Jakub Čížek

A high-resolution BaF2 positron-lifetime spectrometer and experience with its long-term exploitation

Abstract A spectrometer for measuring positron lifetimes in solids is described. It consists of a pair of BaF2 scintillators coupled to Philips photomultipliers XP2020Q and a fast-slow coincidence system composed of standard NIM modules. To achieve adequate stability and the maximum performance of the spectrometer, several improvements on commonly used techniques have been introduced. Of these, modified dynode timing based on the utilization of the negative, “anode-like” fast signal seems to be the most important. At a typical rate of 35–40 coincidences/s, the best recorded timing resolution power of the spectrometer for a 22 Na positron source amounted to 139 ps. Although the photomultipliers used display marked effects of ageing on their dynode systems, the timing resolution of the spectrometer after running for more than five years has deteriorated to only 146 ps. This long-term behaviour has been achieved as a result of measures for compensating the influence of the gradual loss of photomultiplier gain. Important details relating to the spectrometer design, as well as to its optimization and testing, are given. Examples of measured positron-lifetime spectra are given to demonstrate the performance of the spectrometer. The rich data relating to the long-term behaviour of the spectrometer are broadly discussed.

TEM and PAS study of neutron irradiated VVER-type RPV steels

Conventional transmission electron microscopy and positron lifetime and Doppler broadening positron annihilation spectroscopy techniques have been used to investigate the radiation-induced microstructural changes in surveillance specimens of VVER-type reactor pressure vessel (RPV) steels, and RPV steels irradiated in the research reactor. Defects visible in transmission electron microscopy consist of black dots, dislocation loops and precipitates concentrated along the dislocation substructure. Their size and density depend on the neutron flux and fluence. The parallel set of thermally aged specimens, specimens recovery annealed after irradiation and specimens irradiated in a lower neutron flux was investigated too. No defects discernible in transmission electron microscopy were found after accelerated irradiation in the research reactor. In addition to visible defects, the small-volume vacancy clusters were identified by positron annihilation spectroscopy.

Origin of green luminescence in hydrothermally grown ZnO single crystals

Combining photoluminescence and positron annihilation studies of hydrothermally grown ZnO crystals with stoichiometry varied by controlled annealing enabled us to clarify the origin of green luminescence. It was found that green luminescence in ZnO has multiple origins and consists of a band at 2.3(1) eV due to recombination of electrons of the conduction band by zinc vacancy acceptors coupled with hydrogen and a band at 2.47(2) eV related to oxygen vacancies. The as-grown ZnO crystals contain zinc vacancies associated with hydrogen and exhibit a green luminescence at 2.3(1) eV. Annealing in Zn vapor removed zinc vacancies and introduced oxygen vacancies. This led to disappearance of the green luminescence band at 2.3(1) eV and appearance of a green emission at higher energy of 2.47(2) eV. Moreover, the color of the crystal was changed from colorless to dark red. In contrast, annealing of the as-grown crystal in Cd vapor did not remove zinc vacancies and did not cause any significant change of green luminescence nor change in coloration.

Positron Lifetime Study of Reactor Pressure Vessel Steels

Positron lifetime (PL) study of several reactor pressure vessel (RPV) steels is presented. VVER-440 type Cr-Mo-V, VVER-1000 type Cr-Mo-Ni-V and A533-B RPV steels were included in the investigation. It was found that positrons annihilate from delocalized state or trapped at dislocations (lifetime 150 ps) in all specimens. In the case of Cr-Mo-V and Cr-Mo-Ni-V RPV steels a small fraction of positrons may be trapped at misfit defects at the vanadium carbide-matrix interface. The conventional trapping model (CTM) failed in the interpretation of measured PL spectra due to the strongly non-uniform spatial distribution of dislocations in the samples. Therefore, the modified trapping model (MTM) was developed in the present work. Contrary to the CTM, the MTM takes into account the non-uniform distribution of dislocations and it provides a self-consistent interpretation of measured PL spectra. The specific trapping rate vD = 0.36 x 10 -4 m 2 s -1 was determined by comparison of the mean dislocation density obtained from PL spectra using the MTM and that measured by TEM. The component arising from positrons trapped at screw dislocations and the component coming from positrons trapped at edge ones were separated in the PL spectra. This approach allowed us to obtain a measure of fraction of screw and edge dislocations in the samples.

Defect studies of ZnO single crystals electrochemically doped with hydrogen

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...

Microstructure and dislocation density evolutions in MgAlZn alloy processed by severe plastic deformation

Commercial MgAlZn alloy AZ31 was processed by hot extrusion and equal channel angular pressing (ECAP) known as EX-ECAP. Microstructure and defect structure evolution with strain due to ECAP were investigated by TEM, positron annihilation spectroscopy (PAS), and X-ray diffraction. Significant grain refinement was obtained by EX-ECAP. In the extruded condition relatively low density of dislocations was determined by PAS. Sharp increase of dislocation density occurred during the first two passes of ECAP, followed by the saturation and even a decline manifesting the dynamic recovery at higher strains. XRD line profile analysis confirmed the results of PAS with slightly higher values of dislocation densities in individual conditions. Detailed analysis of contrast factors allows to determine the type of dislocations and to draw conclusions about slip activation and its variations with strain. The influence of microstructure evolution on mechanical properties is discussed.

Three-detector setup for positron-lifetime spectroscopy of solids containing 60Co radionuclide

Abstract A three-detector BaF 2 positron-lifetime spectrometer is described. The spectrometer is suited for investigations of the neutron-irradiated reactor pressure vessel steels with a high content of 60 Co. Both timing and energy information about annihilation γ-rays is used for selection of coincidence events. In the triple-coincidence mode, the time resolution of around 220 ps FWHM and the coincidence count rate of 15 s −1 for a 4 MBq 22 Na positron source are simultaneously achieved. Test measurements performed have shown that in this mode the 60 Co prompt-peak contribution in the positron-lifetime spectrum is suppressed to be below 1% when 60 Co activity of a pair of specimens studied does not exceed the level of 4 MBq.

Effect of equal channel angular pressing on microstructure, texture, and high-cycle fatigue performance of wrought magnesium alloys

Abstract The magnesium alloys AZ80 und ZK60 received from Dead Sea Magnesium in as-cast conditions were extruded at T = 350 °C using an extrusion ratio of ER = 22. The extruded bars were severely plastically deformed by equal channel angular pressing (ECAP). Multiple ECAP processing up to 8 passes was done. The ECAP-induced changes in grain size and grain size distribution were measured by transmission electron microscopy while changes in dislocation density and crystallographic textures were determined by positron annihilation spectroscopy and X-ray diffraction analysis, respectively. The strain induced by ECAP was found to influence the microstructural characteristics, in particular the grain size, the dislocation density, and the crystallographic texture, which in turn enhance (or deteriorate) the mechanical or fatigue response of both alloys.

Microstructure and Thermal Stability of Ultra Fine Grained Mg-Based Alloys Prepared by High Pressure Torsion

In the present work we studied microstructure of ultra fine grained (UFG) pure Mg and UFG Mg-based alloys. The initial coarse grained samples were deformed by high pressure torsion (HPT) using pressure of 6 GPa. Such deformation leads to formation of UFG structure in the samples. The severe plastic deformation results in creation of high number of lattice defects. Therefore, we used positron annihilation spectroscopy (PAS) for defect characterizations. PAS represents a well developed non-destructive technique with high sensitivity to open volume defects like vacancies, vacancy clusters, dislocations etc. In the present work we combined PAS with TEM and XRD to obtain complete information about microstructure of the UFG samples studied. We have found that microstructure of HPT-deformed Mg contains two kinds of regions: (a) ”deformed” regions with UFG structure (grain size 100-200 nm) and high number of randomly distributed dislocations, and (b) ”recrystallized” regions with low dislocation density and grain size of few microns. It indicates some kind of dynamic recovery of microstructure already during HPT processing. On the other hand, homogenous UFG structure with grain size around 100 nm and high density of homogeneously distributed dislocations was formed in HPT-deformed Mg-9.33 wt.%Gd alloy. After characterization of the as-deformed microstructure the samples were subsequently isochronally annealed and the development of microstructure with increasing temperature and recovery of defects were investigated.

Vacancy clusters in ultra fine grained metals prepared by severe plastic deformation

Severe plastic deformation of metals introduces not only dislocations, but also a high concentration of vacancies. In the present work we employed positron lifetime spectroscopy for investigation of deformation-induced vacancies in ultra fine grained metals prepared by high pressure torsion. It was found that in all metals studied deformation-induced vacancies agglomerate into small vacancy clusters. Experimental positron lifetime results were combined with ab-initio theoretical calculations of positron parameters for vacancy clusters of various sizes. This new approach described in this paper enables to determine the size distribution of vacancy clusters.