Range Of Electron Densities Research Articles

Commissioning of a powerful electron gun for electron–ion crossed-beams experiments

The sensitivity of an electron–ion crossed-beams experiment is mainly determined by the densities of both beams in the interaction region. Aiming at the extension of the available range of accessible electron energies and densities, a new high-power electron gun has been developed and fabricated. It delivers a ribbon-shaped beam with currents of up to 1A at variable energies reaching 3500eV at the maximum. The design goals of the gun are being met by using a configuration of electrodes, which allows for a variety of operation modes. Here, we report on the results of the ongoing tests of the new electron gun. In particular, electron-impact ionization cross sections of He+ and Xe5+ ions were measured employing the animated crossed-beams technique. The results are in excellent agreement with literature data.

The effects of gap parameter and spin polarization on electronic Hartree and correlation energies of doped graphene nanoribbon

We study the behaviors of both Hartree and correlation energies of undoped gapped armchair graphene nanoribbon using random phase approximation in the context of Hubbard model Hamiltonian. Specially, the effects of spin polarization and gap parameter on electron density dependence of Hartree and correlation energies of armchair graphene nanoribbon has been addressed. Our results show the variation of gap parameter leads to considerable effect on correlation and Hartree energy behavior of spin unpolarized gapped graphene in the middle electron density region. However local Hubbard interaction parameter affects the behaviors of Hartree and correlation energy on the whole range of electron density in zero magnetization case. We also show that a considerable reduction has been observed for density dependence of Hartree and correlation energies of spin polarized gapped graphene nanoribbon.

Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction

A detailed theoretical and experimental analysis is performed for a wide range of laser operating conditions, typical for laser induced breakdown spectroscopy (LIBS) and laser ablation (LA) experiments on copper metallic target. The plasma parameters were experimentally estimated from the line intensities ratio which reflects the relative population of neutral excited species in the plasma. In the case of LA experiments the highest temperature observed was 8210±370K. In case of LIBS measurements, a maximum temperature of 8123K has been determined. The experimental results are in good agreement with a stationary, hydrodynamic model. We have theoretically investigated the plasma emission based on the generalized collisional-radiative model as implemented in the ADAS interconnected set of computer codes and data collections. The ionic population density distribution over the ground and excited states into the cooper plasma is graphically displayed as output from the code. The theoretical line intensity ratios are in good agreement with experimental values for the electron density and temperature range measured in our experiments.

Zeeman splitting of the conduction band of HgTe quantum wells with a semimetallic spectrum

Shubnikov–de Haas oscillations in Hg1-xCdx Te/HgTe/Hg1-xCdxTe structures with the widths of a well of 16 and 20 nm have been investigated in tilted magnetic fields. The spin-to-orbital splitting ratio in the conduction band has been found in a wide range of electron densities. The magnitude and density dependence of the ratio agree fairly well with the calculations of the spectrum in the 8-band kP model. It has been shown that the effective g factor is anisotropic, g|| < g┴. The anisotropy is very high at low densities but decreases rapidly with an increase in the density, approaching unity at ne = (3−4) × 1011 cm-2.

Stark broadening measurements in plasmas produced by laser ablation of hydrogen containing compounds

We present a method for the measurement of Stark broadening parameters of atomic and ionic spectral lines based on laser ablation of hydrogen containing compounds. Therefore, plume emission spectra, recorded with an echelle spectrometer coupled to a gated detector, were compared to the spectral radiance of a plasma in local thermal equilibrium. Producing material ablation with ultraviolet nanosecond laser pulses in argon at near atmospheric pressure, the recordings take advantage of the spatially uniform distributions of electron density and temperature within the ablated vapor. By changing the delay between laser pulse and detector gate, the electron density could be varied by more than two orders of magnitude while the temperature was altered in the range from 6,000 to 14,000K. The Stark broadening parameters of transitions were derived from their simultaneous observation with the hydrogen Balmer alpha line. In addition, assuming a linear increase of Stark widths and shifts with electron density for non-hydrogenic lines, our measurements indicate a change of the Stark broadening-dependence of Hα over the considered electron density range. The presented results obtained for hydrated calcium sulfate (CaSO4⋅2H2O) can be extended to any kind of hydrogen containing compounds.

TH‐AB‐201‐09

Purpose:To determine the k_(Q_msr,Q)^(f_msr,f_ref) factor, introduced in the small field formalism for five common type chambers used in the calibration of Leksell Gamma‐Knife Perfexion model over a range of different phantom electron densities.Methods:Five chamber types including Exradin‐A16, A14SL, A14, A1SL and IBA‐CC04 are modeled in EGSnrc and PENELOPE Monte Carlo codes using the blueprints provided by the manufacturers. The chambers are placed in a previously proposed water‐filled phantom and four 16‐cm diameter spherical phantoms made of liquid water, Solid Water, ABS and polystyrene. Dose to the cavity of the chambers and a small water volume are calculated using EGSnrc/PENELOPE codes. The calculations are performed over a range of phantom electron densities for two chamber orientations. Using the calculated dose‐ratio in reference and machine specific reference field, the k_(Q_msr,Q)^(f_msr,f_ref) factor can be determined.Results:When chambers are placed along the symmetry axis of the collimator block (z‐axis), the CC04 requires the smallest correction followed by A1SL and A16. However, when detectors are placed perpendicular to z‐axis, A14SL needs the smallest and A16 the largest correction. Moreover, an increase in the phantom electron density results in a linear increase in the k_(Q_msr,Q)^(f_msr,f_ref). Depending on the chambers, the agreement between this study and a previous study performed varies between 0.05–0.70% for liquid water, 0.07–0.85% for Solid Water and 0.00–0.60% for ABS phantoms. After applying the EGSnrc‐calculated k_(Q_msr,Q)^(f_msr,f_ref) factors for A16 to the previously measured dose‐rates in liquid water, Solid Water and ABS normalized to the dose‐rate measured with TG‐21 protocol and ABS phantom, the dose‐rate ratios are found to be 1.004±0.002, 0.996±0.002 and 0.998±0.002 (3σ) respectively.Conclusion:Knowing the electron density of the phantoms, the calculated k_(Q_msr,Q)^(f_msr,f_ref) values in this work will enable users to apply the appropriate correction for their own specific phantom material.LM acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290)

Spin-orbit splitting of valence and conduction bands in HgTe quantum wells near the Dirac point

Energy spectra both of the conduction and valence bands of the HgTe quantum wells with a width close to the Dirac point were studied experimentally. Simultaneous analysis of the Shubnikov-de Haas oscillations and Hall effect over a wide range of electron and hole densities gives surprising result: the top of the valence band is strongly split by spin-orbit interaction while the splitting of the conduction band is absent, within experimental accuracy. Astonishingly, but such a ratio of the splitting values is observed as for structures with normal spectrum so for structures with inverted one. These results do not consistent with the results of kP calculations, in which the smooth electric filed across the quantum well is only reckoned in. It is shown that taking into account the asymmetry of the quantum well interfaces within a tight-binding method gives reasonable agreement with the experimental data.

Modification of magicity toward the dripline and its impact on electron-capture rates for stellar core collapse

The importance of microphysical inputs from laboratory nuclear experiments and theoretical nuclear structure calculations in the understanding of the core collapse dynamics, and the subsequent supernova explosion, is largely recognized in the recent literature. In this work, we analyze the impact of the masses of very neutron rich nuclei on the matter composition during collapse, and the corresponding electron capture rate. To this aim, we introduce an empirical modification of the popular Duflo-Zuker mass model to account for possible shell quenching far from stability, and study the effect of the quenching on the average electron capture rate. We show that the preeminence of the $N=50$ and $N=82$ closed shells in the collapse dynamics is considerably decreased if the shell gaps are reduced in the region of $^{78}$Ni and beyond. As a consequence, local modifications of the overall electron capture rate up to 30\% can be expected, with integrated values strongly dependent on the stiffness of magicity quenching and progenitor mass and potential important consequences on the entropy generation, the neutrino emissivity, and the mass of the core at bounce. Our work underlines the importance of new experimental measurements in this region of the nuclear chart, the most crucial information being the nuclear mass and the Gamow-Teller strength. Reliable microscopic calculations of the associated elementary rate, in a wide range of temperatures and electron densities, optimized on these new empirical information, will be additionally needed to get quantitative predictions of the collapse dynamics.

Quadrature laser interferometry in the pulsed plasma diagnostic

The article provides an overview of methods for plasma diagnostics based on the laser interferometry with quadrature recording of informative signals. Quadrature interferometers allow providing a wide dynamic range for measurements of electron densities while maintaining the homogeneous differential sensitivity. The existing range of linear electron density measuring with quadrature interferometers in pulsed plasma installations equals to 1010-1019 cm2.

Theoretical Study of Laser Emission for C-Like (Ar XIII), (Ti XVII) and (Fe XXI)

Energy levels, transition probability and oscillator strengths have been calculated for the Ar XIII, Ti XVII and Fe XXI. The configurations included in the calculations are 2s2 2p2, 2s2 2p 3l (l = s, p & d) and 4l (l = s, p, d, & f) of C-like Ar XIII, Ti XVII & Fe XXI which has 69 fine structures by using the fully relativistic flexible atomic code (FAC) program. These data are used in the determination of the reduced population and gain coefficients over a wide range of electron densities from (10+18 to 10+23) and at various plasmas temperatures. The results show that the transitions in Ar18+, Ti22+, and Fe26+ ions are the most promising laser emission lines in the XUV and soft X-ray spectral regions.

Stark shift of neutral helium lines in low temperature dense plasma and the influence of Debye shielding

In this work, we present the results of an experimental study of the Stark shifts of atomic helium lines at 706.519, 728.135, 471.315, 501.568, 667.815 and 447.148 nm in the electron density range (0.6–7) × 1023 m−3. Stark shift is always presented together with the corresponding electron density and electron temperature. Best-fitting formulas of experimental data enable plasma electron density diagnostics with an accuracy of (15–20 per cent) in the temperature range (10 000–30 000) K. The influence of Debye shielding on the Stark shifts is carefully examined and the experimental results are compared with other experimental and theoretical data.

X-RAYING THE CORONAE OF HD 155555

We present an analysis of the high-resolution Chandra observation of the multiple system, HD 155555 (an RS CVn type binary system, HD 155555 AB, and its spatially resolved low-mass companion HD 155555 C). This is an intriguing system which shows properties of both an active pre-main sequence star and a synchronised (main sequence) binary. We obtain the emission measure distribution, temperature structures, plasma densities, and abundances of this system and compare them with the coronal properties of other young/active stars. HD 155555 AB and HD 155555 C produce copious X-ray emission with log Lx of 30.54 and 29.30, respectively, in the 0.3-6.0 keV energy band. The light curves of individual stars show variability on timescales of few minutes to hours. We analyse the dispersed spectra and reconstruct the emission measure distribution using spectral line analysis. The resulting elemental abundances exhibit inverse first ionisation potential effect in both cases. An analysis of He-like triplets yields a range of coronal electron densities ~10^10-10^13 cm-3. Since HD 155555 AB is classified both as an RS CVn and a PMS star, we compare our results with those of other slightly older active main-sequence stars and T Tauri stars, which indicates that the coronal properties of HD 155555 AB closely resemble that of an older RS CVn binary rather than a younger PMS star. Our results also suggests that the properties of HD 155555 C is very similar to those of other active M dwarfs.

Entropy and thermopower in the 2D t-J model

The entropy of the two-dimensional t-J model is investigated using its 12th order high temperature series. A direct Padé extrapolation of the entropy series doesn't converge well for temperatures below T ∼ J. The series coefficients are exact polynomials so the series convergence can be improved by modifying the series that is extrapolated. By subtracting a scaled version of the series for the entropy of the Heisenberg antiferromagnet from the t-J entropy series the low temperature convergence is greatly improved. Using this technique results are obtained for the full range of electron densities and temperatures. The electron density is an adjustable parameter in the series coefficients allowing the density dependence of the entropy and the density derivative at fixed temperature ∂S/∂n|T to be determined accurately. The density derivative depends strongly on temperature, unlike noninteracting models. The density derivative is also an approximation to the experimentally measured thermopower.

LABORATORY MEASUREMENTS OF WHITE DWARF PHOTOSPHERIC SPECTRAL LINES: Hβ

We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating white dwarf photospheric conditions. Here we present time-resolved measurements of H$\beta$ and fit this line using different theoretical line profiles to diagnose electron density, $n_{\rm e}$, and $n=2$ level population, $n_2$. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from $n_{\rm e}\sim4$ to $\sim30\times10^{16}\,$cm$^{-3}$ throughout a 120-ns evolution of our plasma. Also, we observe $n_2$ to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within $\sim55\,$ns to become consistent with LTE. This supports our electron-temperature determination of $T_{\rm e}\sim1.3\,$eV ($\sim15,000\,$K) after this time. At $n_{\rm e}\gtrsim10^{17}\,$cm$^{-3}$, we find that computer-simulation-based line-profile calculations provide better fits (lower reduced $\chi^2$) than the line profiles currently used in the white dwarf astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.

MO‐F‐CAMPUS‐J‐04: Tissue Segmentation‐Based MR Electron Density Mapping Method for MR‐Only Radiation Treatment Planning of Brain

Purpose:Automatically derive electron density of tissues using MR images and generate a pseudo‐CT for MR‐only treatment planning of brain tumours.Methods:20 stereotactic radiosurgery (SRS) patients’ T1‐weighted MR images and CT images were retrospectively acquired. First, a semi‐automated tissue segmentation algorithm was developed to differentiate tissues with similar MR intensities and large differences in electron densities. The method started with approximately 12 slices of manually contoured spatial regions containing sinuses and airways, then air, bone, brain, cerebrospinal fluid (CSF) and eyes were automatically segmented using edge detection and anatomical information including location, shape, tissue uniformity and relative intensity distribution. Next, soft tissues ‐ muscle and fat were segmented based on their relative intensity histogram. Finally, intensities of voxels in each segmented tissue were mapped into their electron density range to generate pseudo‐CT by linearly fitting their relative intensity histograms. Co‐registered CT was used as a ground truth. The bone segmentations of pseudo‐CT were compared with those of co‐registered CT obtained by using a 300HU threshold. The average distances between voxels on external edges of the skull of pseudo‐CT and CT in three axial, coronal and sagittal slices with the largest width of skull were calculated. The mean absolute electron density (in Hounsfield unit) difference of voxels in each segmented tissues was calculated.Results:The average of distances between voxels on external skull from pseudo‐CT and CT were 0.6±1.1mm (mean±1SD). The mean absolute electron density differences for bone, brain, CSF, muscle and fat are 78±114 HU, and 21±8 HU, 14±29 HU, 57±37 HU, and 31±63 HU, respectively.Conclusion:The semi‐automated MR electron density mapping technique was developed using T1‐weighted MR images. The generated pseudo‐CT is comparable to that of CT in terms of anatomical position of tissues and similarity of electron density assignment. This method can allow MR‐only treatment planning.

The importance of gold‐electrode‐adjacent stationary high‐field Böer domains for the photoconductivity of CdS

When the electron density decreases stronger than linearly with the electric field in photoconductive CdS due to field quenching, high‐field domains must occur that remain attached to either the cathode or anode in slit electrode geometry with blocking cathodes. These Böer domains1 are easily seen by their shift in optical absorption due to the Franz‐Keldysh effect and offer unique opportunities to analyze field dependent parameters within the range of constant electron density and electric field, such as the carrier density or mobility as a function of the field, and give information of the light dependent work function. They also provide insight why a 200 Å thick cover layer of CdS on top of a CdTe solar cell increases its efficiency from 8 to 16% . The behavior of these Böer domains escapes conventional current voltage analyses except for their visual observation, while other high‐field domains with their current fluctuations or oscillations are easily observed and are the subjects of thousands of publications and many books. In this review we will exclude detailed discussion of dynamic domains, but include some new specifics that help to understand the mechanisms of the Böer domains and their applications. Only properties at low optical excitation intensities are discussed that exclude Joules heating. Within the p‐type regime of the anode‐adjacent domain extremely steep electronic quenching signal becomes visible that could signalize an intrinsic donor level slightly above the middle of the band gap that may be responsible for not allowing CdS to ever become p‐type by doping. image

Transmission and Reflection of Terahertz Plasmons in Two-Dimensional Plasmonic Devices

Plasmons in two-dimensional semiconductor devices will be reflected by discontinuities, notably, junctions between gated and non-gated electron channels. The transmitted and reflected plasmons can form spatially- and frequency-varying signals, and their understanding is important for the design of terahertz detectors, oscillators, and plasmonic crystals. Using mode decomposition, we studied terahertz plasmons incident on a junction between a gated and a nongated channel. The plasmon reflection and transmission coefficients were found numerically and analytically and studied between 0.3 and 1 THz for a range of electron densities. At higher frequencies, we could describe the plasmons by a simplified model of channels in homogeneous dielectrics, for which the analytical approximations were accurate. At low frequencies, however, the full geometry and mode spectrum had to be taken into account. The results agreed with simulations by the finite-element method. Mode decomposition thus proved to be a powerful method for plasmonic devices, combining the rigor of complete solutions of Maxwell's equations with the convenience of analytical expressions.

Comparisons and analyses of the aluminum K-shell spectroscopic models

Comparing different collisional-radiative models is of great importance for validating the models for plasma spectroscopy and improving the diagnostic accuracy of plasma parameters. In this paper, the widely applied K-shell spectroscopic models, FAC and FLYCHK, are compared based on their calculation results of the aluminum K-shell emissivity and absorption coefficient. The state abundances, K-shell line ratios, K-shell emissivities and absorption coefficients in a wide range of plasma temperatures and densities are calculated and compared, and the reasons for the differences between these two models are discussed. In an electron temperature range from 200 to 800 eV, and an electron density range from 1017 to 1024 cm-3, the Al ions in the plasma are mainly composed of H-like and He-like ions. The ground-state populations of the H-like and He-like ions, calculated from FAC model, are in good agreement with the results from FLYCHK. Number densities of the excited states are two orders or more less than those of the ground states from both the models, and significant differences are observed in the number densities of n=2 and n=3 states of both the H-like and He-like ions. These differences will further result in the differences in spectral line emissivity and their line emissivity ratio, such as He-IC/He-αup and H-βup/He-βup, which are key parameters used to diagnose the electron temperature and density. The line emissivity ratio Ly-αup/(He-αup+He-IC) is less dependent on the electron density, and the difference in line emissivity ratio between the two models mainly lies in the parameter region where both the electron temperature and density are high. The ratio He-IC/He-αup is less dependent on the electron temperature when the electron density is more than 1019 cm-3 while significant differences are observed at a lower electron density.#br#The reason for the difference between the number densities of the low-energy excited states from FAC and FLYCHK models is analyzed by comparing the rate coefficients of various collisional and radiative processes in the rate equation of each state. The differences in the n=2 excited states of H-like ions come from the fact that FAC and FLYCHK models use the detailed-level model and the super-configuration model respectively to construct the rate equations of these states. The FAC model ignores the collisional excitation and de-excitation processes between the n=3 state and higher excitation states (e.g. n = 4) in H-like and He-like ions, which are responsible for the density difference in the n=3 excited state. Higher Rydberg states considered in FLYCHK model do not have any significant influence on the density of the ground-states. The difference in the absorption coefficient between the two models is smaller than that in the emissivity as discussed above, for the absorption coefficient mainly depends on the number density of the ions in ground state.

Population Inversion and X-Ray Laser Gain by Electron Impact Excitation of Ni-Like Tin

Excitation energies and transition probabilities for the 1s22s22p63s23p63d10, and the 1s22s22p63s23p63d94l (l= s, p, d, and f) configurations in nickel-like tin have been used to calculate the reduced population of 99 fine structure levels over a wide range of electron densities and at different electron plasma temperatures. The gain coefficients for those transitions with positive population inversion factors have been calculated as a function of electron density. This study is concerned with the transitions which produce positive gain at XUV and X-ray spectral regions.

Quantum Hall ice

We show that the chiral kagome ice manifold exhibits an anomalous integer quantum Hall effect (IQHE) when coupled to itinerant electrons. Although electron-mediated interactions select a magnetically ordered ground state, the full ice manifold can coexist with the IQHE over a range of finite temperatures. The degenerate ice states provide a natural realization of power-law correlated flux disorder, for which the spectral gap of the system remains robust. The quantized (up to exponentially small finite-temperature corrections) Hall conductance persists over a wide range of electron densities due to the disorder-induced localization of electronic states.