V. Krsjak

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Papers by V. Krsjak

Semi-insulating GaAs detectors degraded by 8 MeV electrons up to 1500 kGy

Journal of Instrumentation, 2021

Radiation degradation of semi-insulating GaAs detectors by 8 MeV electrons up to doses of 1500 kG... more Radiation degradation of semi-insulating GaAs detectors by 8 MeV electrons up to doses of 1500 kGy is studied in this paper. The influence of irradiation on GaAs material parameters and on spectrometric and electrical properties of fabricated detectors is evaluated. The detector material was degraded before contact preparation, which ensured separation of radiation degradation solely to the bulk material, excluding the contact degradation. The positron annihilation lifetime spectroscopy (PALS) was involved to characterize the substrate material together with galvanometric measurements. Radiation-induced mono-vacancies were clearly identified by PALS in the irradiated materials with increasing concentration up to 2.8 × 1016 cm−3 at maximal applied dose. In correlation with defect concentration the electron Hall mobility decreased with dose down to 3270 cm2 V−1 s−1 and resistivity increased up to 5.22 × 108 Ω cm at 1500 kGy. The bulk material properties influenced the parameters of fa...

28 P 12-3 Irradiation Defects of F 82 H Irradiated at SINQ Using Positron Annihilation Spectroscopy

Irradiation facilities of high-energy particles for neutrons (Material Controlled irradiation Fac... more Irradiation facilities of high-energy particles for neutrons (Material Controlled irradiation Facility), ions (e.g., Heavy ion irradiation facility) and electrons (Low temperature irradiation facility, KUR-LINAC) have been extensively developed at the Research Reactor Institute. The developed facilities have been in operation and opened for joint research projects. One of the objectives of this project is to improve or optimize irradiation facilities for more accurate irradiation experiments. As characterization techniques for irradiated materials, a slow positron-beam system and a focused ion beam system have been developed and introduced, respectively, in addition to previous characterization facilities such as an electron microscope, an electron-spin-resonance spectrometer, a bulk positron annihilation spectrometer and a thermal desorption spectrometer. Another objective is to introduce new techniques or reconsider analytical methods of previously used characterization techniques. Based on these two objectives, we expect promotion of previous research and attraction of new users for the joint research program. The allotted research subject (ARS) and individual co-researchers are listed below. ARS-1: Study on efficient use of positron moderation materials (A. Kinomura et al.) ARS-2: Effects of electron-irradiation on Cu solubility and diffusivity in Fe studied by three dimensional atom probe (K. Nagumo et al.) ARS-3: Irradiation defects of F82H irradiated at SINQ using positron annihilation spectroscopy (K. Sato et al.) ARS-4: Study on irradiation effect and electrical and optical properties of compound semiconductors (K. Kuriyama et al.) ARS-5: Validation of D3×t0.5 TEM disk size miniature test specimens for post-irradiation thermal diffusivity measurement (M. Akiyoshi and A. Kinomura) ARS-6: Positron annihilation study on Fe-Cr binary alloy after electron irradiation (T. Onitsuka et al.) ARS-7: Positron annihilation study on free-spaces in DLC and heteroelement-containing DLC films (K. Kanda et al.) ARS-8 Thermal stability of diamond-like carbon films (S. Nakao et al.

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Microstructural changes of ferritic/martensitic steels after irradiation in spallation target environments

Journal of Nuclear Materials, 2018

changes of ferritic/martensitic steels after irradiation in spallation target environments,

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Microstructural properties of unirradiated RPV model steels revealed by Doppler broadening spectroscopy

APPLIED PHYSICS OF CONDENSED MATTER (APCOM 2019), 2019

The so-called "RPV Model Steels", represented by 12 ferritic steels with the parametric variation... more The so-called "RPV Model Steels", represented by 12 ferritic steels with the parametric variation of alloying elements were developed at EC-JRC Petten (the Netherlands). Their composition was derived from compositions typical for WWER-1000 and PWR RPW materials. In order to understand the role and influence of certain alloying elements and impurities on the behavior of steels during operation of NPP, the set of RPV Model Steels was irradiated in the High Flux Reactor-LYRA irradiation facility (Petten, the Netherlands) up to a neutron fluence of about 2.5×10 19 n.cm-2. In this paper, we present the detailed analyses of DBS (Doppler Broadening spectroscopy) results obtained on an unirradiated set of RPV model steels.

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Sodium-22 source contribution determination in positron annihilation measurements using GEANT4

APPLIED PHYSICS OF CONDENSED MATTER (APCOM 2019), 2019

Positron annihilation spectroscopy (PAS) is a strong material inspection technique, which provide... more Positron annihilation spectroscopy (PAS) is a strong material inspection technique, which provides information not only from the surface, but also from the bulk of the material. Most facilities use unmoderated sodium-22 positron source for purposes of various PAS measurements. Getting precise information from measurements does not depend only on the measured spectrum but the characterization of source and sample is equally important when evaluating PAS data. Source contribution calculation is necessary not only with regards to the evaluation of experimental data but also for reproducibility of the measurement. In this article, a summary of simulations on Kapton influence on positron spectrum is discussed. Simulations have been realized using GEANT4 simulation kit.

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Effects of displacement damage and helium production rates on the nucleation and growth of helium bubbles – Positron annihilation spectroscopy aspects

Journal of Nuclear Materials, 2018

Effects of displacement damage and helium production rates on the nucleation and growth of helium... more

format_quoteCritical radius for growing helium bubbles estimated at 2000-5000 He appm, indicating important insights into defect transformations.format_quote

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ICONE15-10394 Structural Materials for Fusion Technologies Studied by Non Destructive Techniques

The Proceedings of the International Conference on Nuclear Engineering (ICONE), 2007

Non-destructive method, positron annihilation lifetime spectroscopy (PALS) was applied as an eval... more Non-destructive method, positron annihilation lifetime spectroscopy (PALS) was applied as an evaluation tool for microstructure study of the four different Fe-Cr alloys. This paper describes ion implantation technique as a tool for creating microstructure defect in structural materials, similar to radiation damage in nuclear facilities. In present state of the research non implanted specimens were measured using PALS and TEM. The results show the dependency of positron lifetime in defects on the grain size. Measurements also confirmed presence of dislocations, typical for tempered ferritic/martensitic steels, which were the results of thermal treatment in manufacturing of the specimens.

format_quotePALS findings reveal a strong dependency on microstructure for positron lifetime, contrasting with other materials.format_quote

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Model Materials for Reactor Pressure Vessel

Acta Physica Polonica A, 2010

In this paper we report preliminary results from a study focused on investigation of the role of ... more In this paper we report preliminary results from a study focused on investigation of the role of elements as Cr, Ni, Mn and Si in the radiation stability of reactor pressure vessel steels. Twelve model ferritic steels and eight model welds with basic composition derived from Russian VVER-1000 and Western PWR reactor pressure vessel materials were studied by magnetic Barkhausen noise and Charpy-V impact measurements. Comparison of these two model materials led to the conclusion that the ductile-brittle transition temperatures and Barkhausen noise RMS values of model steels are shifted to the lower values as those of model welds. The VVER-1000 as reference point is shifted to lower RMS values in comparison with model steels and model welds. The next stage of the assessment will involve the investigation of the radiation effect on the model steels with accumulated neutron fluence of 10 19 n/cm 2 .

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Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys

Materials Science Forum, 2013

. Binary Fe-11.62wt%Cr alloys were investigated in as-received state as well as after a two step ... more . Binary Fe-11.62wt%Cr alloys were investigated in as-received state as well as after a two step helium ion implantation at different energies (100 keV and 250 keV) with doses up to 3.12×1018 cm-2. In order to study changes in alloys in dependence on the temperature, thermal annealing was performed at temperatures of 400, 475, 525 and 600 °C and specimens were afterwards measured by a pulsed low energy positron system (PLEPS). Annealing out of defects at lower temperatures was not as significant as expected, and we also encountered difficulties with defect identification. However, an apparent decrease of defect size was observed in the specimen annealed at a temperature of 600 °C.

Different Chromium Content and Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys

Physics Procedia, 2012

Reduced activation ferritic/martensitic steels (RAFM) represented by binary Fe-Cr alloys, with di... more Reduced activation ferritic/martensitic steels (RAFM) represented by binary Fe-Cr alloys, with different chromium content, were studied in as-received state as well as after helium ions implantation. In order to study changes in dependence on the temperature, thermal annealing of He ions implanted Fe-11.62%Cr specimens was performed. Measurements by Pulsed Low Energy Positron System (PLEPS) in Garching, Germany were performed afterwards. Annealing out of defects at lower temperatures was not significant as was expected and some uncertainties are present. Extensive decrease of positron lifetime of defects was observed in specimens annealed at temperature 600 ºC.

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Vacancy Type Defects in Oxide Dispersion Strengthened Steels

Materials Science Forum, 2013

This study was focused on commercial oxide-dispersion strengthened (ODS) steels - MA 956 (20%Cr),... more This study was focused on commercial oxide-dispersion strengthened (ODS) steels - MA 956 (20%Cr), PM 2000 (19%Cr), ODM 751 (16%Cr) and MA 957 (14%Cr) developed for fuel cladding of GEN IV reactors. The ODS steels are described in order to comparison of their microstructure features. Vacancy defects were observed by Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). Residual stress proportional to all kinds of defects was investigated by Magnetic Barkhausen Noise (MBN) measurement. The highest presence of open volume defects was found in MA 956 and the lowest defect concentration in MA 957, although this steel contains the largest defects (six-vacancies together with dislocations). Other investigated steels demonstrated probably three- or four-vacancy clusters. Further, results from positron technique indicated proportionality of chromium content to defect concentration. Magnetic Barkhausen noise results also showed that Hpeak value (describ...

Microstructural Evolution of Cr-Rich ODS Steels as a Function of Heat Treatment at 475 °C

In the current research, the effect of heat treatment on the morphology of the dispersoids and th... more In the current research, the effect of heat treatment on the morphology of the dispersoids and their phase composition were investigated in three Cr-rich ferritic oxide dispersion strengthened (ODS) steels: PM2000, MA956, and ODM751. The steels were aged at 475°C for times ranging from 100 to 1,000 h. The microstructure was characterized using transmission electron microscopy. Study of the as-recrystallized samples revealed nano-scale Y-Al-O complex-oxide particles dispersed in the ferritic matrix. These dispersoids, which differ in size (10-160 nm) and geometry (polygonal and spherical), were identified as Y 4 Al 2 O 9 , YAlO 3 , and Y 3 Al 5 O 12. After heat treatment, a significant change in the morphology, size, and distribution of the dispersoids was observed. Changes in the phase composition of the oxide dispersoids were also observed: YAlO 3 (with perovskite structure) was identified as the most dominant phase, indicating that it is probably the most stable phase in the Cr-rich ferritic ODS steels.

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Coincidence Doppler broadening study of Eurofer 97 irradiated in spallation environment

Journal of Nuclear Materials, 2015

The behaviour of transmutation helium during isochronal annealing of irradiated Eurofer 97 was in... more The behaviour of transmutation helium during isochronal annealing of irradiated Eurofer 97 was investigated using coincidence Doppler broadening spectroscopy (CDBS). The investigated ferritic martensitic steel was irradiated in 2000 and 2001 in the frame of the STIP-II project at the Swiss neutron spallation source (SINQ) (irradiation with neutrons and protons) at the Paul Scherrer Institute (PSI). During isochronal annealing experiment, coarsening of vacancy clusters and/or growth of helium bubbles was observed at T  500 °C. This process causes an increase of low-momentum annihilation events and related increase of the S parameter during thermal treatment of material. On the other hand, the maximum concentration of helium in small vacancy clusters (V n) was observed after annealing at 400 °C, where an excellent correlation with the calculated CDBS profiles of V n + He m clusters was found.

format_quoteAfter 16 months of irradiation, helium concentration reached 565 appm and hydrogen concentration 2240 appm, impacting material properties significantly.format_quote

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PALS investigation of chromium effect in ferritic/martensitic steels implanted with helium

physica status solidi (c), 2009

... PACS 61.66.Dk, 61.72.jd, 61.72.up, 61.80.Hg, 78.70.Bj * Corresponding author: e-mailstanislav... more

Microstructural study of He-implanted Fe-Cr alloys with the use of conventional lifetime technique and pulsed low energy positron beam

physica status solidi (c), 2009

... of conventional lifetime technique and pulsed low energy positron beam Vladimír Kršjak*, 1, V... more

Application of fast‐digitizer card Acqiris DP‐240 in positron lifetime spectroscopy

physica status solidi c, 2009

ABSTRACT Based on the available knowledge base in positron annihilation measurements, new softwar... more ABSTRACT Based on the available knowledge base in positron annihilation measurements, new software for digital positron lifetime spectrometry has been designed within the frame of joint activity between STU Bratislava and JRC Petten. The tests of the new positron digital-life time setup have been carried out. Detectors with BaF2 scintillators and fast-dynode outputs were chosen for such studies. As the digitiser unit, the card Acqiris DP240 was fully tested. Originally, start and stop timing signals were sampled to digital waveforms separately at a rate of 1 GS/s in 8 bit resolution. However, due to limited quality of collected waveforms which significantly affected post-processing of data, the joined delayed channel mode with 2 GS/s sampling rate was applied for further studies. The full setup was benchmarked by a series of calibration measurements of two high purity materials, specifically Si and Ni. The inter-comparison of conventional analogue- and digital-based processing are discussed in detail. Further, the optimisation of main parameters for waveform-processing are analysed too. Such a new approach in the digital life-time spectroscopy will help to improve the resolution of existing methodology which might enhance the capabilities of this technique for further applications, especially in materials science. (© 2009 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

NDT study of oxide dispersion strengthened steels

Journal of Physics: Conference Series, 2011

Various oxide dispersion strengthened (ODS) steels (PM2000, MA956, ODM751 and ODS Eurofer) have b... more Various oxide dispersion strengthened (ODS) steels (PM2000, MA956, ODM751 and ODS Eurofer) have been investigated using positron annihilation lifetime spectroscopy. Preliminary characterization of the vacancy type defects and the yttria nanoparticles are reported in this paper. The interpretation of the experimental data considers also the results of magnetic Barkhausen noise (BN) measurements as well as SEM and TEM investigations. Significant differences due to the presence of the yttria oxides (namely Y2O3) were observed in the studied materials in comparison to conventional ferritic/martensitic steels (e.g. Eurofer). Higher positron mean lifetime in ODS steels is, however, not only due to the presence of dispersoids, but also a result of vacancy agglomeration (clusters of 6-8 vacancies) which have been confirmed in recrystallised ODM751 and MA956 materials. On the other hand, positron trapping at dislocations was observed in the as-extruded ODS Eurofer in contrast to the other, recrystallised, materials where the high temperature treatment had led to the static recovery of these defects. It is suggested that some defects which are present (dislocations, vacancy clusters and dispersoids) and affect positron trapping in the materials are also important pinning sites for the magnetic domains. This is reflected by a shift of the signal peak in the BN spectra. The present complementary study provides more comprehensive information about materials microstructure and can support the interpretation of the physical/mechanical testing results (hardness, fracture mechanics etc.) obtained on these materials.

Ion implantation induced defects in Fe-Cr alloys studied by conventional positron annihilation lifetime spectroscopy

Journal of Physics: Conference Series, 2011

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Fe–Cr alloys behavior after helium implantation

Journal of Nuclear Materials, 2011

... [5] SI Porollo, AM Dvoriashin, AN Vorobyev and Yu. V. Konobeev. J. Nucl. Mater., 256 (1998), ... more

SANS investigation of microstructure evolution in high chromium ODS steels after thermal ageing

Journal of Nuclear Materials, 2013

ABSTRACT In the present paper the small-angle neutron scattering (SANS) technique was used to inv... more ABSTRACT In the present paper the small-angle neutron scattering (SANS) technique was used to investigate the phase separation in two oxide dispersion strengthened (ODS) ferritic steels, PM2000 and MA956, during isothermal ageing at 475 °C. The ferritic matrix of the investigated materials separates into Fe-rich α and Cr-rich α′ phases during ageing at this temperature. The samples were measured under magnetic field in order to separate the nuclear and magnetic scattering intensities. The ratio of these two contributions was used to gain information on the chemical composition of the scatterers. The SANS measurements revealed the formation and subsequent growth of Cr-rich precipitates on the nanometer scale. The size distributions and the volume fractions of the precipitates were evaluated from the scattering data. Microhardness was found to significantly increase as a function of ageing time for both materials. The hardening is related to the formation of the Cr-rich α′ phase.

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