Crystal Tungsten Research Articles

Problems of technological use of coherent Bremsstrahlung beams

To determine the possibility of technological use of coherent Bremsstrahlung beams the photonuclear transmutation doping of silicon by aluminum upon γ-quantum irradiation from the aligned single-crystal target was studied. An increase in aluminum concentration by approximately ten times was obtained upon γ-radiation of silicon from the crystalline target aligned by the crystallographic axis 〈111〉 along the direction of the electron beam with the energy of 1200 MeV. The orientation effect of an increase in the yield of γ-radiation from silicon and tungsten crystals in the range of energies of accelerated electrons of 300–1200 MeV was examined. This result is essential for the estimation of a possible minimum energy of electrons at which the use of the orientation effect is reasonable.

Energy loss of electron-induced showers in oriented tungsten crystals

The development of the shower induced by 26- and 28-GeV electrons in the oriented tungsten crystal results in strong absorption of the shower energy by the crystal and an increase in the energy fraction of shower particles scattered between the crystal and detector. The ratios of total shower energy losses in oriented tungsten crystals to total losses in misoriented crystals 1, 2.7, 5.8, and 8.4 mm thick were 3.9, 2.6, 2.1, and 1.9, respectively.

Band mapping in x-ray photoelectron spectroscopy: An experimental and theoretical study of W(110) with 1.25 keV excitation

Angle-resolved photoemission spectroscopy (ARPES) has generally been carried out at energies below \ensuremath{\sim}150 eV, but there is growing interest in going to higher energies so as to achieve greater bulk sensitivity. To this end, we have measured ARPES spectra from a tungsten (110) crystal in a plane containing the [100], [110], and [010] directions with a photon energy of 1253.6 eV. The experimental data are compared to free-electron final-state calculations in an extended zone scheme with no inclusion of matrix elements, as well as highly accurate one-step theory including matrix elements. Both models provide further insight into such future higher-energy ARPES measurements. Special effects occurring in a higher-energy ARPES experiment, such as photon momentum, phonon-induced zone averaging effects, and the degree of cryogenic cooling required are discussed, together with qualitative predictions via appropriate Debye-Waller factors for future experiments with a number of representative elements being presented.

Charged particle multiplicity in showers formed by gamma-rays in oriented tungsten crystal

It is shown that the average charged particle multiplicity in electromagnetic showers induced by γ-rays with energies of 9–26 GeV in the tungsten crystal 1 mm thick (T = 77 K) oriented along the 〈111〉 axis approximately twofold increases in comparison with a misoriented crystal.

Spark plasma sintering of tungsten–yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization

Nano-crystalline W-1%Y 2O 3 (wt.%) powder was produced by a modified solution chemical reaction of ammonium paratungstate (APT) and yttrium nitrate. The precursor powder was found to consist of particles of bimodal morphology i.e. large APT-like particles up to 20 μm and rectangular yttrium containing ultrafine plates. After thermal processing tungsten crystals were evolved from W–O–Y plate like particles. spark plasma sintering (SPS) was used to consolidate the powder at 1100 and 1200 °C for different holding times in order to optimize the sintering conditions to yield high density but with reduced grain growth. Dispersion of yttrium oxide enhanced the sinterability of W powder with respect to lanthanum oxide. W–1%Y 2O 3 composites with sub-micron grain size showed improved density and mechanical properties as compared to W–La 2O 3 composites. Sample sintered in two steps showed improved density, due to longer holding time at lower temperature (900 °C) and less grain growth due to shorter holding time at higher temperature i.e. 1 min at 1100 °C.

Formation of vacancy clusters in tungsten crystals under hydrogen-rich condition

Di-vacancy formation assisted by hydrogen trapping is studied in terms of nucleation free-energies evaluated with density functional theory. Calculations give binding energies for single hydrogen atom as first- and second-nearest-neighbor of di-vacancies of 1.80 and 2.15eV, respectively, which are significantly larger than that for mono-vacancies. At elevated atomic concentrations of interstitial hydrogen atoms, evaluated nucleation free-energies indicate that the hydrogen assisted di-vacancy formation becomes more favorable. It is suggested that the formation would be preceded by VH cluster formation.

A POSITRON SOURCE USING CHANNELING IN CRYSTALS FOR LINEAR COLLIDERS

An alternative way to the conventional positron source using intense electron beams, of some GeV, impinging on thick amorphous targets is presented. This source is using two successive targets: the first one is a tungsten crystal, which <111> axis is aligned with the beam direction, and the second is an amorphous target put at some distance from the crystal. The enhanced radiation due to electron channelling produces a large amount of photons which, consecutively, create high quantity of e +- e - pairs in the amorphous target. Between the two targets a sweeping magnet takes off all or part of the charged particles coming out from the crystal. An optimization procedure is carried out based on two important parameters: a) the distance between the two targets; b) the minimum energy above which the charged particles coming out from the crystal are allowed to hit the amorphous target. This optimization, which first step is presented here, aims to obtain the highest accepted yield (> 1 e +/ e -) after the capture system and the minimum Peak Energy Density Distribution (PEDD) (PEDD < 35 J/g) in order to avoid thermal gradients, which are destructive for the target.

Potential physics measurement with ALICE electromagnetic calorimeters

We present the two electromagnetic calorimeters of the ALICE (A Large Ion Collider Experiment) experiment at LHC (Large Hadron Collider). One is the high-resolution PHOton Spectrometer (PHOS) made of lead tungsten crystals and the other is the ElectroMagnetic Calorimeter (EMCal), a Lead-Scintillator sampling calorimeter. They are dedicated to the measurement and identification of direct photons, light neutral mesons such as π 0 , η and ω ( 782 ) , and jets emitted in proton-proton and heavy-ion collisions at the LHC energies. The PHOS is capable of precisely detecting photons with momentum range between 0.1 GeV/c and 100 GeV/c and the EMCal can extend the prompt photon and light neutral meson momentum measurement beyond 200 GeV/c. The objective of the study is to explore the physics of strongly interacting QCD matter under extreme conditions of energy density.

First-principles study of hydrogen in perfect tungsten crystal

Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies for different interstitial sites were compared to determine the optimal trapping site for the hydrogen atom inside the tungsten lattice. The diffusion barriers for hydrogen atom between nearby trapping sites and the interaction between two interstitial hydrogen atoms were also calculated. The implication of our theoretical results on the hydrogen diffusion and accumulation behavior was discussed.

SU‐FF‐J‐65: Coherent Bremsstrahlung in Image Guided Radiotherapy

Purpose: To determine the suitability of coherent bremsstrahlung (CB) from monocrystalline targets for improving the quality of megavoltage imaging in radiotherapy by selectively enhancing the amount of photons in the energy range useful for diagnostic applications. Method and Materials: A theoretical investigation of CB was carried out in first‐order Born approximation. The interaction cross‐sections derived allow to study features of CB particularly at electron energies in the low MeV regime as used by common clinical linear accelerators. Special emphasis was placed on the comparison with incoherent bremsstrahlung, arising simultaneously. Photon energy distributions were calculated for electron beams hitting various monocrystalline targets at different angles with respect to major crystal axes. The suitability of various elemental crystals for creating low energy photons from megavoltage electrons was determined. Results: Examination of the cross‐sections identified diamond and tungsten crystals to be capable of significantly reducing average photon energies when used as bremsstrahlung targets. The mentioned effect is especially pronounced at electron energies between 10 and 20 MeV and, on a cross‐section level, outperforms ordinary bremsstrahlung produced with 5 MeV electron beams. In case of tungsten a further enhancement can be achieved by cooling the target. Conclusion: Monocrystalline targets carry the potential of creating considerably softer photon spectra than common amorphous or polycrystalline materials. They also offer the possibility of tuning the photon energy distributions by rotating the crystal with respect to the direction of the incident electron beam. Experimental realization, however, appears challenging as the emitted x‐rays are mainly concentrated in the forward direction, thus being unable to illuminate fields of view large enough for IGRT. An array of differently oriented crystals hit by a divergent electron beam is thought to circumvent this limitation.

Quantum Cavity for Spin due to Spin-Orbit Interaction at a Metal Boundary

A quantum cavity for spin is created using a tungsten crystal as substrate of high nuclear charge and breaking the structural inversion symmetry through deposition of a gold quantum film. Spin- and angle-resolved photoelectron spectroscopy shows directly that quantum-well states and the "matrioshka" or Russian nested doll Fermi surface of the gold film are spin polarized and spin-orbit split up to a thickness of at least nine atomic layers. Ferromagnetic materials or external magnetic fields are not required, and the quantum film does not need to possess a high atomic number as analogous results with silver show.

Surface Modification and Deuterium Retention in Tungsten and Molybdenum Exposed to Low-Energy, High Flux Deuterium Plasmas

Deuterium retention in single crystal and polycrystalline tungsten and molybdenum exposed to low-energy (38200 eV/D), high ion flux (10211022 D/m2s) deuterium plasmas at various temperatures were examined with the D(3He,p)4He nuclear reaction at a 3He energy varied from 0.69 to 4.0 MeV, and with thermal desorption spectroscopy. The surface morphology was examined by scanning electron microscope. Blisters formed on the Mo surfaces under plasma exposure are significantly larger in size than those for W. The D retention in the W and Mo samples increases with the exposure temperature, reaching its maximum at about 500 and 530 K (for ion fluxes of 1021 and 1022 D/m2/s), respectively, and then decreases as the temperature grows further. For polycrystalline W and Mo exposed at temperatures above 400 K, the D retention in the bulk (far beyond the ion implanted zone) is dominant. Plastic deformation caused by deuterium super-saturation within the near-surface layer is suggested as a mechanism for blister formation and creation of defects responsible for deuterium trapping at depths up to several micrometers.

Polarized and unpolarized positron sources for electron–positron colliders

A two-step positron facility using a multi-GeV electron beam is under study for electron–positron colliders as ILC or the SuperB factory. In the first step, the electron beam impinges on an oriented tungsten crystal. Channelling radiation provides a large number of γ-rays and consequently creates a large number of pairs in an amorphous target placed after the crystal. Such a method has been successfully tested at CERN and KEK and is now operating at KEK. As a second step and as an upgrade of the system, the electron beam, instead of being sent to the crystal, collides with a circularly polarized laser beam, creating circularly polarized Compton backscattered γ-rays, which impinge on a thin amorphous target where they generate longitudinally polarized positrons. Simulation results are provided for the two steps and the choice of the parameters is discussed for ILC and for the SuperB factory. Particularly, the choice of an ERL instead of a classical linac is recommended for the ILC.

Monochromatic X-ray sources based on a mechanism of real and virtual photon diffraction in crystals

A source of monochromatic X-ray radiation is wanted in industry, science, medicine and so on. Many ways of making such a source are known. The present work describes two mechanisms for the creation of a monochromatic X-ray beam, which are parametric X-ray radiation (PXR) and bremsstrahlung diffraction (DBS). Both the experiments were carried out using an electron beam at a microtron. During the first experiment, the DBS process was investigated as a scattering of the Bremsstrahlung (BS) beam on the crystallographic surfaces of tungsten and pyrolytic graphite crystals. The second experiment consisted in the registration of the PXR and DBS yield during the passage of the electrons through the same crystals as in the first experiment. The spectral and orientation radiation characteristics and simulation results obtained for the DBS and PXR processes are presented. It is shown that the usage of mosaic crystalline targets is rather useful in order to obtain a monochromatic X-ray source based on bremsstrahlung diffraction from moderately relativistic electrons.

Tailoring Self-Assembled Metallic Photonic Crystals for Modified Thermal Emission

We predict that modified thermal emission can result from three-dimensional, self-assembled photonic crystals. In previous tungsten structures, known as inverse opals, strong absorption prevented any influence of the periodicity. We consider the origin of this effect and show how to tailor both absorption and surface coupling in experimentally realizable metallic inverse opals. Calculations for tungsten, molybdenum, and tantalum crystals show that their optical properties can be similar to or even better than the tungsten woodpile, where modified thermal emission has already been seen.

Heterogeneous deformation and spall of an extruded tungsten alloy: plate impact experiments and crystal plasticity modeling

The role of microstructure in the dynamic deformation and fracture of a dual phase, polycrystalline tungsten alloy under high-rate impact loading is investigated via experiments and modeling. The material studied consists of pure tungsten crystals embedded in a ductile binder alloy comprised of tungsten, nickel, and iron. The tungsten crystals are elongated in a preferred direction of extrusion during processing. Plate impact tests were conducted on samples oriented either perpendicular or parallel to the extrusion direction. Spatially resolved interferometric data from these tests were used to extract wave propagation behavior and spall strength dependent upon position in the sample microstructure. Finite element simulations of impact and spall in digitally reproduced microstructural geometries were conducted in parallel with the experiments. Finite deformation crystal plasticity theory describes the behavior of the pure tungsten and binder phases, and a stress- and temperature-based cohesive zone model captures fracture at grain and phase boundaries in the microstructure. In results from both experiments and modeling, the grain orientations affect the free-surface velocity profile and spall behavior. Some aspects of distributions of free-surface velocity and spall strength among different microstructure configurations are qualitatively similar between experimental and numerical results, while others are not as a result of differing scales of resolution and modeling assumptions. Following a comparison of experimental and numerical results for different microstructures, intergranular fracture is identified as an important mechanism underlying the spall event.

Characterisation and application of a SPLEED-based spin polarisation analyser

A commercial electron spin analyser, based on spin-polarised low-energy electron diffraction (SPLEED) from W(1 0 0), has been characterised with incident polarised electron beams from a standard GaAs polarised electron source. The dependence of the Sherman function on the scattering energy and elapse time after CO-flash of the tungsten crystal of the analyser have been measured. The influence of the stray magnetic field on the performance of the analyser has been investigated. The spin analyser has been applied in monitoring the reorientation transition of the easy magnetisation direction of Fe films on W(1 1 0) upon the exposure of CO adsorbent on the surface.

Deuterium trapping in ion-damaged tungsten single crystal

The trapping of plasma-induced deuterium in tungsten single crystal pre-irradiated with high-energy ions of H, D and He and then annealed at 1000K has been studied by thermal desorption spectroscopy. Ion energies well higher and below the threshold of damage production were used for damage and probe irradiation, respectively. It has been shown that D retention in H and D-damaged tungsten crystals correlates with a damage level of these crystals that was calculated by using TRIM program. At the same time, at about the same damage level, D trapping in He-damaged tungsten was about 4 times more than in D-damaged tungsten. The results obtained are connected with some peculiarities of clustering and deuterium trapping in ion-damaged tungsten crystals.

Carbothermic reduction of cobalt and nickel tungstates

Data are presented on the formation sequence of metallic, carbide, and intermediate oxide phases in the carbon reduction of CoWO4 and NiWO4. The surface reaction between CoWO4 particles and solid carbon yields the mixed carbide Co6W6C, while the reaction with the CO originating from carbon vaporization yields the intermetallic phase Co7W6. The initial stage of the solid-state and gas-phase reduction of NiWO4 yields a solid solution of tungsten in nickel (∼10 at % W), Ni4W, and, presumably, the NiWO4-WO3 eutectic. The solid solution reacts with carbon to form Ni2W4C and with CO to form filamentary tungsten crystals.

A field emission microscope in an advanced students' laboratory

This paper describes a university level experiment during which students can observe the surface structure and determine the work function of a clean single tungsten crystal and a crystal covered with barium. The authors used a commercial field emission microscope offered by Leybold Didactic and designed an experiment which can be easily reproduced and performed in a students' laboratory. The use of a digital camera and computer allowed simultaneous observation and imaging of the surface of the body-centred cubic structure of the single tungsten crystal. Some interesting results about the changes in tungsten work function with time and with barium coverage are presented and discussed. The data help to improve knowledge and skills in the calculation of measurement uncertainty.