Positron annihilation lifetime spectroscopy at a superconducting electron accelerator

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA. The electron beam is employed for production of several secondary beams including X-rays from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed in parallel. The adjustable repetition rate of the continuous-wave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films

Introducing artificial length scales to tailor magnetic properties

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale

Positron annihilation lifetime spectroscopy at a superconducting electron accelerator

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA. The electron beam is employed for production of several secondary beams including X-rays from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed in parallel. The adjustable repetition rate of the continuous-wave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films

Positron annihilation lifetime spectroscopy at a superconducting electron accelerator

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA. The electron beam is employed for production of several secondary beams including X-rays from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed in parallel. The adjustable repetition rate of the continuous-wave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films

Vývoj radiačního poškození v čistém W a slitině W-Cr-Hf způsobené 5 MeV Au ionty v širokém rozsahu dpa

Čistý W a slitina W-Cr-Hf, které jsou perspektivními materiály pro jaderné fúzní reaktory, jako je DEMO, byly ozářeny při pokojové teplotě 5 MeV ionty Au2+ s dávkami mezi 4 × 10e14 a 1,3 × 10e16 ionty.cm-2 za účelem vytvoření různých úrovní poškození mřížky od jednotek až po desítky dpa. Odlišný charakter akumulace radiačního poškození, mikrostruktura a povaha defektů byly pozorovány u čistého W i slitin W-Cr-Hf, přičemž tyto slitiny vykazovaly zajímavou schopnost reorganizace poškození a snížení velikosti defektu při vyšších dávkách iontů, jak bylo stanoveno pozitronovou anihilační spektroskopií. (PAS). Vysoká míra radiačního poškození v ozařované vrstvě byla prokázána ve vzorcích W již při nižších dávkách Au-iontů ve srovnání se vzorky W-Cr-Hf, kde se poškození stupňovitě zvyšovalo s rostoucí dávkou Auiontů. Zřetelná akumulace defektů byla doprovázena odlišnou distribucí implantovaných Au-iontů v ozařované vrstvě stanovenou sekundární iontovou hmotnostní spektrometrií (SIMS), stejně jako tepelné vlastnosti ukázaly následné zhoršení hloubky v dobré shodě s Au koncentračními hloubkovými profily. TEM potvrdila výše uvedená zjištění, kde podpovrchová vrstva vykazovala po ozáření uvolnění defektů, maximum hustoty dislokační smyčky bylo identifikováno v hloubce podle predikovaného maxima dpa (posun částic na atom) pro nižší dávku Au-iontů. Kromě toho TEM ukazuje, že struktura pásu hustoty dislokací se objevila ve vzorcích W-Cr-Hf vykazujících pásmo defektů s vysokou hustotou podle projektovaného rozsahu Au-iontů současně s další vrstvou s většími izolovanými dislokacemi vyjádřenými ve větší hloubce jako rostoucí funkce dávky Au-iontů. Takový jev nebyl u W vzorků pozorován.Pure W and W-Cr-Hf alloy which are prospective materials for nuclear fusion reactors, such as DEMO, were irradiated at room temperature with 5 MeV Au2+ ions with fluences between 4 × 10e14 and 1.3 × 10e16 ions.cm-2 to generate various levels of lattice damage from about units up to tens of dpa. The distinct character of radiation damage accumulation, microstructure and defect nature have been observed in both pure W and W-Cr-Hf alloys, the latter exhibited interesting ability of damage reorganisation and defect size decrease at the higher ion fluences as determined by positron annihilation spectroscopy (PAS). High radiation damage rate in the irradiated layer has been evidenced in the W samples already at the lower Au-ion fluences compared to W-Cr-Hf samples, where the damage increased in steps with the increasing Au-ion fluence. The distinct defect accumulation was accompanied with the different Au-ion implanted distribution in the irradiated layer determined by Secondary Ion Mass Spectrometry (SIMS) as well as the thermal properties have shown the consequent worsening in the depth in good agreement with the Au-depth concentration profiles. TEM corroborated above mentioned findings, where the sub-surface layer exhibited defect release after the irradiation, the maximum of dislocation loop density has been identified in the depth according the predicted dpa (displacement particles per atom) maximum for the lower Au-ion fluences. Moreover, TEM shows the dislocation density band structure appeared in W-Cr-Hf samples exhibiting the high density defect band according the projected range of the Au-ions simultaneously with the additional layer with larger isolated dislocations pronounced in the higher depth as a growing function of Au-ion fluence. Such phenomenon was not observed in W samples

Radiation damage evolution in High Entropy Alloys (HEAs) caused by 3–5 MeV Au and 5 MeV Cu ions in a broad range of dpa in connection to mechanical properties and internal morphology

High Entropy Alloys (HEAs) are prospective materials for nuclear fusion reactors and were irradiated in this study at a broad range of energetic ion fluences. Different ion masses (Cu and Au ions) and energies (3 and 5 MeV) were selected to investigate dpa (displacement per atom) development, radiation defect accumulation based on prevailing collision processes (Au ions) and ionization processes (Cu ions) in various HEAs. The studied HEAs differ in terms of elemental composition, internal morphology (grain structure) and other modifiers. Dpa values of 1 to ∼66 were achieved at Cu and Au ion fluences from 4 × 1014 to 1.3 × 1016 ions.cm−2 at room temperature, which generated varying levels of lattice damage. Theoretical simulations were performed to estimate the energy stopping and dpa depth distribution using SRIM code and compared with Au-concentration depth profiles determined by Rutherford backscattering spectrometry for Au-ions with 3 MeV ion energy. The prevailing energy losses of ions via ionization processes for Cu-5 MeV ions were found to increase the damage through lattice strain and probable lattice distortion, although the main defect introduction is expected to occur via collisions during nuclear stopping. Structural modification and defect accumulation were investigated by positron annihilation spectroscopy (PAS), which revealed a broader damaged layer with defects, where HEA-Nb (NbCrFeMnNi) exhibited the least damage accumulation from chosen alloys with no strong relation to the Au-5 MeV ion implantation fluence, whereas strong defect accumulation was recorded in the Au-ion implanted Eurofer97 used for comparison and HEA-Co (CoCrFeMnNi). PAS analysis also allowed defect sizes to be determined as an additional structural characteristic. The observed trends were also confirmed by thermal property analysis, with a worsening of thermal effusivity recorded after the irradiation in HEA-Co and Eurofer97. The worsening of the thermal properties was confirmed by the layer thickness, where the layer identified by PAS was found to be broader than the SRIM theoretical predictions. Nanoindentation measurements confirmed less pronounced radiation hardening of HEA-Nb relative to that observed in HEA-Co and Eurofer97. Transmission Electron Microscopy (TEM) analysis revealed layer thicknesses in reasonable agreement with the dpa depth profiles. The thermal effusivity decreased in the surface-irradiated layer in all investigated samples, the least influenced material was HEA-Nb