Search

The Hall electron mobility in sulfur-doped 4H-SiC over a wide range of S concentration was investigated. Sulfur (S) works as a double donor in SiC. The electron concentration in the S + -implanted layers saturates when the S concentration exceeds 1 × 10 18 cm − 3 and the net donor concentration of the S + -implanted layer with S concentration of 1 × 10 19 cm − 3 is 4 × 10 18 cm − 3 , indicating that the solubility or activation limit of S + -implanted SiC is about 2 × 10 18 cm − 3 . The S + -implanted SiC with a S concentration of 1 × 10 18 cm − 3 exhibits an electron mobility of 598 cm 2 /V s, which is more than twice as high as that in N-doped SiC with the same doping concentration (268 cm 2 /V s). The temperature dependence of the electron mobility in S + -implanted SiC is reproduced in the wide temperature range by the calculation of the electron mobility adopting the helium atom model for neutral-impurity scattering. [ABSTRACT FROM AUTHOR]

The sinking of alkali cations in superfluid 4He nanodroplets is investigated theoretically using liquid 4He time-dependent density functional theory at zero temperature. The simulations illustrate the dynamics of the buildup of the first solvation shell around the ions. The number of helium atoms in this shell is found to linearly increase with time during the first stages of the dynamics. This points to a Poissonian capture process, as concluded in the work of Albrechtsen et al. on the primary steps of Na+ solvation in helium droplets [Albrechtsen et al., Nature 623, 319 (2023)]. The energy dissipation rate by helium atom ejection is found to be quite similar between all alkalis, the main difference being a larger energy dissipated per atom for the lighter alkalis at the beginning of the dynamics. In addition, the number of helium atoms in the first solvation shell is found to be lower at the end of the dynamics than at equilibrium for both Li+ and Na+, pointing to a kinetic rather than thermodynamical control of the snowball size for small and strongly attractive ions. [ABSTRACT FROM AUTHOR]

Helium nanodroplets ("HNDs") are widely used for forming tailor-made clusters and molecular complexes in a cold, transparent, and weakly interacting matrix. The characterization of embedded species by mass spectrometry is often complicated by the fragmentation and trapping of ions in the HNDs. Here, we systematically study fragment ion mass spectra of HND-aggregated water and oxygen clusters following their ionization by charge transfer ionization ("CTI") and Penning ionization ("PEI"). While the efficiency of PEI of embedded clusters is lower than for CTI by about factor 10, both the mean sizes of detected water clusters and the relative yields of unprotonated cluster ions are significantly larger, making PEI a "soft ionization" scheme. However, the tendency of ions to remain bound to HNDs leads to a reduced detection efficiency for large HNDs containing > 1 0 4 helium atoms. These results are instrumental in determining optimal conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated in HNDs. [ABSTRACT FROM AUTHOR]

In this work, the distribution and segregation behaviors of helium (He) and the effect of yttrium (Y) on these behaviors in the symmetrical tilt tungsten (W) Σ5(310)/[001] grain boundary (GB) region were studied using first-principles calculations. The results revealed that the GB has a significant impact on the behaviors of He in W. The solution and segregation energies of He in the W Σ5(310)/[001] GB region increase with increasing the distance from He to the GB and are inversely proportional to the effective electrons of He. The density of states analysis showed that the GB can suppress partial hybridization between He and W atoms. In addition, we find that the strengthening element Y facilitates the dissolution of He in the W GB region. [ABSTRACT FROM AUTHOR]

We present a suite of binary evolution models with massive primaries (10 ≤ M 1 ≤ 40 M ⊙) and periods and mass ratios chosen such that the systems undergo nonconservative mass transfer while the primaries have helium cores. We track the total mass and chemical composition of the ejecta from these systems. This material shows the abundance signatures of hot hydrogen burning that are needed to explain the abundance patterns seen in multiple populations in massive star clusters. We then calculate the total yield of a population of binary stars with masses, mass ratios, and periods consistent with their distribution in a field population. We show that the overall abundance of this material is enriched in helium, nitrogen, sodium, and aluminum, and depleted in carbon, oxygen, and magnesium, by amounts that are consistent with observations. We also show that such a population of binaries will return approximately 25% of its mass in this ejecta (compared to 4% if all the stars were single), over a characteristic timescale of about 12 Myr. We argue that massive binaries must be seriously considered as a contributor to the source of enriched material needed to explain the multiple populations in massive clusters, since essentially all massive stars are formed in binaries or higher-order multiples, massive binaries are primarily formed in clusters, and massive binaries naturally produce material of the right composition. [ABSTRACT FROM AUTHOR]

High entropy ceramics are potential structural materials for advanced reactor concepts where numerous helium atoms would be produced due to the (n, α) nuclear transmutation reaction. However, the irradiation behaviors of high entropy ceramics are not well understood until now. In present work, the responses of high entropy (ZrTiNbTa)C 4 and its binary constituent ZrC ceramics to energetic He-ions irradiation have been studied. X-ray diffraction analysis showed that He-ions irradiation resulted in peak shift and broadening of diffraction peaks in both (ZrTiNbTa)C 4 and ZrC, which denotes lattice expansion and structural damages induced by the irradiation. Transmission electron microscope observation revealed that spherical helium bubbles distribute uniformly in the grain interiors of (ZrTiNbTa)C 4 while "string-like" bubbles with a preferred orientation are observed in ZrC grains. Nanoindentation characterization gives that the irradiation induced hardening of (ZrTiNbTa)C 4 is less definite than that of ZrC. The irradiation mechanisms of high entropy ceramics have been discussed. [ABSTRACT FROM AUTHOR]

The ability to precisely engineer nanostructures underpins a wide range of applications in areas such as electronics, optics, and biomedical sciences. Here we present a novel approach for the growth of nanoparticle assemblies that leverages the unique properties of superfluid helium. Unlike viscous solvents at or near room temperature, superfluid helium provides an unperturbed and cold environment in which weak van der Waals interactions between molecular templates and metal atoms become significant and can define the spatial arrangement of nanoparticles. To demonstrate this concept, diol and porphyrin-based molecules are employed as templates to grow gold nanoparticle assemblies in superfluid helium droplets. After soft-landing on a solid surface to remove the helium, transmission electron microscopy (TEM) imaging shows the growth of gold nanoparticles at specific binding sites within the molecular templates where the interaction between gold atoms and the molecular template is at its strongest. The ability to precisely engineer nanostructures is crucial for a wide range of applications in areas such as electronics, optics, and biomedical sciences. Here, the authors present an approach for templated growth of gold nanoparticle assemblies leveraging the weak van der Waals forces between metal atoms and small molecular templates in superfluid helium. [ABSTRACT FROM AUTHOR]

Three-dimensional mapping of surface structures is important in a wide range of biological, technological, healthcare, and research applications. Neutral helium atom beams have been established as a sensitive probe of topography and have already enabled structural information to be obtained from delicate samples where conventional probes would cause damage. Here, we empirically demonstrate, for the first time, a reconstruction of a complete surface profile using measurements from a modified scanning helium microscope using the heliometric stereo method and a single detector instrument geometry. Results for the surface profile of tetrahedral aluminum potassium sulfate crystals demonstrate that the areas of surfaces and facet orientations can be recovered to within 5% of the expected values. [ABSTRACT FROM AUTHOR]

The influence of the non‐ideality (NI) of the classical plasmas on the doubly excited singlet S states of the helium atom (He) embedded in the plasma has been investigated theoretically. A pseudopotential containing the Debye length and the non‐ideality parameter (NIP) as its characteristics is used to represent the screened interaction potentials in the plasma. Using a large wavefunction within the framework of the stabilization method, it has been possible to recognize six doubly excited singlet S states (five lying below the He+(2S) excitation threshold and one lying below the He+(3S) excitation threshold) for the plasma‐free case. The energies and the autoionization life‐times of those states are computed by fitting the density of states to the Lorentzian form. Convergence of the computed results is corroborated by increasing the number of terms in the employed wavefunction. For the plasma‐free case, these results are in excellent agreement with the established results in the literature. A comprehensive analysis has been made on changes induced on those doubly excited states by varying NI over a wide range. It has been observed that the energies of the states gradually approach the corresponding threshold energies with the increasing NI of the plasma, whereas the change in the life‐times (alternatively the widths of the states) of the states shows distinctive features depending on the angular momentum of an individual electron. [ABSTRACT FROM AUTHOR]

Chloronium, H2Cl+, is detected in astrophysical media. It is key chemical intermediate for understanding of the physical chemistry of chlorine species there. At present, we compute the collision rates for the rotational excitation and de-excitation of ortho- and para- H2Cl+ colliding with He for T  ≤ 150 K, relevant for the interstellar medium (ISM) conditions. Prior to that, we generated the 3D interaction potential of the weakly bound H2Cl+-He complex along the Jacobi coordinates. For electronic structure computations, we used a post-Hartree-Fock explicitly correlated method at the CCSD(T)-F12/aug-cc-pVTZ level. The analytical expansion of this potential was incorporated into close coupling computations of the cross-sections for the rotational excitation and de-excitation of ortho- and para- H2Cl+ colliding with He and for collision kinetic energies E k ≤ 1000 cm−1. The rates were obtained after averaging these cross-sections over a Maxwell–Boltzmann distribution of kinetic energies. Our data show that the ∆j=∆ka=∆kc=−1 de-excitation transitions exhibit the largest values, in particular those used to identify this cation in the surveys. Besides, our results should help for determining more accurate abundances of H2Cl+ in the ISM and thus better modelling the chlorine chemistry there. [ABSTRACT FROM AUTHOR]

Hydrogen exploration in the municipality of Maricá (Rio de Janeiro State, Brazil) indicates that important hydrogen and helium gas fluxes are emitted through reactivated faults. The gas concentrations in the soils, and the modelled gas fluxes, are among the most intense measured so far in the world. This exploration has shown new surface expression of hydrogen micro-seeps, observed along reactivated large tectonic accidents instead of "fairy circles", most common proxy for hydrogen systems in cratonic formation. The geology of Maricá, consisting in Neoproterozoic metamorphic rocks without any already documented evidence of hydrogen-prone formations, appears as a very new field of investigation for natural hydrogen. Results indicate the presence of systematic hydrogen saturation of the usual detectors plugged at 80 cm depth, as well as a permanent recharge of hydrogen through time, without relaxation time. Moreover, positive helium anomalies are associated with this hydrogen soil gas, proving the existence of a gas flux coming from deep horizons of the subsurface. This reduces the risk of misinterpretation of surface hydrogen measurement, since the fear of drill bit metamorphism or superficial biological activity inducing hydrogen artefacts is always a possibility. These hydrogen and helium micro-seeps are associated with positive thorium concentration superficial anomalies, as it was already observed in other places associated with hydrogen exudations. We present a model of soil gas behavior, with a differential process of adsorption/desorption of each gas compound (N 2 , O 2 , Ne, H 2 and He), associated with a deep flux of hydrogen and helium. It is possible to assess a quantification of hydrogen flux of 0.32 m3 m−2.day−1. The surface flux ratio He/H 2 is much higher than any published value of generative flux. This may be explained by a subsurface accumulation, where hydrogen is severely altered whereas helium is preserved, increasing the relative proportion of helium versus hydrogen. [Display omitted] • The Marica municipality (Brazil) reveals important potential for natural hydrogen. • Reactivated faults are excellent proxies for natural hydrogen exploration. • Hydrogen seeping out of reactivated faults may be larger than in fairy circles. • Association of hydrogen and helium in soils indicates that no artefact concerns the hydrogen measurements in soils. • Metamorphic formations may be excellent candidates for hydrogen exploration. [ABSTRACT FROM AUTHOR]

The kinetics of O2− ions were numerically studied in He and Ar with a small admixture of O2 in a strong electric field through a Monte Carlo simulation. Cross sections for elastic scattering between O2− ions and He and Ar atoms were determined by adjusting cross section values to obtain good agreement between calculated and measured ion mobilities in pure helium and argon. Ion transport coefficients, mean energy, and detachment rate were calculated for various mole fractions of O2 and values of the reduced electric field. It was shown that the O2− ion mobility, mean energy, and detachment rate in He:O2 mixtures are much higher than those in Ar:O2 mixtures and air. A great difference between the properties of O2− ions in He–O2 and Ar–O2 mixtures was explained on the basis of the difference in the polarizability and mass for He and Ar atoms. Calculated detachment rates can be used for simulating guided ionization waves in He and Ar flows ejected into ambient air. [ABSTRACT FROM AUTHOR]

We propose the application of helium line intensity ratio spectroscopy in a low-pressure (0.3 mTorr) xenon E × B discharge with an electron temperature of ∼2 eV and a density of 10 10 − 10 11 cm − 3 . We successfully identified the helium atom line emissions at 388.9, 447.1, 501.6, 504.8, and 706.5 nm with helium pressures of up to ∼20 mTorr. The measured electron temperature, density, and I − V characteristics of the discharge remained almost constant in all helium pressures in the present experiment, indicating the suitability of the helium gas as a diagnostic gas. The results of helium line intensity ratio spectroscopy using the line emissions at 388.9, 447.1, and 504.8 nm showed fair agreement with the Langmuir probe measurement. Considering the trade-off relationship between the disturbance introduced by the helium gas and the signal-to-noise ratio, we conclude that a helium pressure of approximately 4 mTorr (approximately 13 times the partial pressure of xenon) represents the optimal pressure range for the application of the helium line emission intensity ratio method to this xenon plasma. It is found that the use of the line emissions at 501.6 and 706.5 nm result in a significant disturbance in the helium line intensity ratio method due to the radiation trapping effect. [ABSTRACT FROM AUTHOR]

Cold three-body recombination between helium and silver atoms is studied using hyperspherical coordinates. The three-body Schrodinger equation, represented in the slow variable discretization approach at short distances and in the adiabatic method at large distances and using the potential-energy surface represented as the addition of realistic He-He and He-Ag pair interaction potentials, is solved using the R-matrix propagation method, in order to numerically calculate the three-body recombination rates for the He+He+Ag → He 2 +Ag and He+He+Ag → HeAg+He processes. Not only zero-angular momentum J = 0 states but also J > 0 states are considered in the calculations, allowing for treating the recombination processes at collision energies beyond the threshold regime. The results of our calculations will be presented and discussed. [ABSTRACT FROM AUTHOR]

New heavy particles with electroweak charges arise in extensions of the standard model. They should take part in sphaleron transitions in the early Universe, which balance baryon asymmetry with the excess of new charged particles. If electrically charged with charge − 2 n , they bind with n nuclei of primordial helium in dark atoms of dark matter. This makes it possible to find the ratio of densities of asymmetric dark matter and baryonic matter. Examples of the model with new, successive, and stable generation of quarks and leptons and the minimal walking technicolor model are considered. [ABSTRACT FROM AUTHOR]

In this article, we study the helium atom confined in a spherical impenetrable cavity by using informational measures. We use the Ritz variational method to obtain the energies and wave functions of the confined helium atom as a function of the cavity radius r0$$ {r}_0 $$. As trial wave functions we use one uncorrelated function and five explicitly correlated basis sets in Hylleraas coordinates with different degrees of electronic correlation. We computed the Shannon entropy, Fisher information, Kullback–Leibler entropy, Tsallis entropy, disequilibrium and Fisher–Shannon complexity, as a function of r0$$ {r}_0 $$. We found that these entropic measures are sensitive to electronic correlation and can be used to measure it. As expected these entropic measures are less sensitive to electron correlation in the strong confinement regime (r0<1$$ {r}_0<1 $$ a.u.). [ABSTRACT FROM AUTHOR]

Absorption of stellar X-ray and extreme ultraviolet (EUV) radiation in the upper atmosphere of close-in exoplanets can give rise to hydrodynamic outflows, which may lead to the gradual shedding of their primordial light element envelopes. Excess absorption by neutral helium atoms in the metastable 23S state [He I(2 3S)], at ~10 830 Å, has recently emerged as a viable diagnostic of atmospheric escape. Here we present a public add-on module to the 1D photoionization hydrodynamic code ATES, designed to calculate the He I(2 3S) transmission probability for a broad range of planetary parameters. By relaxing the isothermal outflow assumption, the code enables a self-consistent assessment of the He I(2 3S) absorption depth along with the atmospheric mass-loss rate and the outflow temperature profile, which strongly affects the recombination rate of He II into He I(2 3S). We investigate how the transit signal can be expected to depend upon known system parameters, including host spectral type, orbital distance, and planet gravity. At variance with previous studies, which identified K-type stars as favorable hosts, we conclude that late M dwarfs with Neptune-sized planets orbiting at ~0.05–0.1 AU can be expected to yield the strongest transit signal, well in excess of 30% for near-cosmological He-to-H abundances. More generally, we show that the physics that regulates the population and depletion of the metastable state, combined with geometrical effects, can yield somewhat counterintuitive results, such as a nonmonotonic dependence of the transit depth on orbital distance. These are compounded by a strong degeneracy between the stellar EUV flux intensity and the atmospheric He-to-H abundance, both of which are highly uncertain. Compared with spectroscopy data, now available for over 40 systems, our modeling suggests either that a large fraction of the targets have helium-depleted envelopes or that the input stellar EUV spectra are systematically overestimated. The updated code and transmission probability module are available publicly as an online repository. [ABSTRACT FROM AUTHOR]

Interstellar aromatic molecules such as polycyclic aromatic hydrocarbons and polycyclic nitrogen and oxygen bearing molecules are thought to be abundant in the interstellar medium (ISM). In this class of molecules, benzonitrile (c-C6H5CN) plays an important role as a proxy for benzene. It has been detected through rotational emission in several astrophysical sources and is one of the simplest N-bearing polar aromatic molecules. Even in the cold ISM, the population of the rotational levels of benzonitrile might not be at equilibrium. Consequently, modelling its detected emission lines requires a prior computation of its quenching rate coefficients by the most abundant species in the ISM (He or H2). In this paper, we focus on the excitation of c-C6H5CN by collision with He. We compute the first potential energy surface (PES) using the explicitly correlated coupled cluster method in conjunction with large basis sets. The PES obtained is characterized by a potential well depth of −97.2 cm−1 and an important anisotropy. Scattering computations of the rotational (de-)excitation of c-C6H5CN by He atoms are performed by means of the coupled states approximation that allow to obtain collisional rates for rotational states up to j  = 9 and temperatures up to 40 K. These rate coefficients are then used to examine the effect of C6H5CN excitation induced by collisions with para-H2 in molecular clouds by carrying out simple radiative transfer calculations of the excitation temperatures and show that non-equilibrium effects can be expected for H2 densities up to 105–106 cm−3. [ABSTRACT FROM AUTHOR]

High harmonic generation is a phenomenon that occurs when an atom is subjected to a strong laser field. The laser field must have an intensity of at least 10 14 W / cm 2 to cause tunnel ionization, which releases an electron from the nucleus and allows it to gain energy by moving in the laser field. Upon recombining with the parent ion, the electron radiates its kinetic energy as photons in the XUV region. In this study, In order to find the optimal intensity of each atom, we first simulate and investigate the ionization probability of the noble gas atom. The simulation results show that the saturation intensity of tunnel ionization is about 3.8, 6, 8.1 and 12.5 × 10 14 W / cm 2 for xenon, argon, neon and helium gases, respectively. Using these results, we have been able to obtain the appropriate intensities to produce optimal attosecond pulses for each atoms. Then, according to the saturation intensity of gas ionization, we simulate the high-order harmonic spectrum in three intensities of 3, 5, and 8 × 10 14 W / cm 2 and investigate the effects of atomic and laser parameters on the cut-off point and plateau region of high harmonic generation spectrum. The simulation results show that the intensity and time profile of attosecond pulses are highly dependent on I P , and only the atoms that are in the range of tunnel ionization produced attosecond pulses. Also, our simulation results indicate that increasing the intensity leads to higher cut-off points and plateau regions for the harmonics. Furthermore, we found that helium had the highest cut-off point and plateau region at all three intensities, making it the most efficient noble gas for high harmonic generation. [ABSTRACT FROM AUTHOR]

Neutral helium atom microscopy is a novel microscopy technique which offers strictly surface-sensitive, non-destructive imaging. Several experiments have been published in recent years where images are obtained by scanning a helium beam spot across a surface and recording the variation in scattered intensity at a fixed total scattering angle θ S D and fixed incident angle θ i relative to the overall surface normal. These experiments used a spot obtained by collimating the beam (referred to as helium pinhole microscopy). Alternatively, a beam spot can be created by focusing the beam with an atom optical element. However up till now imaging with a focused helium beam has only been demonstrated in transmission (using a zone plate). Here we present the first reflection images obtained with a focused helium beam (also using a zone plate). Images are obtained with a spot size (FWHM) down to 4. 7 μ m ± 0. 5 μ m, and we demonstrate focusing down to a spot size of about 1 μ m. Furthermore, we present experiments measuring the scattering distribution from a focused helium beam spot. The experiments are done by varying the incoming beam angle θ i while keeping the beam-detector angle θ S D and the point where the beam spot hits the surface fixed - in essence, a microscopy scale realization of a standard helium atom scattering experiment. Our experiments are done using an electron bombardment detector with adjustable signal accumulation, developed particularly for helium microscopy. • The first reflection images from a zone plate helium atom microscope. • Images with a spot size down to 4. 7 μ m ± 0. 5 μ m are presented. • Focusing down to a spot size of about 1 μ m is demonstrated. • Helium atom scattering on the microscopic scale is demonstrated. [ABSTRACT FROM AUTHOR]

• Parameterized interatomic potential for describing helium-metal interactions among five different refractory metals. • Multi-principal element alloys (MPEAs) exhibit resistance to large helium bubble formation, attributed to the reduced thermodynamical driving force to form large He bubbles and the kinetically suppressed motion of He atoms. • Simulations of helium implantation demonstrate that the distribution of injected helium in MPEAs is minimally influenced by surface orientations, in contrast to the significant dependence observed in tungsten. Tungsten is a leading material considered for nuclear fusion applications. However, W usually suffers from severe surface modification under exposure to He ions generated from fusion reactions. The development of high entropy alloys provides a feasible way to overcome such vulnerabilities. Herein, we explore the response of refractory multi-principal element alloys (MPEAs), WTaTi, WTaCrV, and WTaCrVTi under He implantation and compare the results with pure W. To this end, we have parameterized the interatomic interactions between the constitutive metals and He. With the developed empirical potential, we show that the formation and binding energies of He bubbles in MPEAs are lower than those in W, suggesting a reduced thermodynamical driving force to form large He bubbles in MPEAs. Simulations of He implantation further reveal that the distribution of injected He only weakly depends on the surface orientations in MPEAs. In addition, He bubbles in MPEAs are much smaller and discretely positioned. Our results suggest superior resistance of MPEAs to surface modifications under the impact of energetic He ions, rendering them promising candidate materials for nuclear reactors. [ABSTRACT FROM AUTHOR]

The structural properties of doubly excited 1 , 3 F e metastable-bound and resonance states of neutral helium atom under exponentially screened Coulomb potential are studied using explicitly correlated multi-exponent Hylleraas type basis set. Precise energy eigenvalues of 2 p n f ( 1 , 3 F e) states n = 4 − 15 are estimated in the framework of Ritz variational principle. Stabilization method has been employed to calculate the resonance parameters (energy and width) of 1 , 3 F e states below H e + (3 p) and H e + (4 p) thresholds for different screening conditions. The resonance parameters above H e + (3 p) threshold under screened Coulomb environment are reported for the first time in literature. Furthermore, pioneering calculations for the variation of structural properties such as one- and two-particle moments and inter-electronic angles are carried out for both metastable-bound and resonance 1 , 3 F e states of He atom under screened Coulomb potential. The present results may serve as benchmark for future references. [ABSTRACT FROM AUTHOR]

This study aims to reduce the size and density of He bubbles and dislocation loops during the irradiation process by constructing nanostructure engineering through precipitation. The Ni 35 (CoFe) 55 V 5 Nb 5 high-entropy alloy (HEA) with solid solution and aging states underwent He2+ ions irradiation (500 keV at a fluence of 1017 ions∙cm−2) at 500 °C. The results indicate that the preexisting nanoprecipitates increased the surface area for He atoms adsorption, and improved the ability of the alloy to suppress the coarsening of He bubbles. In the aged HEA, He bubble size decreased (∼3.3 ± 0.4 nm), and density significantly reduced (0.58 ± 0.14 × 1023 m−3) compared to the solid solution HEA (∼6.1 ± 0.3 nm, 2.42 ± 0.15 × 1023 m−3). Moreover, the preexisting nanoprecipitates coordinate the movement and recombination of irradiation defects, which overwhelms the energy driving force that improves the stability and growth of dislocation loops due to the reduction of stacking fault energy, thereby suppressing the formation and growth of dislocation loops. The aged HEA exhibited reduced size and density of dislocation loops (∼6.3 ± 0.5 nm and 0.78 ± 0.15 × 1020 m−3) compared to the solid solution HEA (∼8.5 ± 0.6 nm, 3.83 ± 0.13 × 1020 m−3). • The effect of He2+ ion irradiation at 500 °C on the microstructure evolution of a high entropy alloy was investigated. • The high-density nanoprecipitates can increase the surface area of particles capable of adsorbing helium atoms. • The nanoprecipitates acting as sink points for radiation defects suppress the accumulation of radiation defects. [ABSTRACT FROM AUTHOR]

A scanning helium microscope typically utilises a thermal energy helium atom beam, with an energy and wavelength (¡100 meV, ∼ 0.05 nm) particularly sensitive to surface structure. An angular detector stage for a scanning helium microscope is presented that facilitates the in-situ measurement of scattering distributions from a sample. We begin by demonstrating typical elastic and inelastic scattering from ordered surfaces. We then go on to show the role of topography in diffuse scattering from disordered surfaces, observing deviations from simple cosine scattering. In total, these studies demonstrate the wealth of information that is encoded into the scattering distributions obtained with the technique. • A scanning helium microscope uses a neutral beam of helium atoms to image surfaces. • An angular detector stage is presented to measure helium scattering distributions. • Present elastic and inelastic diffraction of helium atoms from LiF and HOPG. • Demonstrate the role of topography in diffuse scattering from disordered surfaces. • Show the wealth of information encoded into scattering distributions. [ABSTRACT FROM AUTHOR]

A refractory WTaCrVTi high-entropy alloy (HEA) film was prepared by magnetron sputtering, and a pure tungsten (W) film was also prepared in the same way used as a comparison sample. The electrical resistivity of the films in the temperature range of 4 K–300 K was measured by the standard four-probe method and the results showed that the resistivity of the HEA film is about 10 times higher than that of the W film. Furthermore, the thermal conductivity of the film sample is calculated by Wiedemann-Franz law. The results show that the thermal conductivity of the HEA film at 300 K is about 10 times lower than that of the W film. Deuterium and helium retention and desorption behavior induced by deuterium and helium plasma exposure of the two films were investigated. The total deuterium retention measured by thermal desorption spectra (TDS) in the HEA film was approximately 16 times larger than that in the W film, which was attributed to the existence of more grain boundary defects and vacancy-type defects in HEA film, leading to more deuterium trapped. The characteristics of helium bubbles in the two films were compared. The results showed that the helium bubbles in the HEA film had a shallower depth distribution and smaller size than those in the W film, which indicated that the bubbles growth evolution was suppressed in the HEA film compared to the W film. It is suggested that helium atoms in the HEA film experience larger lattice potential energy fluctuations due to the abundant hopping processes during diffusion and aggregation. [ABSTRACT FROM AUTHOR]

FeCoCrNiAlx-based high entropy alloys (HEAs) exhibit excellent high-temperature strength, good radiation tolerance and corrosion resistance. However, studies on light-ion irradiation damage are relatively lacking. The influence of hydrogen and helium effect on HEAs deserves further study, given their complex interaction during different sequence irradiation at different irradiation conditions. In the present study, FeCoCrNiAl 0.3 HEA was irradiated under two different modes at 723 K, one is single He2+ and the other is pre-iradiation of H+ followed by He2+. Microstructure evolution and irradiation hardening were studied by transmission electron microscopy and nanoindentation technique. There is no distinct difference of the irradiation hardening rate between sequential H+-He2+ and single He2+ irradiated samples. A large number of irradiation-induced nano-defects were observed after pre-irradiation of H+, which act as sinks to trap the subsequent helium atoms in the H+-He2+ irradiated sample. Fine helium bubbles and dislocation loops with high density dispersed in both irradiated samples. Compared with single He2+ irradiation, pre-irradiation of H+ resulted in a decrease in bubble density and an increase in loop density in H+-He2+ irradiated sample, but had little effect on their size. It is speculated that the critical concentration of bubble formation originating from H-He-V clusters is supposed to be higher than that of He V clusters at elevated temperatures in FeCoCrNiAl 0.3 alloy, leading to the aggregation and redistribution of He atoms at the H-He-V clusters rather than the formation of stable bubble nucleus after succeeding He2+ irradiation. The influence of pre-irradiation of H+ on FeCoCrNiAl 0.3 alloy on the formation of helium bubbles and the relationship between bubble, loop and irradiation hardening are discussed. • The pre-irradiation of H formed interstitial-type dislocation loops and much SFTs were observed after H-He irradiation. • Fine helium bubbles and dislocation loops with high density dispersed in both irradiated samples. • The average size of loops is similar while the density in H-He irradiated sample is about twice that of He irradiated one. • Little difference in average size of bubbles while density of H-He irradiated sample is much lower than that of He irradiation. [ABSTRACT FROM AUTHOR]

• The He-rich hot spring waters are mainly Na-HCO3-Cl type affected mainly by water-rock interaction and mixing. • The estimated reservoir temperature is 106 – 118 °C. • Snow-melt water and meteoric precipitation from surrounding mountains at elevation ∼5366 m is the derived source for He-rich geothermal fluids. • Cretaceous granites serve as helium and heat sources, and are reservoir rocks. • High-radioactive granite, tectonic system, and surface hydrothermal manifestations are indicators for exploration of He-rich geothermal resources. The Kongchutso geothermal field hosts helium-rich hot springs with helium contents (up to 1.54 vol.%) higher than industrial grade of He resources (0.05 – 0.1 vol.%). However, the geochemical signature of helium-rich geothermal fluids, reservoir feature, and their genesis remain poorly understood. Here, we present element geochemistry (major and trace elements) and multi-isotopes (H, O, Sr, and Li) compositions of hot and cold springs, lake, calcareous sinters, and rocks from Kongchutso to determine the formation mechanism of helium-rich geothermal system. The major elements studies of water samples show that most helium-rich hot spring waters belong to the Na-HCO 3 -Cl type, possibly affected by water-rock interaction in the deep and mixing of geothermal fluids with groundwater in the shallow. Based on chalcedony geothermometers and silica enthalpy mixing model, the reservoir temperature has been calculated to be 106 – 118 °C. The H and O isotopic compositions of water samples manifest that the Kongchutso geothermal system was recharged by snow-melt water and meteoric precipitation from the surrounding mountains at an elevation of 5366 m. The widespread Cretaceous granites in the region featured by high enrichment of uranium (up to 34.15 ppm) and thorium (up to 37.49 ppm) are considered as helium and heat sources, and also are reservoir rocks. The interpretation is supported by consistent signatures of rare earth elements and Sr-Li isotopes for waters, calcareous sinters, and granites. Therefore, we suggest that the helium was initially generated via radioactive decay by U-Th in granites and then dissolved in the deep circulated and heated meteoric waters, and finally ascended to surface through the main fault systems to form the helium-rich hot springs at Kongchutso. Some other hydrothermal manifestations, such as calcareous sinters, formed accompanying the fluids migrated to the surface. Based on these new findings, we further proposed that high radioactive granite, together with tectonic system and surface hydrothermal manifestations can be used as indicators for exploration of helium-rich geothermal resources in Tibet and elsewhere in the world. [Display omitted] [ABSTRACT FROM AUTHOR]

Tungsten, as a kind of material for the plasma-facing wall in fusion reactor and tokamak, is subjected to irradiation by the high-energy neutron and the high fluxes of hydrogen and helium plasmas. Especially, in the presence of helium (He) bubbles, neutron irradiation can significantly exacerbate the deterioration of the material's mechanical properties. However, due to experimental limitations, the evolution of He bubbles in tungsten irradiated by neutrons and the underlying mechanisms remain unclear. Here, molecular dynamics (MD) simulation is used to investigate the behavior of He bubbles in tungsten with different crystal surfaces irradiated by neutrons. We observed the complete evolutionary process of the He bubble, i.e. standing, expanding, and bursting. During this process, high-energy neutron transfers energy to atoms within the He bubble through cascade collision, leading to a rapid increase in energy and pressure of the He bubble, and then the rapid expansion of the He bubble makes the pressure rapidly released, ultimately leading to the rupture of the helium bubble. Furthermore, we also explore the evolution of the He bubble under varying conditions (such as temperature, neutron energy, and He/vacancy ratio) within the same crystal surface. We found that at lower temperatures and neutron energy with a higher He/vacancy ratio, it tends to form pinhole-like channels to release helium atoms, resulting in the He bubble bursting. On the contrary, it tends to form a hill-like expansion to spurt helium atoms out, leading to the bubble bursting. Additionally, it was found that the direction and time of He bubble rupture varied under different crystal surfaces. He bubbles under the (112) surface burst the slowest with excellent corrosion resistance. These findings provide new insights into the behavior of irradiation resistance of fusion reactor materials under extreme conditions and offer significant guidance for the improvement and selection of optimal materials for plasma-facing walls. [ABSTRACT FROM AUTHOR]

Understanding the effect of the interaction between hydrogen (H) and helium (He) in tungsten (W)-based plasma-facing materials is essential for predicting materials performance in the context of fusion environments. In this work, the effect of H-He interaction on their segregation, accumulation and desorption behaviors at typical W/HfC interfaces were systematically investigated by first-principles calculations. Both H and He atoms tend to segregate at the W/HfC interfaces, where H atoms are discretely distributed and He atoms aggregate in a closely packed arrangement between W(110) planes. At the interface with a vacancy, the pre-existing He atoms can mitigate the accumulation of H atoms and promote their diffusion, which accounts for the reduction of H retention in W-carbide composites as observed in experiments. However, the pre-captured H has a minimal impact on the trapping of He. Furthermore, the maximum desorption temperatures of H and He from the interface are evaluated, ranging from 400 to 550 K and 700 to 1600 K, respectively. The pre-existing He atoms at the interfacial W vacancy also lower the desorption temperature of H. This work provides deep insight at the atomic scale towards a better understanding of the effect of H-He interaction on their retention and desorption in plasma facing-materials in the fusion environment. [ABSTRACT FROM AUTHOR]

Metallic glasses commonly demonstrate softening due to excessive free volume or hardening through partial nanocrystallization when subjected to irradiation. However, this study demonstrates a unique strengthening effect on plasticity without sacrificing hardness in Zr-based metallic glass under low-dose helium ion irradiation in the absence of nanocrystals. Nanoindentation hardness measurements reveal an initial increase in hardness with low irradiation doses, followed by a rapid decrease at higher doses. In situ mechanical transmission electron microscopy experiments confirmed the switch from localized shear banding to homogeneous shear flow with low-dose helium ion irradiation. Notably, a remarkable increase in tensile plasticity ∼18 % was achieved without compromising the strength ∼1.74 GPa. The improvement can be attributed to the formation of local dense packing regions filled with helium atoms, which elevate the critical stress for plastic deformation. These regions interact with the loosely packed surroundings and facilitate shear band formation. Our findings offer invaluable atomic-scale insight into the structure-property relationships in irradiated metallic glasses, thereby supporting their potential applications in advanced fission and fusion reactors. [ABSTRACT FROM AUTHOR]

In this study, molecular dynamics (MD) simulations were used to systematically investigate the behaviour of hydrogen (H) and helium (He) around 1/2<111> screw dislocations (SDs) and 1/2<111>{110} edge dislocations (EDs) in tungsten (W). The binding energy distributions of H and He around these two dislocations were obtained by molecular statics methods. The results revealed that both types of dislocation cores strongly attracted H and He atoms, and capture strengths were stronger for EDs than SDs. The simulations predicted that two factors influenced the interaction: strain fields of dislocations (effects of dislocations on H/He atoms) and residual stresses of H/He atoms (influences of H/He atoms on dislocations), which were validated. In the selection of trapping sites, H was synergistically affected by hydrostatic compressive and shear stress fields, and He was influenced only by shear stress fields. Conversely, the insertion of H atoms reduced the local lattice distortion of the SDs and induced the EDs to glide. He atoms repelled neighbouring atoms to increase the free volume of the trapping site. The diffusion behaviour of H/He atoms around dislocation cores was studied through nudged elastic band (NEB) and MD methods. Both H and He atoms tended to diffuse along screw channels around SD lines and one-dimensional channels inclined to ED lines, preferentially migrating along the dislocation line. The results of this study provide new insight into the retention and diffusion of H and He atoms in W. However, we have not been able to determine the clear influence of dislocations on the retention amount and diffusion tendency of H or He atom. [ABSTRACT FROM AUTHOR]

A gas discharge is simulated and numerically studied. The discharge is excited by a tubular relativistic electron beam in gaseous helium in a coaxial gas-discharge chamber. A magnetic field in the form of a cusp was imposed on the chamber. The code based on the PiC/MC-method was used. Electron beam dynamics, time evolution of phase portraits of particles, physical parameters, and spatial-time characteristics of plasma were calculated. It is shown that a dense plasma appears in the cavity under the action of an electron beam in the form of a thin-walled tube of a large length; its concentration can be n H e + ≈ 2 × 1 0 10 cm−3. At the same time, it turned out that plasma is considerably non-uniform in length; its denser part corresponds to the parts where beams stop. Time dependence of the charge compensation of electrons on the helium atom concentration in the chamber was calculated. The dependence turned out to be decreasing since the rate of the discharge development is increasing with the growth of the helium pressure in the cavity. [Display omitted] • A relativistic electron-beam discharge in He near magnetic cusp is simulated. • The discharge is excited by a tubular electron beam in He gas in a coaxial chamber. • A gas discharge in cusp and its kinetic characteristics is studied by PiC-code KARAT. • Dynamic and kinetic characteristics of such a plasma-beam He discharge were studied. [ABSTRACT FROM AUTHOR]

This paper is about a " number [which] has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it. " [R. P. Feynman, QED: The strange theory of light and matter , Princeton 1985]. This number —Sommerfeld's fine-structure constant α = e 2 / ħ c ∼ 1 / 137 — appears quite unexpectedly in the nonlinear mean-field description of a non-relativistic quantum system; namely, quantum-dot helium constituted by a pair of 2d parabolically-confined opposite-spin electrons entangled in the same singlet state. I show by an iteration procedure provided by a quickly convergent series of nonlinear eigenstates that the non-orthogonality (or inner product) of the two lowest " s " orbitals approaches asymptotically, within less than 0.5% relative error, QED's e i π α electronic probability amplitude for photon emission/absorption. I explain the appearance of α in the non-relativistic mixed-state quantum system by stimulated transitions between the two non-orthogonal eigenstates due to the presence of QED virtual photons generated by the nonlinearity of the system. • The numerical value ∼ 1 / 137 of the fine-structure constant α is " one of the greatest damn mysteries in physics. " [Feynman, QED , Princeton, 1985]. • A nonlinear mixed-state integro-differential description of quantum dot helium provides a mathematical solution with 99% accuracy to this riddle. • Quantum nonlinearity defines eigenstate overlap and generates virtual QED photons. • These photons stimulate transitions between eigenstates with probability proportional to α. • Comparing QED's virtual photon transitions with eigenstate overlap yields the 99% accurate numerical value of α. [ABSTRACT FROM AUTHOR]