Compositional and Structural Modifications by Ion Beam in Graphene Oxide for Radiation Detection Studies.

In the present study, graphene oxide foils 10 µm thick have been irradiated in vacuum using same charge state (one charge state) ions, such as protons, helium and oxygen ions, at the same energies (3 MeV) and fluences (from 5 × 1011 ion/cm2 to 5 × 1014 ion/cm2). The structural changes generated by the ion energy deposition and investigated by X-ray diffraction have suggested the generation of new phases, as reduced GO, GO quantum dots and graphitic nanofibers, carbon nanotubes, amorphous carbon and stacked-cup carbon nanofibers. Further analyses, based on Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis, have indicated a reduction of GO connected to the atomic number of implanted ions. The morphological changes in the ion irradiated GO foils have been monitored by Transmission Electron, Atomic Force and Scanning Electron microscopies. The present study aims to better structurally, compositionally and morphologically characterize the GO foils irradiated by different ions at the same conditions and at very low ion fluencies to validate the use of GO for radiation detection and propose it as a promising dosimeter. It has been observed that GO quantum dots are produced on the GO foil when it is irradiated by proton, helium and oxygen ions and their number increases with the atomic number of beam gaseous ion.

Density functional theory modeling of C-Au chemical bond formation in gold implanted polyethylene.

We have studied processes of gold ion implantation in polyethylene (PE) by theoretical chemistry methods. Car-Parrinello molecular dynamics (CPMD) simulations of collisions and following chemical kinetics considerations lead to the conclusion that chemical bonds between gold atoms and PE chains are formed. We have identified and characterized by a DFT method various stable structures with C-Au, C-Au-C, C-Au-H and C-AuH2 types of chemical bonds. The binding energies (BE) of C-Au bonds are as high as 227 kJ mol-1 and the bond analysis reveals a covalent bonding character. For the experimental detection of these structures in gold implanted PE, we predicted characteristic infra-red (IR) frequencies. The C-Au stretching vibrational modes lie around 500 cm-1. Other characteristic frequencies lie in a band between 730 cm-1 and 1500 cm-1.

Application of Proton Irradiation in the Study of Accelerated Radiation Ageing in a GaAs Semiconductor.

Proton irradiation experiments have been used as a surrogate for studying radiation effects in numerous materials for decades. The abundance and accessibility of proton accelerators make this approach convenient for conducting accelerated radiation ageing studies. However, developing new materials with improved radiation stability requires numerous model materials, test samples, and very effective utilization of the accelerator beam time. Therefore, the question of optimal beam current, or particle flux, is critical and needs to be adequately understood. In this work, we used 5 MeV protons to introduce displacement damage in gallium arsenide samples using a wide range of flux values. Positron annihilation lifetime spectroscopy was used to quantitatively assess the concentration of radiation-induced survived vacancies. The results show that proton fluxes in range between 1011 and 1012 cm-2.s-1 lead to a similar concentration of monovacancies generated in the GaAs semiconductor material, while a further increase in the flux leads to a sharp drop in this concentration.

High-Fluence Multi-Energy Ion Irradiation for Testing of Materials.

Structural materials of the new generation of nuclear reactors, fission as well as fusion, must often cope with high production rates of transmutation helium. Their testing hence requires either a powerful source of fast neutrons or a high-fluence ion-irradiation facility providing sufficient amounts of high-energy helium to investigate its effect on the material. Most ion irradiation studies, however, concentrate on basic effects such as defect evolution or bubble swelling in narrow near-surface regions modified by ion bombardment. Studies on bulk samples with a relatively thick implanted region, which would enable, for instance, micromechanical testing, are underrepresented. This gap might be filled by high-fluence multi-energy ion irradiations modifying several tens of micrometres of the investigated substrate. High-energy ion accelerators providing reasonable currents with energies of tens of MeV are rarely employed in such studies due to their scarcity or considerable beamtime costs. To contribute to this field, this article reports a unique single-beam He implantation experiment aimed at obtaining quasi-uniform displacement damage across >60 µm with the He/dpa ratio roughly one order of magnitude above the typical spallation neutron target irradiation conditions. Some technical aspects of this irradiation experiment, along with recent developments and upgrades at the 6 MV Tandetron accelerator of the Slovak university of technology in Bratislava, are presented.

Bubble Swelling in Ferritic/Martensitic Steels Exposed to Radiation Environment with High Production Rate of Helium.

Reduced-activativon ferritic/martensitic (RAFM) steels are prospective structural materials for fission/fusion nuclear applications because their radiation and swelling resistance outperforms their austenitic counterparts. In radiation environments with a high production rate of helium, such as fusion or spallation applications, these materials suffer from non-negligible swelling due to the inhibited recombination between vacancy and interstitial-type defects. In this work, swelling in helium-implanted Eurofer 97 steel is investigated with a focus on helium production rates in a wide range of helium/dpa ratios. The results show virtually no swelling incubation period preceding a steady-state swelling of about 2 × 10-4%/He-appm/dpa. A saturation of swelling above 5000 He-appm/dpa was observed and attributed to helium bubbles becoming the dominant sinks for new vacancies and helium atoms. Despite a relatively low irradiation temperature (65 ± 5 °C) and a rather high concentration of helium, transmission electron microscope (TEM) results confirmed a microstructure typical of ferritic/martensitic steels exposed to radiation environments with high production rates of helium.

A new approach to near-surface positron annihilation analysis of ion irradiated ferritic alloys.

The present work provides an innovative approach to the near-surface slow-positron-beam (SPB) study of structural materials exposed to ion-beam irradiation. This approach enables the use of variable-energy positron annihilation lifetime spectroscopy (PALS) to characterise a wide range of microstructural damage along the ion implantation profile. In a typical application of the SPB PALS technique, positron lifetime is used to provide qualitative information on the size of vacancy clusters as a function of the positron energy, i.e., the probing depth of the spectrometer. This approach is limited to a certain defect concentration above which the positron lifetime gets saturated. In our experiments, we investigated the back-diffusion of positrons and their annihilation at the surface. The probability of such an event is characterised by the positron diffusion length, and it depends on the density of lattice defects, even in the saturation range of the positron lifetime. Until now, the back-diffusion experiments were reported only in connection with Doppler broadening spectroscopy (DBS) of positron-annihilation radiation. To verify the validity of the used approach, we compared the obtained results on helium-implanted Fe9Cr alloy and its oxide dispersion strengthened variant with the transmission electron microscopy and "conventional" slow positron DBS analysis.

Application of Positron Annihilation Spectroscopy in Accelerator-Based Irradiation Experiments.

Positron annihilation spectroscopy (PAS) is widely recognized as a powerful characterization technique in all types of radiation damage studies in nuclear materials. In the past, fission reactor irradiation of reactor pressure vessel (RPV) steels was a primary aim in most studies, while today's applications of PAS in this field are centered around ion implantation experiments in advanced structural materials. These experiments use hydrogen, helium, heavy ions, and their combination to simulate various radiation environments of future nuclear reactors or nuclear research facilities. The spectrum of ion energies used ranges from a few tens of keV to tens or even hundreds of MeV in proton irradiation or spallation neutron source irradiation experiments. The variety of ion energies, irradiation temperatures, and other experimental conditions poses a major challenge to researchers, who often fail to successfully incorporate the lessons learned from their research. In this paper, we review and supplement recent PAS studies in which structural materials irradiated under a variety of irradiation conditions were investigated using positron annihilation spectroscopy. It summarizes the most important conclusions and lessons learned from the application of PAS in accelerator-based irradiation experiments.