Browsing by Subject "IRRADIATION"

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  • Urraca, Ruben; Antonanzas, Javier; Sanz-Garcia, Andres; Javier Martinez-de-Pison, Francisco (2019)
    Different types of measuring errors can increase the uncertainty of solar radiation measurements, but most common quality control (QC) methods do not detect frequent defects such as shading or calibration errors due to their low magnitude. We recently presented a new procedure, the Bias-based Quality Control (BQC), that detects low-magnitude defects by analyzing the stability of the deviations between several independent radiation databases and measurements. In this study, we extend the validation of the BQC by analyzing the quality of all publicly available Spanish radiometric networks measuring global horizontal irradiance (9 networks, 732 stations). Similarly to our previous validation, the BQC found many defects such as shading, soiling, or calibration issues not detected by classical QC methods. The results questioned the quality of SIAR, Euskalmet, MeteoGalica, and SOS Rioja, as all of them presented defects in more than 40% of their stations. Those studies based on these networks should be interpreted cautiously. In contrast, the number of defects was below a 5% in BSRN, AEMET, MeteoNavarra, Meteocat, and SIAR Rioja, though the presence of defects in networks such as AEMET highlights the importance of QC even when using a priori reliable stations.
  • Granberg, Fredric; Byggmästar, Jesper; Nordlund, Kai (2019)
    In order to understand the effect of irradiation on the material properties, we need to look into the atomistic evolution of the system during the recoil event. The nanoscale features formed due to irradiation will ultimately affect the macroscopic properties of the material. The defect production in pristine materials have been subject to investigation previously, but as the dose increases, overlap will start to happen. This effect of cascades overlapping with pre-existing debris has only recently been touched, and mainly been investigated for interstitial-type defects. We focus on vacancy-type defect clusters in BCC Fe and start a recoil event in their near vicinity. The final defect number as well as the transformation of the defect clusters are investigated, and their behaviour is related to the distance between the defect and the cascade centre. We found that for vacancy-type defects, the suppression of defect production is not as strong as previously observed for interstitial-type defects. The cascade-induced transformation, such as change in Burgers vector or creation of dislocations, was determined for all initial defect structures.
  • Byggmästar, J; Granberg, F; Sand, A E; Pirttikoski, A; Alexander, Rebecca; Marinica, M. C.; Nordlund, K (2019)
    Overlap of collision cascades with previously formed defect clusters become increasingly likely at radiation doses typical for materials in nuclear reactors. Using molecular dynamics, we systematically investigate the effects of different pre-existing self-interstitial clusters on the damage produced by an overlapping cascade in bcc iron and tungsten. We find that the number of new Frenkel pairs created in direct overlap with an interstitial cluster is reduced to essentially zero, when the size of the defect cluster is comparable to that of the disordered cascade volume. We develop an analytical model for this reduced defect production as a function of the spatial overlap between a cascade and a defect cluster of a given size. Furthermore, we discuss cascade-induced changes in the morphology of self-interstitial clusters, including transformations between 1/2<111> and <100> dislocation loops in iron and tungsten, and between C15 clusters and dislocation loops in iron. Our results provide crucial new cascade-overlap effects to be taken into account in multi-scale modelling of radiation damage in bcc metals.
  • Poganitsch-Korhonen, M.; Masliukaite, I.; Nurmio, M.; Lahteenmaki, P.; van Wely, M.; van Pelt, A. M. M.; Jahnukainen, K.; Stukenborg, J-B (2017)
  • Hinks, J. A.; Hibberd, F.; Hattar, K.; Ilinov, A.; Bufford, D. C.; Djurabekova, F.; Greaves, G.; Kuronen, A.; Donnelly, S. E.; Nordlund, K. (2018)
    Nanostructures may be exposed to irradiation during their manufacture, their engineering and whilst in-service. The consequences of such bombardment can be vastly different from those seen in the bulk. In this paper, we combine transmission electron microscopy with in situ ion irradiation with complementary computer modelling techniques to explore the physics governing the effects of 1.7 MeV Au ions on gold nanorods. Phenomena surrounding the sputtering and associated morphological changes caused by the ion irradiation have been explored. In both the experiments and the simulations, large variations in the sputter yields from individual nanorods were observed. These sputter yields have been shown to correlate with the strength of channelling directions close to the direction in which the ion beam was incident. Craters decorated by ejecta blankets were found to form due to cluster emission thus explaining the high sputter yields.
  • Pliatsikas, N.; Karabinaki, O.; Zarshenas, M.; Almyras, G. A.; Shtepliuk, I.; Yakimova, R.; Arvanitidis, J.; Christofilos, D.; Sarakinos, K. (2021)
    In the present work, we elucidate the interplay among energetic bombardment effects in magnetron sputtering and defect generation in two-dimensional (2D) materials. Using deposition of gold (Au) layers on single-layer graphene (SLG) as a model system, we study the effect of pressure-distance (pd) product during magnetron sputtering on the pristine SLG properties. Raman spectroscopy, complemented by X-ray photoelectron spectroscopy, shows that for pd = 8.2 Pa center dot cm, Au layer deposition causes defects in the SLG layer, which gradually diminish and eventually disappear with increasing pd to 82.5 Pa center dot cm. Stochastic and deterministic simulations of the sputtering process, the gas-phase transport, and the interaction of sputtered and plasma species with the substrate surface suggest that defects in SLG primarily emanate from ballistic damage caused by backscattered Ar atoms with energies above 100 eV. With increasing pd, and thereby gas-phase scattering, such high energy Ar species become thermalized and hence incapable of causing atomic displacements in the SLG layer. The overall results of our study suggest that control of backscattered Ar energy is a potential path toward enabling magnetron sputtering for fabrication of multifunctional metal contacts in devices founded upon 2D materials.
  • Lu, Eryang; Zhao, Junlei; Makkonen, Ilja; Mizohata, Kenichiro; Li, Zhiming; Hua, Mengyuan; Djurabekova, Flyura; Tuomisto, Filip (2021)
    We present evidence of homogenization of atomic diffusion properties caused by C and N interstitials in an equiatomic single-phase high entropy alloy (FeMnNiCoCr). This phenomenon is manifested by an unexpected interstitial-induced reduction and narrowing of the directly experimentally determined migration barrier distribution of mono-vacancy defects introduced by particle irradiation. Our observation by positron annihilation spectroscopy is explained by state-of-the-art theoretical calculations that predict preferential localization of C/N interstitials in regions rich in Mn and Cr, leading to a narrowing and reduction of the mono-vacancy size distribution in the random alloy. This phenomenon is likely to have a significant impact on the mechanical behavior under irradiation, as the local variations in elemental motion have a profound effect on the solute strengthening in high entropy alloys. (C) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.
  • Zhao, Junlei; Cao, Lu; Palmer, Richard E.; Nordlund, Kai; Djurabekova, Flyura (2017)
    In this paper, we study the mechanisms of growth of Ag nanoclusters in a solid Ar matrix and the emission of these nanoclusters from the matrix by a combination of experimental and theoretical methods. The molecular dynamics simulations show that the cluster growth mechanism can be described as "thermal spike-enhanced clustering" in multiple sequential ion impact events. We further show that experimentally observed large sputtered metal clusters cannot be formed by direct sputtering of Ag mixed in the Ar. Instead, we describe the mechanism of emission of the metal nanocluster that, at first, is formed in the cryogenic matrix due to multiple ion impacts, and then is emitted as a result of the simultaneous effects of interface boiling and spring force. We also develop an analytical model describing this size-dependent cluster emission. The model bridges the atomistic simulations and experimental time and length scales, and allows increasing the controllability of fast generation of nanoclusters in experiments with a high production rate.
  • Kupka, K.; Leino, A. A.; Ren, W.; Vazquez, H.; Åhlgren, E. H.; Nordlund, K.; Tomut, M.; Trautmann, C.; Kluth, P.; Toulemonde, M.; Djurabekova, F. (2018)
    Stable C-C bonds existing in several sp hybridizations place carbon thin films of different structural compositions among the materials most tolerant to radiation damage, for applications in extreme environments. One of such applications, solid state electron stripper foils for heavy-ion accelerators, requires the understanding of the structural changes induced by high-energy ion irradiation. Tolerance of carbon structure to radiation damage, thermal effects and stress waves due to swift heavy ion impacts defines the lifetime and operational efficiency of the foils. In this work, we analyze the consequences of a single swift heavy ion impact on two different amorphous carbon structures by means of molecular dynamic simulations. The structures are constructed by using two different recipes to exclude the correlation of the evolution of sp2-to-sp3 hybridization with the initial condition. Both initial structures contain approximately 60% of sp2-bonded carbon atoms, however, with different degree of clustering of atoms with sp3 hybridization. We simulate the swift heavy ion impact employing an instantaneous inelastic thermal spike model. The analysis of changes in density, bonding content and the number and size of carbon primitive rings reveals graphitization of the material within the ion track, with higher degree of disorder in the core and more order in the outer shell. Simulated track dimensions are comparable to those observed in small angle x-ray scattering measurements of evaporation-deposited amorphous carbon stripper foils irradiated by 1.14 GeV U ions.
  • Hamedani, A.; Byggmästar, J.; Djurabekova, F.; Alahyarizadeh, G.; Ghaderi, R.; Minuchehr, A.; Nordlund, K. (2020)
    We develop a silicon Gaussian approximation machine learning potential suitable for radiation effects, and use it for the first ab initio simulation of primary damage and evolution of collision cascades. The model reliability is confirmed by good reproduction of experimentally measured threshold displacement energies and sputtering yields. We find that clustering and recrystallization of radiation-induced defects, propagation pattern of cascades, and coordination defects in the heat spike phase show striking differences to the widely used analytical potentials. The results reveal that small defect clusters are predominant and show new defect structures such as a vacancy surrounded by three interstitials. Impact statement Quantum-mechanical level of accuracy in simulation of primary damage was achieved by a silicon machine learning potential. The results show quantitative and qualitative differences from the damage predicted by any previous models.
  • Zhang, Yanwen; Sachan, Ritesh; Pakarinen, Olli H.; Chisholm, Matthew F.; Liu, Peng; Xue, Haizhou; Weber, William J. (2015)
    A long-standing objective in materials research is to effectively heal fabrication defects or to remove pre-existing or environmentally induced damage in materials. Silicon carbide (SiC) is a fascinating wide-band gap semiconductor for high-temperature, high-power and high-frequency applications. Its high corrosion and radiation resistance makes it a key refractory/structural material with great potential for extremely harsh radiation environments. Here we show that the energy transferred to the electron system of SiC by energetic ions via inelastic ionization can effectively anneal pre-existing defects and restore the structural order. The threshold determined for this recovery process reveals that it can be activated by 750 and 850 keV Si and C self-ions, respectively. The results conveyed here can contribute to SiC-based device fabrication by providing a room-temperature approach to repair atomic lattice structures, and to SiC performance prediction as either a functional material for device applications or a structural material for high-radiation environments.
  • Jansson, V.; Baibuz, E.; Djurabekova, F. (2016)
    Sharp nanoscale tips on the metal surfaces of electrodes enhance locally applied electric fields. Strongly enhanced electric fields trigger electron field emission and atom evaporation from the apexes of nanotips. Together, these processes may explain electric discharges in the form of small local arcs observed near metal surfaces in the presence of electric fields, even in ultra-high vacuum conditions. In the present work, we investigate the stability of nanoscale tips by means of computer simulations of surface diffusion processes on copper, the main material used in high-voltage electronics. We study the stability and lifetime of thin copper (Cu) surface nanotips at different temperatures in terms of diffusion processes. For this purpose we have developed a surface kinetic Monte Carlo (KMC) model where the jump processes are described by tabulated precalculated energy barriers. We show that tall surface features with high aspect ratios can be fairly stable at room temperature. However, the stability was found to depend strongly on the temperature: 13 nm nanotips with the major axes in the <110 > crystallographic directions were found to flatten down to half of the original height in less than 100 ns at temperatures close to the melting point, whereas no significant change in the height of these nanotips was observed after 10 mu s at room temperature. Moreover, the nanotips built up along the <110 > crystallographic directions were found to be significantly more stable than those oriented in the <100 > or <111 > crystallographic directions. The proposed KMC model has been found to be well-suited for simulating atomic surface processes and was validated against molecular dynamics simulation results via the comparison of the flattening times obtained by both methods. We also note that the KMC simulations were two orders of magnitude computationally faster than the corresponding molecular dynamics calculations.
  • Hooda, Sonu; Avchachov, Konstantin; Khan, S. A.; Djurabekova, Flyura; Satpati, B.; Nordlund, Kai; Bernstorff, Sigrid; Ahlawat, Sarita; Kanjilal, D.; Kabiraj, D. (2017)
    The formation of nanoscale voids in amorphous-germanium (a-Ge), and their size and shape evolution under ultra-fast thermal spikes within an ion track of swift heavy ion, is meticulously expatiated using experimental and theoretical approaches. Two step energetic ion irradiation processes were used to fabricate novel and distinct embedded nanovoids within bulk Ge. The 'bow-tie' shape of voids formed in a single ion track tends to attain a spherical shape as the ion tracks overlap at a fluence of about 1 x 10(12) ions cm(-2). The void assumes a prolate spheroid shape with major axis along the ion trajectory at sufficiently high ion fluences. Small angle x-ray scattering can provide complementary information about the primary stage of void formation hence this technique is applied for monitoring simultaneously their formation and growth dynamics. The results are supported by the investigation of cross-sectional transmission and scanning electron micrographs. The multi-time-scale theoretical approach corroborates the experimental findings and relates the bow-tie shape void formation to density variations as a result of melting and resolidification of Ge within the region of thermal spike generated along an ion track, plus non-isotropic stresses generated towards the end of the thermal spike.
  • Sahi, Helka; Their, Jenny; Gissler, Mika; Koljonen, Virve (2020)
    Merkel cell carcinoma (MCC) is a rare cutaneous carcinoma that has gained enormous interest since the discovery of Merkel cell polyoma virus, which is a causative oncogenic agent in the majority of MCC tumours. Increased research has focused on effective treatment options with immuno-oncology. In this study, we reviewed the real-world data on different treatments given to MCC patients in Finland in 1986-2016. We used the Finnish Cancer Registry database to find MCC patients and the Hospital Discharge Register and the Cause-of-Death Register to obtain treatment data. We identified 376 MCC patients and 33 different treatment entities and/or combinations of treatment. An increase was noted in the incidence of MCC since 2005. Therefore, the cohort was divided into two groups: the "early" group with time of diagnosis between years 1986 and 2004 and the "late" group with time of diagnosis between 2005 and 2016. The multitude of different treatment combinations is a relatively new phenomenon; before the year 2005, only 11 treatments or treatment combinations were used for MCC patients. Our data show that combining radiation therapy with simple excision provided a survival advantage, which was, however, lost after adjustment for stage or age. Our registry study serves as a baseline treatment efficacy comparison as we move into the age of immunotherapy in MCC. Standardizing the treatment of MCC patients in Finland requires more work on awareness and multidisciplinary co-operation.
  • Javanainen, Arto; Muinos, Henrique Vazquez; Nordlund, Kai; Djurabekova, Flyura; Galloway, Kenneth F.; Turowski, Marek; Schrimpf, Ronald D. (2018)
    Heavy ion irradiation increases the leakage current in reverse-biased SiC Schottky diodes. This letter demonstrates, via molecular dynamics simulations, that a combination of bias and ion-deposited energy is required to produce the degradation.
  • Granberg, F.; Byggmastar, J.; Nordlund, K. (2021)
    Tungsten has been chosen as the plasma-facing wall material in fusion reactors, due to its high density and melting point. The wall material will not only be sputtered at the surface, but also damaged deep inside the material by energetic particles. We investigate the high-dose damage production and accumulation by computational means using molecular dynamics. We observe that the choice of interatomic potential drastically affects the evolution. The structure and stability of the obtained defect configurations are validated using a quantum-accurate Gaussian approximation potential. (c) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
  • Henriksson, K. O. E. (2016)
    The number of point defects formed in spherical cementite and Cr23C6 inclusions embedded into ferrite (alpha-iron) has been studied and compared against cascades in pure versions of these materials (only ferrite, Fe3C, or Cr23C6 in a cell). Recoil energies between 100 eV and 3 keV and temperatures between 400 K and 1000 K were used. The overall tendency is that the number of point defects-such as antisites, vacancy and interstitials-increases with recoil energy and temperature. The radial distributions of defects indicate that the interface between inclusions and the host tend to amplify and restrict the defect formation to the inclusions themselves, when compared to cascades in pure ferrite and pure carbide cells. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
  • Penna, Suprasanna; Jain, S. Mohan (2017)
    Agricultural sustainability and food security are major challenges facing continued population growth. Integration of existing and new technologies for the induction and exploitation of genetic diversity towards developing healthier, nutritious and productive crops is the need of the hour. Mutagenesis is a proven technology for the development of improved or novel varieties with desirable traits. Several mutant genes have been successfully explored, either directly or indirectly, to complement crop productivity. The advent of genomics approaches and plant genome sequencing has benefitted mutation discovery and mutant characterization. Plant mutant repositories are being established to serve as platforms for basic and applied research in crop improvement. This review briefly outlines the impact and molecular/genomic characterization of induced mutations in crop improvement.
  • Mota-Santiago, P.; Vazquez, H.; Bierschenk, T.; Kremer, F.; Nadzri, A.; Schauries, D.; Djurabekova, F.; Nordlund, K.; Trautmann, C.; Mudie, S.; Ridgway, M. C.; Kluth, P. (2018)
    The cylindrical nanoscale density variations resulting from the interaction of 185 MeV and 2.2 GeV Au ions with 1.0 mu m thick amorphous SiNx:H and SiOx:H layers are determined using small angle x-ray scattering measurements. The resulting density profiles resembles an under-dense core surrounded by an over-dense shell with a smooth transition between the two regions, consistent with molecular-dynamics simulations. For amorphous SiNx:H, the density variations show a radius of 4.2 nm with a relative density change three times larger than the value determined for amorphous SiOx:H, with a radius of 5.5 nm. Complementary infrared spectroscopy measurements exhibit a damage cross-section comparable to the core dimensions. The morphology of the density variations results from freezing in the local viscous flow arising from the non-uniform temperature profile in the radial direction of the ion path. The concomitant drop in viscosity mediated by the thermal conductivity appears to be the main driving force rather than the presence of a density anomaly.
  • Jin, Xin; Crocombette, Jean-Paul; Djurabekova, Flyura; Zhang, Shuo; Nordlund, Kai; Garrido, Frederico; Debelle, Aurelien (2020)
    As Rutherford backscattering spectrometry in channeling mode (RBS/C) is an efficient technique for characterizing crystallographic defects, its computational simulation has drawn attention over the past several decades. Recently, a RBS/C simulation code based on the binary collision approximation called Rutherford backscattering simulation in arbitrary defective crystals has been suggested and successfully applied to predict the RBS/C spectra from different damaged materials, whose structures were generated in high-dose ion irradiation atomistic simulations. In the present paper, we introduce new developments improving the flexibility of the developed software and its applicability to different types of materials. More precisely, we modified the algorithm describing the slowdown process of backscattered ions, added fitting parameters in the collision partner search routine, modified the routine taking into account target atom thermal vibrations and provided new descriptions of the ion beam divergence. As an example, the effect of the modifications on simulated RBS/C spectra is shown for an <011 & rang;-oriented UO(2)crystal analyzed with a 3.085 MeV He(2+)ion beam. Some of these changes proved necessary to achieve satisfying agreement between simulations and experimental data. Similar observation was made for <001 & rang;-oriented Si and <001 & rang;-oriented GaAs crystals analyzed with a 1.4 MeV He(+)ion beam. In these simulations, the modifications have also resulted in good agreement with experiment.