Pulsed Low Energy Positron System Research Articles

Indentation response of model oxide dispersion strengthening alloys after ion irradiation up to 700 °C

This paper presents an experimental investigation of irradiation-induced evolutions in three different oxide dispersion strengthening (ODS) alloys. High-dose, dual beam Ni–He ion irradiations are carried out up to 700 °C. The significant dose-dependent changes in the ODS particle size and number density are documented and interpreted in terms of specific point defect transport mechanisms, from small angle neutron scattering, TEM, and pulsed low-energy positron system measurements combined. The corresponding micro-mechanical changes in the alloys are evaluated based on the indentation response, which is, in turn, interpreted in terms of related, sub-grain plasticity mechanisms. The room temperature tests (without dwell time) reveal that the microscale work-hardening rate increases with decreasing the particle number density and pronounced strain localization effect. The elevated temperature tests (up to 600 °C, with dwell time) show that the indentation creep compliance is mostly temperature-independent after irradiation up to 25 dpa at Tirr = 500 °C and markedly temperature-dependent, after irradiation beyond 40 dpa at Tirr = 600 °C. This effect is ascribed to particular creep mechanisms associated with indent-induced plasticity, i.e., high stress and high dislocation density conditions.

On the Limitations of Positron Annihilation Spectroscopy in the Investigation of Ion-Implanted FeCr Samples

New materials for advanced fission/fusion nuclear facilities must inevitably demonstrate resistance to radiation embrittlement. Thermal and radiation ageing accompanied by stress corrosion cracking are dominant effects that limit the operational condition and safe lifetime of the newest nuclear facilities. To study these phenomena and improve the current understanding of various aspects of radiation embrittlement, ion bombardment experiments are widely used as a surrogate for neutron irradiation. While avoiding the induced activity, typical for neutron-irradiated samples, is a clear benefit of the ion implantation, the shallow near-surface region of the modified materials may be a complication to the post-irradiation examination (PIE). However, microstructural defects induced by ion implantation can be effectively investigated using various spectroscopic techniques, including slow-positron beam spectroscopy. This method, typically represented by techniques of positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy, enables a unique depth-profile characterisation of the near-surface region affected by ion bombardment or corrosion degradation. One of the best slow-positron beam facilities is available at the pulsed low-energy positron system (PLEPS), operated at FRM-II reactor in Munich (Germany). Bulk studies (such as high energy ion implantation or neutron irradiation experiments) can be, on the other hand, effectively performed using radioisotope positron sources. In this paper, we outline some basics of the two approaches and provide some recommendations to improve the validity of the positron annihilation spectroscopy (PAS) data obtained on ion-irradiated samples using a conventional 22Na positron source.

PLEPS study of ions implanted RAFM steels

Current nuclear power plants (NPP) require radiation, heat and mechanical resistance of their structural materials with the ability to stay operational during NPP planned lifetime. Radiation damage much higher, than in the current NPP, is expected in new generations of nuclear power plants, such as Generation IV and fusion reactors. Investigation of perspective structural materials for new generations of nuclear power plants is among others focused on study of reduced activation ferritic/martensitic (RAFM) steels. These steels have good characteristics as reduced activation, good resistance to volume swelling, good radiation, and heat resistance. Our experiments were focused on the study of microstructural changes of binary Fe-Cr alloys with different chromium content after irradiation, experimentally simulated by ion implantations. Fe-Cr alloys were examined, by Pulsed Low Energy Positron System (PLEPS) at FRM II reactor in Garching (Munich), after helium ion implantations at the dose of 0.1 C/cm2. The investigation was focused on the chromium effect and the radiation defects resistivity. In particular, the vacancy type defects (monovacancies, vacancy clusters) have been studied. Based on our previous results achieved by conventional lifetime technique, the decrease of the defects size with increasing content of chromium is expected also for PLEPS measurements.

Development of a new time and position resolving detector for the pulsed low energy positron system PLEPS

The pulsed low energy positron system PLEPS at the Munich research reactor FRM-II is a user facility for depth resolved positron lifetime measurements. Besides positron lifetime measurements 2D-AMOC (Two Dimensional Age Momentum Correlation) experiments are also possible. 2D-AMOC provides in coincidence the lifetime of the positron and the longitudinal momentum distribution of the annihilated electron. It would be of great scientific concern to measure simultaneously the entire 3D-momentum distribution of the electron annihilating with the positron and the corresponding lifetime of the positron (4D-AMOC). To perform 4D-AMOC measurements a time and position resolving detector is required in coincidence with a pixelated Germanium detector. Therefore a time and spatially resolving detector is currently developed at our institute with envisaged time resolution of 100 ps (FWHM) and a spatial resolution of about 2.6 mm (FWHM) over an area of 12 cm2. First test measurements have been carried out with a 25 mm diameter MCP (Micro Channel Plate) image intensifier and with special delay-line anode readout for the spatial information. Up to now 178 ps (FWHM) time resolution and on average 3.4 mm (FWHM) position resolution have been achieved with BaF2 as scintillator material and a 60Co source.

PLEPS study of ion implanted and annealed Fe-11.62%Cr alloys

Reduced activation ferritic/martensitic (RAFM) steels are one of the candidate structural materials for application in future nuclear facilities. These steels fulfill demands on radiation, thermal, and mechanical resistance during their operational lifetime. Our experiments were focused on study of microstructural changes of binary Fe-Cr alloys after annealing and irradiation, experimentally simulated by ion implantations. Alloys with 11.62% Cr were examined after helium ion implantations at different doses (0.1; 0.3; 0.5 C/cm2). Thermal annealing, motivated by literature and our previous work in the field of reactor steels [1,2], was performed at temperatures of 400, 475, 525 and 600 °C after implantations with aim to study changes of the defect size/amount. The Pulsed Low Energy Positron System (PLEPS) at FRM II reactor in Garching (Munich) was applied for lifetime studies [3]. Damage introduced into the microstructure caused problems with defect interpretations by the PLEPS technique and therefore SEM was applied as well. Positron lifetimes measured at annealing temperatures in range of 400 – 525 °C did not show an expected decrease. Only in the case of temperature of 600 °C the mean lifetime (MLT) decreased significantly and assumptions about the defect size decrease were made according to the literature [2].

Geant4 simulation of the effect of backscattered positrons on the lifetime spectra of PLEPS

The Pulsed Low Energy Positron System (PLEPS) allows the measurement of positron lifetime spectra of very high quality with peak-to-background ratios up to 3 · 104. At those peak-to-background ratios small structures appear in the lifetime spectra due to backscattered positrons. Despite their small overall contribution – less than 2 % of the total events in the spectrum even with backscattering coefficient as high as 40 % – those satellite structures can render the data analysis difficult. To understand the origin of those satellite structures and to further improve the performance of the system, comprehensive simulations of the target chamber of PLEPS have been undertaken. The results reproduce fairly well the background of the lifetime spectrum. It is now possible to identify the origin of the background structures and also plan some additional countermeasures.

Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys

Abstract. 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

Abstract 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°C.

The status of the positron beam facility at NEPOMUC

The NEutron induced POsitron source MUniCh NEPOMUC provides a high intensity positron beam with 9·108 moderated positrons per second with a primary beam energy of 1keV. After remoderation, the positron beam is magnetically guided to five experimental setups: a coincident Doppler-broadening spectrometer (CDBS), a positron annihilation induced Auger-electron spectrometer (PAES), a pulsed low-energy positron system (PLEPS) as well as an interface for providing a pulsed beam with further improved brightness. An apparatus for the production of the negatively charged positronium ion Ps− is currently in operation at the open multi-purpose beam port, where additional experiments can be realized. Within this contribution, an overview of the positron beam facility NEPOMUC with its instrumentation at the research reactor FRMII is given.

Microstructural study of high irradiated reactor steels

AbstractPositron Annihilation Spectroscopy (PAS) techniques in combination with other techniques were effectively used in the testing and selection process of optimal reactor steels for use in Generation III and IV reactors or thermonuclear fusion facilities. Conventional PAS lifetime technique and pulsed low energy positron system were applied on wide spectrum of reactor steels together with other techniques viz., Transmis sion Electron Microscopy and Mossbauer Spectroscopy focused on the role of Nickel in the steel microstructure. Experimental experiences in this area collected over the last twenty years were very useful in the actual study by avoiding many mistakes in handling with specimens or in careful interpretation of the results. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC

The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam ( E = 15–1000 eV) of high intensity in the range between 4 × 10 7 and 5 × 10 8 moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps −. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC's status, experimental results and recent developments at the running spectrometers are presented.

Pulsed low energy positron system (PLEPS) at the Munich research reactor FRM II

AbstractCurrently, the Munich pulsed low energy positron beam‐system (PLEPS) is transferred to the high‐intensity positron source (NEPOMUC) at the Munich research Reactor FRM II. We expect count‐rates up to 106 cps. Several improvements to enhance the performance of the system have been implemented. Until now, PLEPS was employed exclusively for lifetime measurements. To enable also Doppler‐broadening, coincident Doppler‐broadening and AMOC experiments, two additional detector ports have been installed. An additional chopper allows to suppress pulses and, therefore, to extend the standard time window of 20 ns for precise measurements of longer lifetimes. The high event‐rate supports the use of smaller scintillators. Consequently an improvement in time resolution is expected. Also envisaged is the use of new detector materials. Various measures to further reduce background from back‐scattered positrons and a reduction of the beam diameter down to 1 mm will improve the overall performance of the system. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation study of alloys for fission and fusion technology

AbstractPositron annihilation spectroscopy (PAS) based on positron lifetime measurements using the conventional positron annihilation lifetime set‐up and the Pulsed Low Energy Positron System (PLEPS) was applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels as well as for investigation of new materials foreseen for Generation‐IV and fusion facilities. PAS results obtained in framework of several scientific projects are summarised and correlated to other techniques. Possibilities for experimental modelling of irradiation effects using ion implantation instead of neutron irradiation are discussed in the paper. The actually running program of micro structural study of Fe–Cr binary alloys is presented in detail. For the next effective study of radiation degradation of fusion as well as fission materials, the experimental micro structural study of model alloys in combination to computer simulations is recommended. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of Mossbauer and positron annihilation spectroscopies for monitoring of WWER RPV steel degradation

This paper describes the application of Mossbauer Spectroscopy (MS) and Positron Annihilation Spectroscopy (PAS) to evaluate the microstructure parameters of materials. These methods are used to collect additional characteristics for understanding of degradation processes in Reactor Pressure Vessel (RPV) steels. Samples from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 reactors, were irradiated near the core at NPP Bohunice (Slovakia) to neutron fluences in the range 7.8 ? 1023 m−2 to 2.5 ? 1024 m−2. Systematic changes in the MS and PAS spectra were observed mainly during the early period of irradiation. These could be due to changes caused by the precipitation of elements such as Cu, P or Cr, mainly in carbides to the surface. The MS results confirm that the close environment of Fe atoms in the body-centred cubic lattice of RPV steels remain almost stable after initial changes and can be correlated with the ductile?brittle transition temperature curve from mechanical tests. Positron annihilation lifetime measurements using the Pulsed Low Energy Positron System (PLEPS) were applied for investigation of defects of irradiated and thermally treated RPV steels. Results showed that the changes in the microstructure of the RPV-steel properties caused by neutron irradiation and postirradiation thermal treatment can be detected in the surface as well as the bulk region.

Electrical properties of MOS structures on nitrogen-doped Czochralski-grown silicon: A positron annihilation study

Measurements of interface trap density, effective generation lifetime (GL) and effective surface generation velocity have been performed using different methods on selected MOS structures prepared on nitrogen-doped Czochralski-grown (NCz) silicon. The application of the positron annihilation technique using a pulsed low energy positron system (PLEPS) focused on the detection of nitrogen-related defects in NCz silicon in the near surface region. In the case of p-type Cz silicon, all the results could be used for the testing of homogeneity. In n-type Cz silicon, positron annihilation was found insensitive to nitrogen doping.

Defects investigation in neutron irradiated reactor steels by positron annihilation

Positron annihilation spectroscopy (PAS) based on positron lifetime measurements using the pulsed low energy positron system (PLEPS) were applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in microstructure of the RPV-steel properties caused by neutron irradiation and post-irradiation heat treatment can be well detected. From the lifetime measurements in the near-surface region (20–550 nm) the defect density in Russian types of RPV-steels was calculated using the diffusion-trapping model. The post-irradiation heat treatment studies performed on non-irradiated specimens are also presented.

Czochralski-grown nitrogen-doped silicon: Electrical properties of MOS structures; A positron annihilation study

Czochralski-grown nitrogen-doped (NCZ) silicon was studied using different methods. Measurements of interface traps density, effective generation lifetime and effective surface generation velocity were performed on selected Metal-Oxide-Semiconductor (MOS) structures. Application of the positron annihilation technique (PAS)—pulsed low energy positron system (PLEPS)—was focused on the detection of nitrogen-related defects in NCZ silicon in the near surface region. PAS—PLEPS technique gave relevant results on p-type NCZ silicon. Low sensitivity in the application to n-type NCZ silicon discriminates the PAS—PLEPS technique and should be alternated by other experimental technique. On the other hand, more pertinent measurement of generation lifetime was performed on MOS structures with n-type Si. Although the generation lifetime decreases in NCZ silicon, considerable lateral homogenization of the relaxation time was observed on the wafer.

Positron annihilation Investigations of defects in copper alloys selected for nuclear fusion technology

A positron annihilation spectroscopy (PAS) based on positron lifetime measurements, using the pulsed low energy positron system (PLEPS), is used to investigate defects create by hydrogen implantation and thermal treat of copper alloys. These alloys are designated for the use in the international thermonuclear experimental reactor (ITER). The results show that the changes in the microstructure of selected copper alloys (CuCrZr, CuAl25) depend strongly on the preparing technology of alloys as well as on the implantation dose experimentally simulating the neutron treatment. The full recovering of the structure after isochronal annealing in vacuum is observed in all implanted specimens at the temperature of 450 °C. Using the PLEPS technique, for the first time, depth profiling of the positron lifetime spectra in the near-surface region (20–500 nm) of hydrogen implanted copper alloys was performed and compared with the TRIM calculations and transmission electron microscopy (TEM) studies. Possible annihilation channels in CuCrZr and CuAl25 materials are discussed in details together with corresponding annihilation characteristics determined theoretically and using computer simulations. We discuss the results of positron lifetime measurements of the irradiated and non-irradiated CuZrCr and CuAl25 specimens. We identified the most probable types of positron trapping sites. Finally, the results are discussed in terms of microstructural changes of the studied materials upon irradiation and subsequent heat treatment.

Investigation of Defects in Copper Alloys Selected for Nuclear Fusion Technology

Abstract. Positron lifetime measurements using a Pulsed Low Energy Positron System (PLEPS) were applied to the investigation of defects in hydrogen implanted and subsequently thermally treated copper alloys which are designated for the use in the International Thermonuclear Experimental Reactor (ITER). PLEPS results showed that the changes in the microstructure of selected copper-alloys (CuCrZr, CuAl25) depend strongly on the preparing technology of alloys and on the implantation dose. The full recovery of the structure after isochronal annealing in vacuum in a region of 100-600 ”C was observed in all implanted specimens at a level of about 450 ”C. With the PLEPS technique, for the first time, depth profiling of the near-surface region (20-500 nm) of hydrogen implanted copper alloys was performed and compared with the transmission electron microscopy (TEM) studies. Introduction The precipitation hardened alloy (CuCrZr) and the dispersion strengthened alloy (CuAl25) are studied here by positron annihilation spectroscopy (PAS). These Cu-alloys should be applied in the ITER as a cooler and should diffuse the heat [1]. Due to the particle bombardment of ITER first-wall materials (selected copper alloys) various changes in mechanical properties can be induced. Investigation of defects created in this way is an interesting issue for solid-state science as well as for applied research. For the simulation of the radiation damage due to a neutron flux, the ion implantation of protons has been applied. The protons were chosen because they have approximately the same mass as that of neutrons. Although the ballistic influence of protons on the primary-knock-on atom (PKA) production is different from that of D-T fusion reactor conditions, nevertheless it is instructive to investigate fundamental displacement damage effects using proton irradiation.

Hydrogen implantation effect in copper alloys selected for ITER investigated by positron annihilation spectroscopy

Defects in the form of vacancies (loops, voids, etc) created by hydrogen implantation into selected Cu alloys foreseen for the International Thermonuclear Experimental Reactor (ITER) first wall cladding were studied using positron annihilation spectroscopy. The pulsed low-energy positron system, which enables depth profiling of the positron lifetime spectra in the near-surface region (20–460 nm) of hydrogen-implanted copper alloys, was applied, and its results were compared with TRIM calculations and transmission electron microscopy studies. The selected specimens were implanted in the Ion Beam Laboratory of FEI STU Bratislava. The energy of implantation was EH = 2 × 95 keV for the molecular ion beam. The temperature during this process was lower than 90°C. Two implantation doses were chosen for both the alloys: 1.3 × 1019 ions cm−2 (1.1 C cm−2) and 5 × 1018 ions cm−2 (0.4 C cm−2). Although the influences of neutrons with energy 14 MeV and protons with energy 95 keV are not the same (differences in energy and existence of proton charge), experimental simulation of radiation damage of ITER construction materials was successfully performed. The results are discussed in terms of microstructural changes of the studied materials upon irradiation. The CuAl25 alloy seems to be more resistant to proton bombardment than CuCrZr.