Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 2012"

The Nanoscale Ordered MAterials Diffractometer NOMAD at the Spallation Neutron Source SNS

Abstract: The Nanoscale Ordered MAterials Diffractometer (NOMAD) is neutron time-of-flight diffractometer designed to determine pair distribution functions of a wide range of materials ranging from short range ordered liquids to long range ordered crystals. Due to a large neutron flux provided by the Spallation Neutron Source SNS and a large detector coverage neutron count-rates exceed comparable instruments by one to two orders of magnitude. This is achieved while maintaining a relatively high momentum transfer resolution of a δ Q / Q ∼ 0.8 % FWHM (typical), and a possible δ Q / Q of 0.24 % FWHM (best). The real space resolution is related to the maximum momentum transfer; a maximum momentum transfer of 50 A−1 can be obtained routinely and the maximum momentum transfer given by the detector configuration and the incident neutron spectrum is 125 A - 1 . High stability of the source and the detector allow small contrast isotope experiments to be performed. A detailed description of the instrument is given and the results of experiments with standard samples are discussed.

Nanometric transformation of the matter by short and intense electronic excitation: Experimental data versus inelastic thermal spike model

Abstract: Experimental investigations of ion tracks and sputtering phenomena with energetic heavy projectiles in the electronic energy loss regime are re-examined in metallic and insulating materials. An overview of track data such as the velocity dependence of the track size and the critical electronic energy loss for track formation is presented. Different physical characterizations of the material transformation are listed in order to deduce a track size which is independent of the observations. It will point out the differences of damage creation by electronic energy loss compared to nuclear energy loss. In the second part, we present a theoretical description of track formation based on the inelastic thermal spike model. This thermodynamic approach combines the initial size of the energy deposition with the subsequent diffusion process in the electronic and lattice subsystems of the target. The track size, resulting from the quench of a molten phase, is determined by the energy density deposited on the atoms around the ion path governed by the electron–phonon strength. Finally, we discuss the general validity of this model in metallic materials and its suitability to describe track formation in amorphizable and non-amorphizable insulators.

Effect of gamma irradiation on the properties of plastic bottle sheet

Abstract: In this article, the effects of gamma irradiation on the optical and structural properties of plastic bottle sheets have been studied. Bottle sheets were exposed with 1.25 MeV 60 Co γ-ray’s source at various dose levels within the range from 0 to 670 kGy. The changes so induced were analyzed by using UV–Vis and X-ray diffraction spectroscopy. UV–Vis spectra show the peaks, their shifting and broadening as a result of gamma irradiation. With increasing γ-dose, the values of the direct and indirect band gap are found to be decreased. These results are in good agreement with the corresponding results published previously for polyethylene terephthalate polymer. We also calculated numbers of carbon atoms per conjugation length. The X-ray diffraction spectra exhibited an increase in peak intensity after gamma irradiation. Furthermore, the percentage crystallinity and crystallite size for pristine and irradiated sample have been calculated. It has been found that both crystallinity and crystallite size increase due to irradiation. In addition, interchain distances, micro strain, inter planar distance, dislocation density and distortion parameters were calculated. The analysis revealed there is the significant decrease in micro strain, dislocation density and distortion parameters with an increase of gamma dose, which is in line with the crystallinity calculation. Moreover, Interchain and Interplanar distances were marginally changed. These results demonstrated the applicability of sheets as a cost-effective dosimeter.

Radiation synthesis of superabsorbent CMC based hydrogels for agriculture applications

Abstract: A series of superabsorbent hydrogel based on carboxymethylcellulose (CMC) and polyvinylpyrrolidone (PVP) crosslinked with gamma irradiation have been proposed for agriculture application. The effect of preparation conditions such as feed solution composition and absorbed irradiation dose on the gelation and swelling degree was evaluated. The structure and the morphology of the superabsorbent CMC/PVP hydrogel were characterized using Fourier transform infrared spectroscopy technique (FTIR), and scanning electron microscope (SEM). Effect of ionic strength and cationic and anionic kinds on the swelling behavior of the obtained hydrogel was investigated. Urea as an agrochemical model was loaded onto the obtained hydrogel to provide nitrogen (N) nutrients. The water retention capability and the urea release behavior of the CMC/PVP hydrogels were investigated. It was found that, the obtained CMC/PVP hydrogels have good swelling degree that greatly affected by its composition and absorbed dose. The swelling was also extremely sensitive to the ionic strength and cationic kind. Owing to its considerable slow urea release, good water retention capacity, being economical, and environment-friendly, it might be useful for its application in agriculture field.

“Total IBA” – Where are we?

Abstract: The suite of techniques which are available with the small accelerators used for MeV ion beam analysis (IBA) range from broad beams, microbeams or external beams using the various particle and photon spectrometries (including RBS, EBS, ERD, STIM, PIXE, PIGE, NRA and their variants), to tomography and secondary particle spectrometries like MeV-SIMS. These can potentially yield almost everything there is to know about the 3-D elemental composition of types of samples that have always been hard to analyse, given the sensitivity and the spacial resolution of the techniques used. Molecular and chemical information is available in principle with, respectively, MeV-SIMS and high resolution PIXE. However, these techniques separately give only partial information – the secret of “Total IBA” is to find synergies between techniques used simultaneously which efficiently give extra information. We here review how far “Total IBA” can be considered already a reality, and what further needs to be done to realise its full potential.

Statistically sound evaluation of trace element depth profiles by ion beam analysis

Abstract: This paper presents the underlying physics and statistical models that are used in the newly developed program NRADC for fully automated deconvolution of trace level impurity depth profiles from ion beam data. The program applies Bayesian statistics to find the most probable depth profile given ion beam data measured at different energies and angles for a single sample. Limiting the analysis to % level amounts of material allows one to linearize the forward calculation of ion beam data which greatly improves the computation speed. This allows for the first time to apply the maximum likelihood approach to both the fitting of the experimental data and the determination of confidence intervals of the depth profiles for real world applications. The different steps during the automated deconvolution will be exemplified by applying the program to artificial and real experimental data.

Effect of H and He irradiation on cavity formation and blistering in ceramics

Abstract: Single- or poly-crystalline specimens of SiC, Si3N4, MgO, Al2O3 and MgAl2O4 were implanted with 0.4–1 MeV H+ or He+ ion beams at room temperature and 650 °C up to fluences of ∼1 × 1022/m2. This produced peak implanted gas and displacement damage levels as high as ∼50 at.% and 34 displacements per atom (dpa). The specimens were subsequently examined optically, and in cross-section using transmission electron microscopy. Subsurface blistering occurred for specimens irradiated to H or He fluences greater than about 3 × 1021/m2 (∼15 at.% peak implanted gas concentration), and surface exfoliation occurred for fluences above ∼1 × 1022/m2 (∼40 at.% implanted gas). Both helium and hydrogen had comparable effectiveness for inducing blistering and exfoliation on an atomic basis. The threshold blistering and exfoliation fluences for both ions were weakly dependent on temperature between 25 and 650 °C. Both H and He were found to be very effective in inducing matrix cavity formation, due to their low solubility in these ceramics. The implanted gas concentrations that resulted in visible cavity formation generally ranged from 1 to 5 at.%. Visible cavity formation was readily induced during room temperature irradiation despite the limited vacancy mobility in these ceramics at room temperature. Three general types of cavity morphologies were observed: isolated cavities, clusters of small cavities (typically associated with dislocation loops), and two-dimensional platelets. Cavity formation was observed to initiate at the periphery of dislocation loops in some cases. During elevated temperature irradiation, cavity formation was often observed to be preferentially associated with certain low-index habit planes, particularly if the habit plane was oriented nearly parallel to the irradiated surface: (0 0 0 1) and { 1 1 ¯ 0 0 } for Al2O3, (0 0 0 1) for α–SiC, {0 0 1} and {1 1 0} for MgO, and {1 1 0} and {1 1 1} for MgAl2O4. The bubble formation and blistering behavior of the ceramics was similar to that observed in other studies of metals irradiated at comparable homologous temperatures. Ionization-induced diffusion effects associated with dual-beam light ion irradiation appeared to exert only a weak effect on cavity and dislocation loop growth compared to the single ion irradiation conditions.

ESTRA-FitEXA: A software package for EXAFS data analysis

Abstract: ESTRA and FitEXA are two programs for processing X-ray absorption spectroscopy data, extraction of extended X-ray absorption fine structure (EXAFS) signal, and EXAFS data analysis via least square refinement procedure (shell fitting). ESTRA and FitEXA propose useful options such as the analysis of the noise on the raw χ(k) data and a high flexibility in the choice of the model distribution function: harmonic, anharmonic (cumulants) and hard sphere models. The minimization routines underneath the FitEXA code allow ample choice/control of the non-linear minimization procedure and check of the correlation among the parameters.

Oxygen-excess-related point defects in glassy/amorphous SiO2 and related materials

Abstract: An insight is given into recent experimental advances in the spectroscopic studies of oxygen-excess intrinsic defects, in glassy SiO 2 and α-quartz. By controlling excess oxygen in a-SiO 2 , and the conditions of F 2 -laser irradiation, SiO 2 glass samples can be obtained with optical absorption almost exclusively dominated by single defect, oxygen dangling bonds (“non-bridging oxygen hole centers” or NBOHCs), without the presence of complementary Si dangling bonds (generic “E′-centers”). This allows for a more accurate determination of the spectral shape of NBOHC optical absorption in UV and vacuum UV spectral regions. The temperature dependence of NBOHC electron paramagnetic resonance (EPR) signal intensity is stronger than predicted by Curie’s law (1/ T ) even at temperatures at and below 77 K. Dangling bonds are characteristic of an amorphous state and do not exist in a regular crystal lattice. However, site-selective luminescence shows that highly ordered NBOHCs exist in particle-irradiated α-quartz, evidently either on the border between the damage tracks and the crystalline phase or as a part of Si vacancies. They are different from the common “glass-like” NBOHCs in a-SiO 2 by giving distinct sharp zero-phonon lines with characteristic energies in luminescence spectra instead of a continuous distribution of lines. Two distinct types of luminescent NBOHCs, associated with the long and short Si–O bonds in α-quartz are suggested. EPR data corroborate the presence of oriented NBOHCs in neutron-irradiated α-quartz and confirm distinct NBOHCs at the sites of “long” and “short” Si–O bonds in α-quartz.

Modelling the effects of electronic excitations in ionic-covalent materials

Abstract: High energy radiation events in ionic and covalent materials can lead to highly excited electronic configurations which, over time, relax to the ground state, either radiatively by emitting photons, or non-radiatively. Non-radiative relaxation involves the transfer of energy to the lattice and this can result in lattice heating, defect formation or even phase changes. The effects of the relaxation mechanisms on the atomic configuration are challenging to model accurately by standard methods. The situation is further complicated by interactions between electronic excitations and pre-existing defects, possibly created by other radiation events. In this paper we describe a range of mechanisms by which the electronic energy is transferred to the lattice and the resulting effects on the atomic configuration, along with the different techniques that are used to model these effects.

XPS and RBS investigation of TiNxOy films prepared by vacuum arc discharge

Abstract: Three titanium oxynitride films have been prepared by vacuum arc discharge technique at different chamber temperatures (50 °C, 150 °C and 300 °C). X-ray photoelectron spectroscopy was used to reveal the elemental and chemical compositions by analyzing high resolution spectra of Ti 2p3/2, N 1s and O 1s. Higher temperatures were found to promote the nitride components and to produce nitrogen-rich films. Homogeneity and thickness of the films have been estimated by means of Rutherford Back Scattering technique, which showed that the film thickness increased with the increasing of temperature. A significant improvement in the crystalline quality and texture when increasing the temperature was found by X-ray diffraction technique. Electrical resistivity of the films was measured at room temperature and was found to decrease from 46.6 μΩ cm down to 26.3 μΩ cm for the samples prepared at 50 °C and 300 °C, respectively.

Investigation of activation cross-sections of proton induced nuclear reactions on natMo up to 40 MeV: New data and evaluation

Abstract: Cross-sections of proton induced nuclear reactions on natural molybdenum have been studied in the frame of a systematic investigation of charged particle induced nuclear reactions on metals for different applications. The excitation functions of 93mTc, 93gTc(m+), 94mTc, 94gTc, 95mTc, 95gTc, 96gTc(m+), 99mTc, 90Mo(cum), 93mMo, 99Mo(cum), 90Nb(cum), 92mNb, 95mNb, 95gNb, 96Nb and 88Zr(cum), 89Zr(cum) were measured up to 40 MeV proton energy by a using stacked foil technique and activation method. The main goals of this work were to study the production possibility of the medically important 99mTc and its 99Mo parent nucleus, to get experimental data for accelerator technology, for monitoring of proton beam, for thin layer activation technique and for testing nuclear reaction theories. The experimental data were compared with critically analysed published data and with the results of model calculations, obtained by using the ALICE-IPPE, EMPIRE-II and TALYS codes.

Effect of gamma irradiation on biopolymer composite films of poly(vinyl alcohol) and bacterial cellulose

Abstract: Composite materials containing in different ratios poly(vinyl alcohol) (PVA), bacterial cellulose (BC) and glycerol (G) as plasticizer were obtained and exposed to different γ radiation doses using an irradiator GAMMATOR provided with 137Cs source. These films successively received up to 50 kGy absorbed doses at a dose rate of 0.4 kGy/h at room temperature. In order to study the chemical and structural changes during γ irradiation, Fourier-transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and UV–Vis spectroscopy were used. Water vapour permeability (WVP), Hunter colour parameters and hardness were also measured for the irradiated samples. Investigation revealed that WVP was not significantly affected by the irradiation. Colour measurements indicated a slight decrease of pure PVA films transparency and it made clear that all samples became more reddish and yellowish after irradiation. The samples hardness was not affected by the irradiation doses used. However, the results showed no drastic structural or chemical changes of the irradiated samples, which prove, in consequence, a good durability. These composite materials could be used as packaging materials for γ irradiated products.

Free radical kinetics in a plasma immersion ion implanted polystyrene: Theory and experiment

Abstract: The kinetics of free radicals in polystyrene (PS) treated by plasma immersion ion implantation (PIII) in nitrogen plasma are investigated by means of electron paramagnetic resonance (EPR). EPR spectra are recorded for PS samples with various PIII treatment times ranging from 40 to 1600 s (corresponding to ion fluences of 5 × 1014–2 × 1016 ions/cm2) after storage times ranging from 45 min to 3 weeks (room temperature). An unexpected, non-linear behaviour at short treatment times is observed. As treatment time increased, the EPR signal first remains constant then shows a sharp increase and a subsequent saturation at long treatment times. An analytical model for the processes of formation and decay of free radicals in PIII treated polymers is proposed. The model describes the kinetics of short-lived and long-lived radicals using a system of linear differential equations. The model is in good agreement with experimental data.

Investigations of 89Y(p,x)86,88,89gZr, 86m+g,87g,87m,88gY, 85gSr, and 84gRb nuclear processes up to 42 MeV

Abstract: Production cross-sections of the 89Y(p,x)86,88,89gZr, 86m+g,87g,87m,88gY, 85gSr, and 84gRb nuclear processes were investigated up to 42-MeV proton energy by using a stacked-foil activation technique at the MC-50 cyclotron of the Korea Institute of Radiological and Medical Sciences. Cumulative cross-sections of the 88gY radionuclide have been reported here for the first time from proton activation on natural yttrium target. The present data were compared with the available experimental data as well as theoretical calculations based on the TALYS and the ALICE-IPPE codes, and found in general good agreement among them. Due to the mono-isotopic characteristics of the natural yttrium, the investigated 89Y(p,x)86,88,89gZr, 86m+g,87g,87m,88gY, 85gSr, and 84gRb processes are suitable for testing of nuclear reaction theories and/or mechanism. A quantitative analysis has also done to understand more accurately the predictive power of the model codes by performing calculations of deviation factors between the measured data and respective model calculations. The thick target integral yields, i.e., induced radioactivity per unit fluence of 42-MeV protons were also deduced from the measured cross-sections of the investigated radionuclides. Optimal production pathway for the 89Zr-PET radionuclide using a cyclotron is discussed elaborately. The measured cross-sections of 87gY, 88gY, 88Zr, and 89gZr radionuclides find importance due to their suitable decay characteristics leading to medical and thin layer activation applications. In addition, the present experimental results will play an important role in enrichment of the literature data base for proton-induced reactions on natural yttrium leading to various applications.

Geant4 physics processes for microdosimetry simulation: Very low energy electromagnetic models for electrons in silicon

Abstract: The Energy-Loss Function (ELF) of silicon is used to calculate differential and total inelastic cross-sections of incident electrons. The model is validated in the 50 eV–50 keV incident energy range by comparing the inelastic cross-sections, stopping powers, and ranges to experimental and calculated data from the literature. It is applicable down to 16.7 eV. The cross sections are then used to simulate low-energy electron tracks in silicon with Geant4, using a similar implementation as the Geant4-DNA extension; this new Geant4 extension is called MuElec. Generation of low-energy electrons is clearly seen. The obtained ranges are consistent with experimental data.

Femtosecond laser-induced subwavelength ripples on Al, Si, CaF2 and CR-39

Abstract: The formation of self-organized subwavelength ripples on Al, Si, CaF 2 and CR-39 induced by 25 fs laser pulses at central wavelength of 800 nm has been observed under certain experimental conditions. In case of Al subwavelength gratings with periodicities ranging from 20 to 220 nm are reported. For CaF 2 the periodicity goes up to 625 nm. In case of Si, nano-gratings have the periodicity of 10–100 nm. The interspacing of these gratings is 60 nm in case of CR-39. These features which are significantly shorter than incident laser wavelength are observed at the irradiation fluence slightly higher than the ablation threshold regardless of the target material. In addition to these nanoripples, classical or microripples with an average spacing of 1–2 μm have also been registered on irradiated surfaces of Al and Si. These microripples have appeared at fluence higher than that is required for nanoripple-formation. It has been found that the formation of the laser-induced ripples is strongly dependent and quite sensitive to the incident laser fluence and the selection of material.

Review of dynamic recovery effects on ion irradiation damage in ionic-covalent materials

Abstract: Single crystalline samples of highly ionic Ca2La8(SiO4)6O2 and covalent 6H–SiC have been irradiated with different ions/energies to study the effects of dose, temperature, damage-energy density, and in-cascade ionization rate on the dynamics of irradiation-induced amorphization. Above temperatures of 100–150 K, the dose for complete amorphization, D, increases with temperature in a single stage and exhibits a strong dependence on the ratio of in-cascade recovery to displacement cross sections, σr/σd. A fit of a dynamic model for amorphization to these data indicates that irradiation-induced dynamic recovery occurs with an activation energy of 0.15 ± 0.02 and 0.12 ± 0.01 eV for Ca2La8(SiO4)6O2 and 6H–SiC, respectively. Analysis of these data reveals that ionization processes are the dominant contributor to in-cascade recovery in Ca2La8(SiO4)6O2; while in 6H–SiC, ionization processes are less dominant.

Calculations of electron stopping powers for 41 elemental solids over the 50 eV to 30 keV range with the full Penn algorithm

Abstract: We present mass collision electron stopping powers (SPs) for 41 elemental solids (Li, Be, graphite, diamond, glassy C, Na, Mg, Al, Si, K, Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Ge, Y, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Cs, Gd, Tb, Dy, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Bi) that were calculated from experimental energy-loss-function data with the full Penn algorithm for electron energies between 50 eV and 30 keV. Improved sets of energy-loss functions were used for 19 solids. Comparisons were made of these SPs with SPs calculated with the single-pole approximation, previous SP calculations, and experimental SPs. Generally satisfactory agreement was found with SPs from the single-pole approximation for energies above 100 eV, with other calculated SPs, and with measured SPs.

FTIR analysis of polyethylene terephthalate irradiated by MeV He

Abstract: The degradation induced by MeV He+ ions onto polyethylene terephthalate (PET) is investigated as a function of fluence and electronic stopping power (dE/dx)e. Stack samples of four 3.6 μm PET films are irradiated by 3.5 MeV He+ under vacuum and room temperature at fluence ranging from 1013 to 1.5 × 1015 He+ cm−2. The entrance energies range from 3.5 MeV for the front film to 0.573 MeV for the rear one at the beginning of the irradiation and the corresponding mean (dE/dx)e lie for pristine PET between 158 and 264 keV/μm. A Fourier transform infrared spectroscopy (FTIR) analysis is undertaken. The different stack films present similar damages with magnitude increase as the fluence goes up. Moreover, higher is the electronic stopping power (dE/dx)e more important is the damage. The evolution of the normalized integrated absorbance A( ϕ )/A0 of the internal reference 1410 cm−1 band as a function of the fluence ϕ presents a complex behavior characterized by a two-exponential decay associated to low and high fluence range. The analysis reveals clearly that the A( ϕ )/A0 of the characteristic benzene ring 1577 cm−1 band remains steady till fluence around 3 × 1014 He+ cm−2 and then decreases smoothly as fluence increases. The band appearing at 1610 cm−1, assigned to mono substituted benzene, presents an integrated absorbance A( ϕ ) which rises progressively and levels off beyond a critical fluence around 4 × 1014 He+ cm–2. The cross section of the creation of the monosubstituted benzene lies between 3 × 10−15 and 6 × 10−15 cm2 for (dE/dx)e = 158 and 264 keV/μm, respectively. Drastic loss of crystallinity is induced by the MeV He+ ion irradiation in the polymer. The amorphization is monitored by the evolution of the specific bands of the trans and gauche ethylene glycol residue conformations. A sudden drop in the integrated absorbance of the 1340 cm−1 band which characterizes the trans conformation is observed at fluence around 2 × 1014 He+ cm−2. On the other hand, a drastic increase is present in the integrated absorbance of the gauche conformation 1370 cm−1 band that pairs up with 1340 cm−1 band. This observation is a clear indication of the conversion of the trans ethylene glycol residue to gauche one under irradiation.

Hyperfine spectroscopy of muonic hydrogen and the PSI Lamb shift experiment

Abstract: The recent Lamb shift experiment at PSI and the discussions about the incompatibility of the proton radii extracted using different methods have revived the interest in the measurement of the hyperfine splitting of the ground state in muonic hydrogen. We summarize the existing experimental ideas for this measurement and analyze quantitatively the main methodological difficulties, expected to be related to the required power of the IR laser source, the signal-to-noise ratio and other systematic factors. An elaborate model is developed to estimate the statistical uncertainty of the experimental value of the hyperfine splitting.

The JANNUS Saclay facility: A new platform for materials irradiation, implantation and ion beam analysis

Abstract: The third accelerator of the multi-ion irradiation platform JANNUS (Joint Accelerators for Nanosciences and NUclear Simulation), a 6SDH-2 Pelletron from National Electrostatic Corporation, Middleton was installed at Saclay in October 2009. The first triple beam irradiation combining Fe, He and H ion beams has been performed in March 2010. In the first part of this paper, we give a technical description of the triple beam facility, its performances and experimental capabilities. Typically, damage dose up to 100 dpa can be reached in 10 h irradiation with heavy ion beams, with or without simultaneous bombardment by protons, helium-4 ions or any other heavy ion beam. In the second part of this paper, we illustrate some IBA results obtained after irradiation and implantation experiments.

High electronic excitation induced modifications by 100 MeV O 7+ and 150 MeV Ni 11+ ions in Makrofol KG polycarbonate film

Abstract: Makrofol-KG polycarbonate (M-KG PC) films are irradiated with 150 MeV Ni 11+ and 100 MeV O 7+ ion beams at various fluences ranging from 1 × 10 11 to 3 × 10 12 ions/cm 2 . We have investigated the effect of irradiation parameters such as energy and ions fluence on optical, structural and chemical properties of the studied polymer, respectively. UV–Visible, Fourier Transform Infrared (FTIR) and X-ray diffraction (XRD) spectral studies have been employed in the present investigation. UV–Visible spectra exhibit a shift towards the higher wavelength regime after irradiation. This shift clearly reflects decrease in optical band gap after irradiation. The FTIR spectrum shows a decrease in intensity of the typical bands whereas the formation of new bands indicates the degradation of the polymer after irradiation. The XRD pattern of M-KG PC shows the decreasing intensity of peak positions with increase in ions fluence, which suggests loss of crystallinity of the films due to irradiation. Observed results indicate the formation of disordered system in the irradiated films. Magnitude of effect of irradiation is greater in case of 150 MeV Ni 11+ rather than 100 MeV O 7+ ions due to high electronic energy loss of Ni 11+ ions in M-KG PC. The results can be correlated on the basis of linear energy transfer (LET) of the irradiated ions.

Doubly differential diffraction at a time grating in above-threshold ionization: Intracycle and intercycle interferences

Abstract: We analyze the doubly differential electron distribution in atomic above-threshold ionization by a linearly-polarized short-laser pulse. We generalize the one-dimensional (1D) simple man’s model (SMM) of Arbo et al. [19], to a three dimensional (3D) description by using the saddle-point approximation (SPA). We prove that the factorization of the photoelectron spectrum in terms of intracycle and intercycle interference patterns can be extended to the doubly differential momentum distribution. Intercycle interference corresponds to the well-known ATI peaks of the photoelectron spectrum arising from the superposition of electron trajectories released at complex times during different optical cycles, whereas intracycle interference comes from the coherent superposition of trajectories released within the same optical cycle. We verify the SPA predictions by comparison with time-dependent distorted wave calculations and the solutions of the full 3D time-dependent Schrodinger equation (TDSE). An analytical expression for the complete interference pattern within the SPA is presented showing excellent agreement with the numerical calculations. We show that the recently proposed semiclassical description based on the SMM in terms of a diffraction process at a time grating remains unchanged when considering the full 3D problem within the SPA.

An in situ TEM study of the evolution of Xe bubble populations in UO2

Abstract: Transmission electron microscopy (TEM) experiments were carried out on the JANNUS platform (Joint Accelerators for Nano-science and NUclear Simulation) at CSNSM (Center of Nuclear Spectrometry and Mass Spectrometry) laboratory in Orsay. The experiment was devoted to the study of the evolution of the xenon aggregate population with increasing implantation fluence. A thin UO2 foil was implanted at fluences ranging from 3 x 10(12) to 7 x 10(14) at cm(-2) with 390 key Xe3+ ions at an irradiation temperature of 873 K. The TEM results indicate the presence of nanometer size bubbles above a fluence of 6 x 10(12) Xe cm(-2) and an increase in the bubble number density was observed between 6 x 10(12) Xe cm(-2) and 2 x 10(14) Xe cm(-2). Above 2 x 10(14) Xe cm(-2), the number density levels off at 4 x 10(23) +/- 0.5 x 10(23) m(-3). (C) 2011 Elsevier B.V. All rights reserved.

Geant4 physics processes for microdosimetry simulation: Very low energy electromagnetic models for protons and heavy ions in silicon

Abstract: The Geant4-DNA extension of the Geant4 Monte Carlo simulation toolkit aims at modeling early biological damages induced by ionizing radiation at the DNA scale, and it can now track particles down to very low energies in liquid water. New models, called “MuElec”, have been implemented for microelectronic applications following the same initial theory, to track low energy electrons in silicon. This paper presents the extension of these MuElec models to incident protons and heavy ions in silicon. First, the theory of the model is presented. The resulting cross sections and stopping powers are compared with data from the literature. The model is then implemented in Geant4 and used to simulate proton tracks. Various physical quantities are extracted from the simulation, and compared with data from the literature and with results from simulation using other Geant4 models. It is shown that the generation of low-energy electrons results in more physically meaningful low-energy secondary electron tracks, which significantly modifies the proton and ion track core on the nanometer scale.

Response behavior of ZrO2 under swift heavy ion irradiation with and without external pressure

Abstract: In this study, we demonstrate that the combination of relativistic heavy ions with pressure can influence the phase behavior of ZrO2 in ways none of those two extreme conditions alone could. The response behavior of ZrO2 towards ion irradiation under different pressure conditions is investigated. ZrO2 exposed to energetic particles is known to undergo a crystalline-to-crystalline phase transition from the monoclinic to the tetragonal phase. In agreement with earlier findings, this structural change requires also for heaviest ions, such as Au, Pb, and U, a multiple ion impact. If the irradiation is performed under high pressure, the monoclinic-to-tetragonal transformation occurs at a fluence that is more than one order of magnitude lower suggesting a single impact process. Raman measurements at ambient conditions and X-ray diffraction analysis of the samples irradiated under pressure revealed that the monoclinic-to-tetragonal transformation under pressure is not a direct process but involves a transition into the cubic high-temperature structure, before the tetragonal structure becomes stable under decompression. At even higher pressures, the additional ion irradiation forces ZrO2 to transform to the higher orthorhombic-II phase that is far away from its stability field.

Plasma impregnation of wood with fire retardants

Abstract: The efficacy of chemical and plasma treatments with phosphate and boric compounds, and nitrogen as flame retardants on wood are compared in this study. The chemical treatment involved the conventional method of spraying the solution over the wood surface at atmospheric condition and chemical vapor deposition in a vacuum chamber. The plasma treatment utilized a dielectric barrier discharge ionizing and decomposing the flame retardants into innocuous simple compounds. Wood samples are immersed in either phosphoric acid, boric acid, hydrogen or nitrogen plasmas or a plasma admixture of two or three compounds at various concentrations and impregnated by the ionized chemical reactants. Chemical changes on the wood samples were analyzed by Fourier transform infrared spectroscopy (FTIR) while the thermal changes through thermo gravimetric analysis (TGA). Plasma-treated samples exhibit superior thermal stability and fire retardant properties in terms of highest onset temperature, temperature of maximum pyrolysis, highest residual char percentage and comparably low total percentage weight loss.

Effect of gamma irradiation on the optical properties of UHMWPE (Ultra-high-molecular-weight-polyethylene) polymer

Abstract: The UV–Visible absorption spectra of virgin and high dose gamma irradiated (up to 2000 kGy) UHMWPE polymer have been studied by using UV–Visible spectrophotometer (JASCO, V-570). The existence of the peaks, their shifting and broadening as a result of gamma irradiation has been observed. In the present work the Urbach energy is calculated by using Urbach edge method. Also the direct and indirect energy band gap in virgin and gamma irradiated UHMWPE polymer has been calculated. The values of indirect energy band gap have been found to be lower than the corresponding values of direct energy band gap. A decrease in the optical energy band gap with increasing gamma irradiation has been discussed on the basis of gamma-irradiation-induced modification in the UHMWPE polymer. The correlation between optical energy band gap and the number of carbon atoms in a cluster with modified Tauc’s equation has been discussed in this polymer. We have also observed the color formation which became more and more prominent with increasing dose and at the highest dose the fully transparent sample became completely opaque. Looking at the trend of the absorption curve this polymer can be used as a very good dosimeter for the gamma ray irradiation.

Damage evolution and amorphization in semiconductors under ion irradiation

Abstract: The interaction of energetic ions with crystalline semiconductors may cause defect formation and amorphization both by nuclear and electronic energy deposition which occur simultaneously with different strength. In the energy range of dominating nuclear energy loss the transition to the amorphous state depends on the energy received by the target atoms through nuclear interactions, i.e. on ion mass and energy, and on the irradiation temperature which can favour dynamic defect annealing. Regarding the damage evolution, different groups of semiconductors are observed. In many materials (e.g. elemental and binary III–V semiconductors, SiC) a continuous increase of the damage concentration up to amorphization is found below characteristic critical irradiation temperatures. In some other materials (AlAs, GaN) the damage concentration increases stepwise until amorphorphization occurs, and in few other cases (ZnO, CdTe) amorphization is prevented even at 15 K. In case of swift heavy ion (SHI) irradiation with dominating electronic energy loss, heavily damaged tracks are formed above critical values of the energy deposition in some materials (e.g. InP, InAs, InSb, GaSb), which may agglomerate to continuous amorphous layers. In other semiconductors (e.g. Si, Ge, GaP, GaAs, AlAs) so far no track formation but only the formation of point defects and point defect complexes was observed. In the present contribution the state of the art of damage evolution and amorphization occurring at temperatures equal to and below room temperature in covalent and ionic-covalent semiconductors is summarized and possible mechanisms are discussed to understand the experimental results.