Optical absorption in fused silica during TRIGA reactor pulse irradiation
Spectral transmission characteristics of fused silica before, during, and after exposure to reactor irradiation pulses
De Crescente, M. A.↗
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Photothermal transformation of Au–Ag nanocages under pulsed laser irradiation
Pulsed laser irradiation can photothermally transform Au–Ag nanocages into pseudo-spherical, solid nanoparticles. The results may have implications for the future use of Au–Ag nanocages in biomedicine, catalysis, and sensing.
Hood, Zachary D.↗
Determination of Refractive Indices of a Layered Medium under Pulsed Irradiation
Problems related to determination of refractive indices of layered strongly scattering biological media by means of an optical tomography are investigated. Methods of reconstruction of refractive indices of layers, which are based on the smoothness properties of the solution to the initial-boundary problem, are proposed within the framework of the nonstationary radiative-transfer model. The influence of the probe-pulse duration on the accuracy of recovery of the sought-for characteristics of the medium is investigated numerically.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Optical absorption and fluorescence in fused silica during TRIGA pulse irradiation
Spectral transmission measurements on fused silica during and after exposure to pulsed neutron irradiation
Gagosz, R.↗
Effect of focused nanosecond laser pulse irradiation on microtribological properties of diamond-like films
We have examined the effect of focused nanosecond laser pulses on the formation and tribological properties of microstructures in the form of a matrix of craters produced in a – C : H : Si : O diamond-like films (DLFs), both undoped and doped with tungsten. The studies are performed by scanning probe microscopy (SPM) using SPM probes of standard design with a tip and probes without a tip (tipless-type probes). A special procedure is developed for assessing the wettability (contact angle) of individual sections of the laser-induced microstructure by the local adhesion force. According to estimates, the material inside the crater becomes softer and more hydrophobic as a result of laser irradiation. This is manifested to the greatest extent in tungsten-doped DLFs, where the crater walls are arranged like terraces formed by ‘nanoflakes’, i.e. grains of the substance. Scanning of microstructures with a tipless probe in the regime of lateral force microscopy demonstrates a decrease in friction in the crater area. (interaction of laser radiation with matter. laser plasma)
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Godiva II—An Unmoderated Pulse-Irradiation Reactor
Design features of Godiva II, the improved pulsed reactor successor to Lady Godiva, are discussed together with characteristics of power excursions, and performance is compared with that of the original Godiva. Measurements of the wait time between stepwise reactivity insertion and the occurrence of a burst are presented and compared with theory based on a statistical model of fission chains. Analytical and numerical solutions of the reactor equations are developed to reproduce experimental data and extrapolate to higher energy release. Here, consideration is also given to perturbations arising from room-returned neutrons. Two different modes of operation are discussed and some design problems of Godiva-type pulsed reactors are briefly mentioned. Typical bursts are illustrated with peak powers up to 13,000 Mw and widths at half-maximum down to 35 μsec.
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
Target optimisation for the yield of X-rays of desired hardness under femtosecond pulse irradiation
Different regimes of electron acceleration from solid foils and low-density targets are investigated using three-dimensional numerical simulations. The size of the plasma corona is shown to be the main parameter characterising the temperature and number of hot electrons, which determine the yield of X-ray radiation and its hardness. Also studied is the generation of X-ray radiation by laseraccelerated electrons, which bombard the converter target located behind the laser target. (extreme laser radiation: physics and fundamental applications)
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Primary radical yields in pulse irradiated alkaline aqueous solution
Primary radical yields of hydrated electrons, H atoms, and OH radicals are determined by measuring hydrated electron formation following a 4 microsecond pulse of X rays. The pH dependence of free radical yields beyond pH 12 is determined by observation of the hydrated electrons.
Fielden, E. M.↗
Laser damage of crazed electron-beam high-reflectors following infrared and ultraviolet irradiation in the nanosecond pulse regime
Laser damage of optical components can be a limiting factor in scaling the energetics of high-peak and average power laser systems. Specifically for optical coatings, damage under nanosecond pulsed irradiation is initiated by pre-existing defects in the coating layers, including those that cause discontinuities in the structure, like craze lines. Crazing or cracking in a multilayer dielectric optical coating is induced when the overall coating stress is sufficiently tensile, and is an occasionally observed issue when employing more porous deposition techniques like electron-beam evaporation. Here, in this study, electron-beam high-reflectors were fabricated utilizing process parameters that are known to induce crazing based on prior processing history to systematically evaluate the impact of crazing on reflector damage performance for 1064 and 355 nm lasers. The crazing that was observed was apparently nucleated at nodular defects. When the cross-section of these nodules was investigated, it was observed that there were cracks into the fused silica substrate of approximately 5 µm in depth. The craze lines were irradiated with 1064 and 355 nm light at fluences slightly above the onset of damage initiation fluence of the coating. The 1064 nm irradiated sub-apertures exhibit laser damage but with no spatial correlation with the craze line, whereas the 355 nm irradiated area exhibited many damage sites along the craze line. Finite-difference time-domain electric-field simulations were conducted, and ∼2× field amplification in hafnia was observed for the 355 nm wavelength case. The laser damage can be attributed to a slight electric-field intensification coincidental with an area where UV damage-prone precursors are known to occur. The 355 nm laser damage in uncoated fused silica substrates has been previously correlated to initiate through localized UV absorption at the broken silica bonds in the tips of fractures.
Harthcock, Colin [Lawrence Livermore National Labo↗
Compositional and Microstructural Evolution of Olivine Under Multiple-Cycle Pulsed Laser Irradiation as Revealed by FIB/Field-Emission TEM
The use of pulsed laser irradiation to simulate the short duration, high-energy conditions characteristic of micrometeorite impacts is now an established approach in experimental space weathering studies. The laser generates both melt and vapor deposits that contain nanophase metallic Fe (npFe(sup 0)) grains with size distributions and optical properties similar to those in natural impact-generated melt and vapor deposits. There remains uncertainty, however, about how well lasers simulate the mechanical work and internal (thermal) energy partitioning that occurs in actual impacts. We are currently engaged in making a direct comparison between the products of laser irradiation and experimental/natural hypervelocity impacts. An initial step reported here is to use analytical SEM and TEM is to attain a better understanding of how the microstructure and composition of laser deposits evolve over multiple cycles of pulsed laser irradiation.
Christoffersen, R.↗
Compositional and Microstructural Evolution of Olivine During Pulsed Laser Irradiation: Insights Based on a FIB/Field-Emission TEM Study
Introduction: The use of pulsed laser irradiation to simulate the short duration, high-energy conditions characteristic of micrometeorite impacts is now an established approach in experimental space weathering studies. The laser generates both melt and vapor deposits that contain nanophase metallic Fe (npFe(sup 0)) grains with size distributions and optical properties similar to those in natural impact-generated melt and vapor deposits. There remains uncertainty, however, about how well lasers simulate the mechanical work and internal (thermal) energy partitioning that occurs in actual impacts. We are currently engaged in making a direct comparison between the products of laser irradiation and experimental/natural hypervelocity impacts. An initial step reported here is to use analytical TEM is to attain a better understanding of how the microstructure and composition of laser deposits evolve over multiple cycles of pulsed laser irradiation. Experimental Methods: We irradiated pressed-powder pellets of San Carlos olivine (Fo(sub 90)) with up to 99 rastered pulses of a GAM ArF excimer laser. The irradiated surface of the sample were characterized by SEM imaging and areas were selected for FIB cross sectioning for TEM study using an FEI Quanta dual-beam electron/focused ion beam instrument. FIB sections were characterized using a JEOL2500SE analytical field-emission scanning transmission electron microscope (FE-STEM) optimized for quantitative element mapping at less than 10 nm spatial resolutions. Results: In the SEM the 99 pulse pressed pellet sample shows a complex, inhomogeneous, distribution of laser-generated material, largely concentrated in narrow gaps and larger depressions between grains. Local concentrations of npFe0 spherules 0.1 to 1 micrometers in size are visible within these deposits in SEM back-scatter images. Fig. 1 shows bright-field STEM images of a FIB cross-section of a one of these deposits that continuously covers the top and sloping side of an olivine grain. The deposit has 3 microstructurally distinct sub-layers composed of silicate glass with varying modal fractions and size distributions of npFe( sup 0) spherules, along with nanocrystalline silicate material. A relatively thin (50-300 nm) topmost surface layer has a high-concentration of npFe0 spherules 5-20 nm in size. Element mapping shows the layer to be enriched in Fe by a factor of 2.5 relative to the olivine substrate, with Mg and Si depleted by 20% and 10% respectively. This is compositionally complementary to the underlying, middle layer of the deposit that is depleted in Fe, enriched in Mg and has a much lower npFe0 concentration. A third layer of nanocrystalline olivine occurs at the substrate interface. Discussion: The FE-STEM results suggest the topmost layer is a vapor deposit, underlain by a thicker microstructurally complex melt-generated layer. The compositional relations suggest the melt layer was partially vaporized, preferentially losing more volatile elements (e.g., Fe). The vaporized material re-condensed to form the thin, npFe(sup 0)-rich surface deposit during or immediately after the scan cycle. Nanocrystalline olivine that grew within the melt layer as it formed and cooled is similar in volume and microstructure to what we have observed in the impact melt lining of a micrometeorite impact crater in olivine. This suggest the time-temperature relations attained in the laser sample may not be too different from a micrometeorite impact. Our TEM observations, however, do not show evidence for the same level of mechanical dam-age (e.g., fracturing) seen around the natural micrometeorite crater.
Christoffersen, R.↗
Pulsed-Laser Irradiation Space Weathering Of A Carbonaceous Chondrite
Grains on the surfaces of airless bodies experience irradiation from solar energetic particles and melting, vaporization and recondensation processes associated with micrometeorite impacts. Collectively, these processes are known as space weathering and they affect the spectral properties, composition, and microstructure of material on the surfaces of airless bodies, e.g. Recent efforts have focused on space weathering of carbonaceous materials which will be critical for interpreting results from the OSIRIS-REx and Hayabusa2 missions targeting primitive, organic-rich asteroids. In addition to returned sample analyses, space weathering processes are quantified through laboratory experiments. For example, the short-duration thermal pulse from hypervelocity micrometeorite impacts have been simulated using pulsed-laser irradiation of target material e.g. Recent work however, has shown that pulsed-laser irradiation has variable effects on the spectral properties and microstructure of carbonaceous chondrite samples. Here we investigate the spectral characteristics of pulsed-laser irradiated CM2 carbonaceous chondrite, Murchison, including the vaporized component. We also report the chemical and structural characteristics of specific mineral phases within the meteorite as a result of pulsed-laser irradiation.
Thompson, M. S.↗
Fractal analysis of collision cascades in pulsed-ion-beam-irradiated solids [Collision cascade densities in ion pulsed ion irradiated solids { a fractal approach]
The buildup of radiation damage in ion-irradiated crystals often depends on the spatial distribution of atomic displacements within collision cascades. Although collision cascades have previously been described as fractals, the correlation of their fractal parameters with experimental observations of radiation damage buildup remains elusive. Here, we use a pulsed-ion-beam method to study defect interaction dynamics in 3C-SiC irradiated at 100 °C with ions of different masses. These data, together with results of previous studies of SiC and Si, are analyzed with a model of radiation damage formation which accounts for the fractal nature of collision cascades. Our emphasis is on the extraction of the effective defect diffusion length from pulsed beam measurements. Results show that, for both Si and SiC, collision cascades are mass fractals with fractal dimensions in the range of ~1–2, depending on ion mass, energy, and the depth from the sample surface. Within our fractal model, the effective defect diffusion length is ~10 nm for SiC and ~20 nm for Si, and it decreases with increasing cascade density. These results demonstrate a general method by which the fractal nature of collision cascades can be used to explain experimental observations and predict material’s response to radiation.
36 MATERIALS SCIENCE↗
Microscopy X-ray imaging enriched with small angle X-ray scattering for few nanometer resolution reveals shock waves and compression in intense short pulse laser irradiation of solids
Understanding how laser pulses compress solids into high-energy-density states requires diagnostics that simultaneously resolve macroscopic geometry and nanometer-scale structure. Here we present a combined X-ray imaging (XRM) and small-angle X-ray scattering (SAXS) approach that bridges this diagnostic gap. Using the Matter in Extreme Conditions end station at LCLS, we irradiated 25 μm copper wires with 45 fs, 0.9 J, 800 nm pulses at 3.5 × 10 19 W/cm 2 while probing with 8.2 keV XFEL pulses. XRM visualizes the evolution of ablation, compression, and inward-propagating fronts with ∼ 200 nm resolution, while SAXS quantifies their nanometer-scale sharpness via the time-resolved evolution of scattering streaks. The joint analysis reveals that an initially smooth compression steepens into a nanometer-sharp shock front after t sh ≈ (18 ± 3) ps, consistent with an analytical steepening model and hydrodynamic simulations. The front reaches a velocity of c sh ≈ 25 k m / s and a lateral width of several tens of microns, demonstrating direct observation of shock formation and decay at solid density for the first time with few-nanometer precision. This integrated XRM–SAXS method establishes a quantitative, multi-scale diagnostic of laser-driven shocks in dense plasmas relevant to inertial confinement fusion, warm dense matter, and planetary physics.
Kluge, Thomas [Helmholtz-Zentrum Dresden-Rossendor↗
Demonstration of periodic nanostructure formation with less ablation by double-pulse laser irradiation on titanium
By pairing femtosecond laser pulses (duration ∼40 fs and central wavelength ∼810 nm) at an appropriate time interval, a laser-induced periodic surface structure (LIPSS) is formed with much less ablation than one formed with a single pulse. On a titanium plate, a pair of laser pulses with fluences of 70 and 140 mJ/cm{sup 2} and a rather large time interval (>10 ps) creates a LIPSS with an interspace of 600 nm, the same as that formed by a single pulse of 210 mJ/cm{sup 2}, while the double pulse ablates only 4 nm, a quarter of the ablation depth of a single pulse.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Spallation-induced roughness promoting high spatial frequency nanostructure formation on Cr
Interaction of ultrafast laser pulses with metal surfaces in the spallation regime can result in the formation of anisotropic nanoscale surface morphology commonly referred to as laser-induced periodic surface structures (LIPSS) or ripples. The surface structures generated by a single pulse irradiation of monocrystalline Cr samples are investigated experimentally and computationally for laser fluences that produce high spatial frequency nanostructures in the multi-pulse irradiation regime. Electron microscopy reveals distinct response of samples with different crystallographic surface orientations, with (100) surfaces exhibiting the formation of more refined nanostructure by a single pulse irradiation and a more pronounced LIPSS after two laser pulses as compared to (110) surfaces. A large-scale molecular dynamics simulation of laser interaction with a (100) Cr target provides detailed information on processes responsible for spallation of a liquid layer, redistribution of molten material, and rapid resolidification of the target. The nanoscale roughness of the resolidified surface predicted in the simulation features elongated frozen nanospikes, nanorims and nanocavities with dimensions and surface density similar to those in the surface morphology observed for (100) Cr target with atomic force microscopy. The results of the simulation suggest that the types, sizes and dimensions of the nanoscale surface features are defined by the competition between the evolution of transient liquid structures generated in the spallation process and the rapid resolidification of the surface region of the target. The spallation-induced roughness is likely to play a key role in triggering the generation of high-frequency LIPSS upon irradiation by multiple laser pulses.
Materials Science↗
Heating rate dependent ignition of Al/Pt nanolaminates through pulsed laser irradiation
Direct laser irradiation of sputter deposited Al/Pt nanolaminate multilayers results in rapid local heating and exothermic mixing of reactant layers. Milli- and microsecond pulsed laser irradiation under certain test conditions leads to single-point ignition of rapid, self-propagating, formation reactions. Multilayers having bilayer thicknesses of 328 nm, 164 nm, and 65 nm are characterized by their ignition onset times and temperatures. Smaller bilayer thickness multilayers require less laser intensity for ignition compared with larger bilayer designs (when utilizing a particular pulse duration). The relationship between laser intensity and ignition onset time is used to calibrate an activation energy for ignition within a finite element reactive heat transport model. The local heating rate is varied from 10 4 K/s to 10 6 K/s by selecting a laser intensity. Kissinger analysis was performed on the heating rate-dependent ignition temperatures measured with high speed pyrometry to experimentally determine an activation energy in the foils of (6.2 ± 1.6 × 10 4 J/mole atoms). Furthermore, this value is then compared to an activation energy produced from model fits to an ignition onset time of 7.2 × 10 4 J/mole atoms.
36 MATERIALS SCIENCE↗
Pulse number controlled laser annealing for GeSn on insulator structure with high substitutional Sn concentration
Crystalline GeSn-on-insulator structures with high Sn concentration (>8%), which exceeds thermal equilibrium solid-solubility (∼2%) of Sn in Ge, are essential to achieve high-speed thin film transistors and high-efficiency optical devices. We investigate non-thermal equilibrium growth of Ge{sub 1−x}Sn{sub x} (0 ≤ x ≤ 0.2) on quartz substrates by using pulsed laser annealing (PLA). The window of laser fluence enabling complete crystallization without film ablation is drastically expanded (∼5 times) by Sn doping above 5% into Ge. Substitutional Sn concentration in grown layers is found to be increased with decreasing irradiation pulse number. This phenomenon can be explained on the basis of significant thermal non-equilibrium growth achieved by higher cooling rate after PLA with a lower pulse number. As a result, GeSn crystals with substitutional Sn concentration of ∼12% are realized at pulse irradiation of single shot for the samples with the initial Sn concentration of 15%. Raman spectroscopy and electron microscopy measurements reveal the high quality of the grown layer. This technique will be useful to fabricate high-speed thin film transistors and high-efficiency optical devices on insulating substrates.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗