiconductor–metal (MSM) photodetectors grown on aluminum mirrors/pseudo-substrates were fabricated and tested. Surface characterization measurements revealed the good quality of the surface morphology, while x-ray diffraction measurements showed several crystallographic orientations of the GaAs layer. The material exhibited a 50 fs carrier lifetime due to growth-induced defects. The response of the photodetectors showed a full width at half maximum of 300 fs. These results demonstrate that the growth of GaAs layers on lattice-mismatched metallic substrates with high thermal conductivity is a promising approach for low-cost and large-area fabrication of electronic and ultrafast photonic devices that require a highly effective thermal drain.

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.