Results 1 - 10 of 781
Results 1 - 10 of 781. Search took: 0.033 seconds
|Sort by: date | relevance|
[en] In nuclear safeguards, precise and accurate isotopic analyses are needed for two major elements from the nuclear fuel cycle: uranium and plutonium. This can be achieved by Isotope Dilution Mass Spectrometry (IDMS), which is one of the most reliable analytical techniques for the determination of plutonium amount content to a high level of accuracy. In order to achieve reliable isotope measurements isotopic reference materials with certified amount of plutonium and isotopic composition are required. At the Institute for Reference Materials and Measurements (IRMM) various plutonium spike reference materials for isotopes 239Pu, 240Pu, 242Pu and 244Pu are available. This enabled the setup of an inter-calibration campaign inter-linking selected plutonium spikes on a metrological basis applying state-of-the-art measurement procedures. The aim of this campaign is threefold: firstly to perform measurements on selected plutonium spike isotopic reference materials for quality control purposes, secondly to verify the amount content and the isotopic composition of the recently produced IRMM-1027m large sized dried (LSD) spikes and thirdly to demonstrate IRMM's measurement capabilities for plutonium analysis via external quality tools. The obtained results using various spike isotopic reference materials will be presented and discussed in this paper. The measurement uncertainties of the IDMS results were calculated according to the guide to the expression of uncertainty in measurement (GUM). (author)
[en] Recent experiments on the erosion of condensed gases and alkali halides by incident ions have renewed interest in the description of thermal spikes. In such spikes a localized, transiently heated region produced by incident radiation may induce activated processes like evaporation of atoms or molecules from a surface. In this paper are presented useful expressions to describe the thermal sputtering for materials having a temperature dependent thermal diffusivity, using a heat capacity and thermal conductivity which vary as C = C0Tsup(n-1) and K = K0Tsup(m-1), respectively, and assigning a width to the initial temperature distribution. (author)
[en] Elongation of metal nanoparticles (NPs) embedded in silica, induced by swift heavy ion irradiation, has received much attention since the discovery. While the mechanism is still under debate, one of the most likely mechanisms is the synergy effect of transient melting of NPs by thermal spike and in-plane stress from silica matrix by the ion hammering. This paper aims at a critical evaluation of this mechanism. As reported by Benyagoub et al., the ion hammering effect is strongly affected by the radiation-induced compaction of the silica matrix at low fluences where the track coverage ratio CR = πR"2ϕ < 1, where R and ϕ denote the track radius and the ion fluence, respectively. A silica film of ∼ 10 μ m thick showed shrinkage at the low fluences (CR < 1) and then turned to in-plane dilation with increasing the fluence (CR > 1)
[en] Many natural signals EEG are comprised frequency overlapping of oscillatory and transient components. In our study the intracranial EEG signals of epilepsy are composed of the superposition of oscillatory signals (HFOs: High Frequency oscillations) and a transient signals (spikes and sharp waves, etc.). The oscillatory components (HFOs) exist in the frequency band 80-500Hz. The transient components comes from nonrhythmic brain activities (spikes, sharp waves and vertex waves of varying amplitude, shape and duration) and cover a continuous wide bandwidth from low to high frequencies and resemble an HFOs events when filtered using a band pass classical filter. The classical filtering methods based on FIR filters, Wavelet transforms and the Matching Pursuit cannot separate the oscillatory from transient activities. This paper describes an approach for decomposing an iEEG signals of epilepsy into the sum of oscillatory components and a transient components based on overcomplete rational dilation wavelet transforms (overcomplete RADWT) in conjunction with morphological component analysis (MCA).
[en] A number of studies have suggested that the irradiation behavior and damage processes occurring during sequential and simultaneous particle irradiations can significantly differ. Currently, there is no definite answer as to why and when such differences are seen. Additionally, the conventional multiparticle irradiation facilities cannot correctly reproduce the complex irradiation scenarios experienced in a number of environments like space and nuclear reactors. Therefore, a better understanding of multiparticle irradiation problems and possible alternatives are needed. This study shows ionization induced thermal spike and defect recovery during sequential and simultaneous ion irradiation of amorphous silica. The simultaneous irradiation scenario is shown to be equivalent to multiple small sequential irradiation scenarios containing latent damage formation and recovery mechanisms. The results highlight the absence of any new damage mechanism and time-space correlation between various damage events during simultaneous irradiation of amorphous silica. This offers a new and convenient way to simulate and understand complex multi-particle irradiation problems. (authors)
[en] One investigated both experimentally and theoretically into spike structure of spontaneous radiation of praseodymium ions in LaF3 matrix at 3P0-3H6 transition frequency under coherent pulse excitation of 3H4-3P0 contact transition. The experimental results are in line with the theoretical calculations based on the analysis of radiation dynamics of the excited group of three-level systems in the mean field approximation. The spike structure generation is interpreted as a cnoidal wave pulse radiation. The recorded random nature of time intervals between spikes reflects sensitivity of cnoidal wave period to fluctuations within the system. One revealed the relation of the elaborated theory with determinate chaotic dynamics models
[ru]Экспериментально и теоретически исследована пичковая структура спонтанного излучения ионов празеодима в матрице LаF3 на частоте перехода3Р0-3Н6 при когерентном импульсном возбуждении смежного перехода 3Н4-3Р0. Экспериментальные результаты хорошо согласуются с теоретическими выводами, основанными на анализе динамики излучения возбужденного ансамбля трехуровневых систем в приближении среднего поля. Генерация пичковой структуры интерпретирована как излучение импульса кноидальной волны. Зарегистрированный случайный характер интервалов времени между пичками отражает чувствительностъ периода кноидальной волны к флуктуациям в системе. Выявлена связь развиваемой теории с моделями детерминированной хаотической динамики
[en] A more quantitative approach to the concept of shock associated processes is proposed, and to show how both orders of magnitude and collision system parameter dependences of measured sputtering yields and disordered atom rates can be predicted with reasonably accuracy. (author)
[en] We use first principles molecular dynamics to investigate unit displacement processes in amorphous silicon oxycarbides (SiOC) and demonstrate that the introduction of hydrogen (H) into these materials enhances their radiation resistance. Our simulations show that 100 eV knock-ons in H-free SiOC redistribute C through the formation of C−O and C−C bonds. H counteracts this trend by passivating dangling O bonds and preventing C from coming out of solution. This effect arises from the exceptionally high mobility of H, which leads to the saturation of radiation-induced O and C radicals even before the thermal-spike induced by a high-energy knock-on dies down. Our work suggests that fully hydrogenated SiOC is a promising radiation-resistant material for future nuclear energy applications.
[en] Complete text of publication follows. Clarke et al. (1976) described a new method based on mass spectrometric measurement of 3He to determine low level tritium concentrations of water samples. The method consists of three major steps: 1) The water samples are put into glass bulbs. The dissolved gases including helium are removed from the water by vacuum pumping. 2) The sample are stored for several months or years so that 3He atoms are produced from tritium decay. 3) The amount of the tritiogenic 3He is measured mass spectrometrically. Since then numerous laboratories adopted this method as noble gas mass spectrometers became commercially available. The measurements are usually calibrated by means of well known air aliquots, which in size can be compared to the helium amount from the tritium sample. The 3He/4He ratio of samples can differ considerably from that of air used for standardization. For this reason it has to be kept in mind that a possible discrimination of 3He by 4He is not necessarily corrected by the air standards. Additionally, the mass spectrometric sensitivities of the different helium isotopes depend on the pressure at the ion source. In case of tritium measurements, the overall pressure of the helium in the mass spectrometer is much lower than that of the helium from the air calibration. We use air aliquots that contain 7 x 10-9 to 2 x 10-7 ccSTP of 4He and 5 x 10-15 to 1.5 x 10-13 ccSTP of 3He. The relative standard deviations of the calibration measurements vary between 1-2%, and the non-linearity effect is always taken into account. The overall helium amount in the mass spectrometer in case of a tritium sample is usually less than 3 x 10-10 ccSTP that derives, of course, from the 4He. As the amount of the calibration sample converges to that of tritium samples, we lose the good statistical error of the 3He measurement being counted by an electro-multiplier. It seems that the large difference of the helium pressure in the ion source between a sample and the calibration makes a different ionization efficiency for the tritiogenic 3He. In our case, even if the air calibration seems to be good, this pressure-dependent effect caused a changing systematic error of 10 to 40% for the standard water samples of known tritium concentration. Within a short period of a few weeks, the systematic difference between the expected and measured values of standard samples of known tritium concentration was quite stable allowing us to use a correction factor, that was usually used to correct the measured values of a sample. To solve this problem, we have decided to increase the helium pressure in the ion source for a tritium sample by adding an ultrapure 4He spike to each tritium sample during the inlet. We built a reservoir of 6230 cm3 that was filled with 4He at about 1 x 10-5 mbar, and CO2 at 1030 mbar. Pipetting aliquots from this spike reservoir we can prepare 4He samples of 7.6 x 10-8 ccSTP that is always added to the tritium samples. Hence the helium pressures are always within the same order of magnitude. This ultrapure 4He was prepared by Yoshiki et al. using a superleak made of compacted aluminium oxide nano powder of 200 to 500 A. Liquid helium of 1.5 K was let through this superleak. The superfluid component, that composed of solely 4He bozons passed easily the superleak while the non-superfluid and gas components were retained. Our measurements confirmed that this helium hardly contained any 3He: the measured 3He/4He isotope ratio is lower than 5 x 10-10, that is extremely perfect for our purpose. Since we use this isotope dilution technique, namely the 4He spiking, the standard measurement became very precise: the measured values are within 2.3 % around the expected values (see Fig). The significance of this new method is beyond the tritium measurements. In all cases of helium measurements where precise helium isotope ratio measurements is needed, but the pressures are different, the 4He isotope dilution can be a way.