Effect of the sample annealing temperature and sample crystallographic orientation on the charge kinetics of MgO single crystals subjected to keV electron irradiation

This paper focuses on the effect of sample annealing temperature and crystallographic orientation on the secondary electron yield of MgO during charging by a defocused electron beam irradiation. The experimental results show that there are two regimes during the charging process that are better identified by plotting the logarithm of the secondary electron emission yield, lnσ , as function of the total trapped charge in the material Q_T. The impact of the annealing temperature and crystallographic orientation on the evolution of lnσ is presented here. The slope of the asymptotic regime of the curve lnσ as function of Q_T, expressed in cm² per trapped charge, is probably linked to the elementary cross section of electron–hole recombination, σ_hole, which controls the trapping evolution in the reach of the stationary flow regime.     

Recent developments and new strategies in scanning electron microscopy*

In addition to improvements in lateral resolution in scanning electron microscopy, recent developments of interest here concern extension of the incident beam energy, E₀, over two decades, from ≈ 20 keV to ≈ 0.1–0.5 keV and the possibility of changing the take-off emission, α, of detected secondary electrons. These two degrees of freedom for image acquisition permit a series of images of the same field of view of a specimen to be obtained, each image of the series differing from the others in some aspect. The origins of these differences are explored in detail and they are tentatively interpreted in terms of the change in the secondary electron emission yield δ vs. E₀, δ = f(E₀), and also of the change in δ vs. α, ∂δ/∂α. Various origins for the chemical contrast and topographic contrast have been identified. Illustrated by correlating a secondary electron image and a backscattered electron image, use of the scatter diagram technique facilitates image comparison. The difference between the lateral resolution and the size of the minimum detectable detail is outlined to avoid possible errors in nanometrology. Some aspects related to charging are also considered and possible causes of contrast reversal are suggested. Finally, the suggested strategy consists of the acquisition of various images of a given specimen by changing one parameter: primary beam energy and take-off angle for conductive specimens; working distance or beam intensity for high-resolution experiments; scanning frequency for insulating specimens.     

Mechanisms of radiation damage in beam‐sensitive specimens,for TEM accelerating voltages between 10 and 300 kV

Ionization damage (radiolysis) and knock‐on displacement are compared in terms of scattering cross section and stopping power, for thin organic specimens exposed to the electrons in a TEM. Based on stopping power, which includes secondary processes, radiolysis is found to be predominant for all incident energies (10–300 keV), even in materials containing hydrogen. For conducting inorganic specimens, knock‐on displacement is the only damage mechanism but an electron dose exceeding 1000 C cm^?2 is usually required. Ways of experimentally determining the damage mechanism (with a view to minimizing damage) are discussed. Microsc. Res. Tech., 2012. © 2012 Wiley Periodicals, Inc.     

Measurement of specimen charging in scanning electron microscopy with a kelvin probe

A vibrating Kelvin probe in form of a platinum wire loop is used to measure the surface potential U_s on electron-irradiated free-floating metal and insulator specimens as a function of electron energy E. This allows an accurate measurement of the critical electron energy E₂ for no charging. At energies below E₂, the positive charging increases with decreasing energy to U_s=2–5 eV at E=0.5 keV and switching off the collector bias of the Everhart-Thornley detector. A two-to threefold increase of U_s is observed when the bias is switched on. For E > E₂, the strong increase of a negative surface potential can be measured. Insulating films free-supported on a conductive substrate show a steep decrease to small positive and negative U_s when the film thickness becomes lower than the electron range at a critical energy E₃ > E₂. At insulating specimen the temporal decrease of charging can be measured when the electron beam is switched off.     

Contrast in the transmission mode of a low-voltage scanning electron microscope

The contrast thicknesses (x_k) of thin carbon and platinum films have been measured in the transmission mode of a low-voltage scanning electron microscope for apertures of 40 and 100 mrad and electron energies (E) between 1 and 30 keV. The measured values overlap with those previously measured for E (≥ 17keV) in a transmission electron microscope. Differences in the decrease of x_k with decreasing E between carbon and platinum agree with Wentzel-Kramer-Brillouin calculations of the elastic cross-sections. Knowing the value of x_k allows the exponential decrease ∝ exp(—x/x_k) in transmission with increasing mass-thickness (x = ρt) of the specimen and the increasing gain of contrast for stained biological sections with decreasing electron energy to be calculated for brightfield and darkfield modes.     

About the role of the various types of secondary electrons (SE1; SE2; SE3) on the performance of LVSEM

The relative weight, δ_Β, of the yield of secondary electrons, SE₂, induced by the backscattered electrons, BSE, with respect to that, δ_P, of secondary electrons, SE₁, induced by the primary electrons, PE, is deduced from simple theoretical considerations. At primary energies E₀ larger than E_M (where the total SE yield δ = δ_P + δ_B is maximum), the dominant role of the backscattering events is established. It is illustrated in SEM by a direct comparison of the contrast between SE images and BSE images obtained at E₀ ～ 5 keV and E₀ ～ 15 keV on a stratified specimen. At energies E₀ less than E_M, the dominant role of SE₁ electrons with respect to SE₂ (and SE₃) is established. It is illustrated by the better practical resolution of diamond images obtained with an in‐lens detection in low voltage SEM E₀ ～ 0.2–1 keV range compared with that obtained with a lateral detector. The present contribution illustrates the improved performance of LVSEM in terms of contrast and of practical resolution as well as the importance of variable voltage methods for subsurface imaging. The common opinion that the practical lateral resolution is given by the incident spot diameter is also reconsidered in LVSEM.     

A large-area continuous accelerator with the extraction of a high-density electron beam

The design features of the electron accelerator with a large-area 200-keV beam and results of its investigation are described. The accelerator is based on a set of discrete longitudinal filament cathodes and operates in the continuous mode. The cross section of the beam extracted into the atmosphere is 40 × 50 cm², and the maximal current density of the extracted electron beam is up to 100 μA/cm². The nonuniformity of the current density distribution over the electron beam cross section is 10% or less.     

Determining Generation–Recombination Characteristics of a Semiconductor–Dielectric Interface from the Current Kinetics of Surface Minority Carrier Generation

Conditions for the one-to-one characterization of the generation (G _s) and surface recombination (R _s) rates of minority charge carriers (MCCs) in a metal–oxide–semiconductor (MOS) structure (in the case of strong nonequilibrium depletion) by the MCC surface generation current (I(t)) flowing in an external circuit of this structure are revealed. These conditions are the following: (1) the generation current I is independent of the time t (until the structure enters an equilibrium state) and the voltage V _{g 0} corresponding to the initial nonequilibrium depletion and (2) the duration of current steps I(V _{g 0}) = const and, consequently, the equilibrium surface charge increase with increasing V _{g 0}. The observed kinetics of the MCC generation current for the MCCs induced in an n-Si MOS structure at 293 K experimentally confirms the realization of these conditions. The values of the generation and recombination rates G _s = 2.84 × 10¹⁰ cm^–2s^–1 and R _s = 6.82 cm s^–1 obtained from current levels I(V _{g 0}) = const are typical of high-quality Si MOS structure. Additionally measured capacitance–voltage characteristics were used to determine the interface state density at the Si/SiO₂ contact near the middle of the Si gap (N _ss(E) 6.4 × 10¹⁰ cm^–2eV^–1), which allowed the estimation of the effective capture cross section of these states _eff 1.4 × 10^–16 cm².     

CsPbBr3纳米片的三光子非线性吸收性质研究

为了研究不同层数纳米片在近红外二区的非线性光学吸收性质的变化,采用配体辅助再沉淀方法制备了不同层数(3~5层)的CsPbBr₃纳米片。以中心波长为1030 nm、脉宽为6 ps、重复频率为25 kHz的激光作为激发光源,利用Z?扫描技术研究了CsPbBr₃纳米片的非线性三光子吸收光学性质。研究结果表明:CsPbBr₃纳米片的非线性三光子吸收截面随层数减小而增大,量子限域效应增强,三层纳米片的三光子吸收截面高达4.1×10^(?71 )cm⁶s²photon^?2。CsPbBr₃纳米片在近红外二区具有优良的非线性光学吸收性质,可应用于多光子激发荧光成像领域。     

A source of broad electron beams with a self-heated hollow cathode for plasma nitriding of stainless steel

The principle of operation and characteristics of a broad electron beam source based on the discharge with a self-heated hollow cathode and widened anode part are described. The source is intended for the ion nitriding of metals in the electron beam plasma. The influence of the current density (1–7 mA/cm²) and ion energy (0.1–0.3 keV) on the nitriding rate of the 12X18H10T austenitic stainless steel is studied. It is shown that the maximal nitriding rate is reached by the combining of the minimal bias voltage across the samples (100 V) and maximal ion current density, which ensures the dynamic oxide layer sputtering on the sample surface. The electron source, in which electrons are extracted through a stabilizing grid in the direction normal to the axis of the hollow cathode, ensures the radially divergent electron beam formation with a 700-cm² initial cross section, a current of up to 30 A, and initial electron energy of 0.1–0.5 keV. The source stably operates at nitrogen-argon mixture pressures of up to 3 Pa.     

Control of charging in low-voltage SEM

Charging of the specimen under electron beam irradiation is a common problem in scanning electron microscopy (SEM). It results in unstable imaging conditions and a loss in resolution due to defocus of the beam. In addition, it can cause permanent changes in some specimens from translocation of mobile ions under the influence of the induced electrostatic field. To minimize charging and its associated problems, the incident beam energy must be carefully chosen to be the value E₂ at which a dynamic charge balance is obtained. This article presents data on E₂ values for a variety of materials and demonstrates how E₂ is affected by the choice of angle of beam incidence.     

Basic questions related to electron-induced sputtering in the TEM

Although the theory of high-angle elastic scattering of fast electrons is well developed, accurate calculation of the incident-energy threshold and cross section for surface-atom sputtering is hampered by uncertainties in the value of the surface-displacement energy E_d and its angular dependence. We show that reasonable agreement with experiment is achieved by assuming a non-spherical escape potential with E_d=(5/3) E_sub, where E_sub is the sublimation energy. Since field-emission sources and aberration-corrected TEM lenses have become more widespread, sputtering has begun to impose a practical limit to the spatial resolution of microanalysis for some specimens. Sputtering can be delayed by coating the specimen with a thin layer of carbon, or prevented by reducing the incident energy; 60 keV should be sufficiently low for most materials.     

Ultimate characteristics of an X-ray prism spectrometer

X-ray prism spectrometry schemes for experimental investigations of fast processes are considered. Diamond and beryllium prism dispersion parameters and special features of transmission spectra for crystal prisms are analyzed. It is shown that relative energy resolution E/ΔE at photon energies E ∼ 10 keV may reach 10³–10⁴ and the total working spectral band is ∼100 keV. This opens unique possibilities for measuring fine structures of single-shot absorption spectra for quasi-parallel beams and continuous monitoring of the fundamental and high-frequency harmonics of an X-ray free electron laser. Original Russian Text ? A.G. Tur’yanskii, 2009, published in Pribory i Tekhnika Eksperimenta, 2009, No. 4, pp. 150–158.     

Low-energy electron/atom elastic scattering cross sections from 0.1–30 keV

Empirical forms for electron/atom scattering cross sections predict backscattering factors that compare well with those calculated using tabulated Mott data from 0.1 to 30 keV. The form of the empirical total cross section is similar to the screened Rutherford cross section. The fit to the tabulated differential Mott cross sections is decomposed into two parts, one part being of the same mathematical form as the screened Rutherford cross section (σ_R), and the second part being an isotropic distribution (σ_I). The ratio of the total cross sections (σ_R/σ_I) between the screened Rutherford part of the differential scattering cross section and the isotropic part of the distribution is fitted to give the same ratio of forward to backscattered currents as the tabulated Mott differential cross sections. The three equations, one for the total elastic cross section and two describing the differential cross section—one for the Rutherford screening parameter and one for the ratio σ_R/σ_I—give backscattering results covering all the major trends with energy and atomic number compared with the backscattering coefficients calculated using tabulated Mott cross sections. However, agreement with experiment is poor for some well-researched examples such as Au. Monte Carlo calculations using the empirical cross sections show that surface effects may be critical in interpreting experimental results.     

Structural observations and biomechanical measurements of clarinet reeds made from Arundo donax

Plant anatomy was examined for two clarinet reeds made out of Arundo donax by different means of microscopy: light microscopy, low‐energy secondary electron scanning electron microscopy (SEM), backscattered electron SEM, and helium ion microscopy (HiM). The local indentation hardness H_IT and Young's modulus E_IT of different tissues on their cross sections were measured. A vascular bundle (Vb) (H_IT = 60–100 MPa, E_IT = 1,500–2,000 MPa) that includes soft tissues of phloem and xylem and a vascular bundle sheath (Bs) (H_IT = 300–500 MPa, E_IT = ～7,000 MPa) form a pipe of the strong string along the longitudinal direction of the cane. This Vb/Bs string is connected transversally with a net of thin cell‐walls of parenchyma cells (Pa) (H_IT = 70–200 MPa, E_IT = 2,000–3,000 MPa) that also range along the longitudinal direction of the cane. It was turned out that the acoustic quality of a reed is mainly ascribed to the shape and configuration of Vb and the size of Pa. A reed where Vb bundles with continuous Bs rings are homogeneously distributed with higher proportion among a softer network of small Pa cells enables musical performance.     

EVALUATION OF SELF-ATTENUATION COEFFICIENTS OF OIL-PRODUCED SCALES FOR GAMMA-RAY SPECTROSCOPIC ANALYSIS

This study focuses on the evaluation of self-attenuation correction factors (C_f) and linear?attenuation coefficients (μ) of scale samples, produced from the oil fields. This was performed using ¹⁵²Eu point source at the optimal geometry found (i.e., 3 mm). The scale samples were of apparent density ranging 1.04 ? 3.08 g cm^?3. The results illustrated that self-attenuation correction values were as high as 5.08 for the most dense samples at γ-energy line of 45.5 keV. High correlation was observed by Pearson matrix between self-attenuation correction factor and sample density (correlation coefficient of 0.967 at γ-energy of 45.5 keV). A simplified model of the relationship between these variables was proposed. The remarkable finding was that when scale sample density increased self-absorption extended to occur to a higher energy line. Hence, self-attenuation correction was negligible at γ-energies of 122, 222, and 344 keV for samples with density ranging 1.04–1.41, 1.45–2.04, and 2.12–3.08 g cm^?3, respectively. Since linear attenuation coefficient is material sensitive, it was calculated for each sample. Analysis of variance (ANOVA) reflected the linear relationship between linear-attenuation coefficient and sample density up to 344 keV which disappeared thereafter. The data obtained allowed an accurate determination of the concentrations of γ-emitters in scale samples within the energy range of 45.5–1408 keV.     

CASINO: A new monte carlo code in C language for electron beam interaction —part I: Description of the program

This paper is a guide to the ANSI standard C code of CASINO program which is a single scattering Monte CArlo SImulation of electroN trajectory in sOlid specially designed for low-beam interaction in a bulk and thin foil. CASINO can be used either on a DOS-based PC or on a UNIX-based workstation. This program uses tabulated Mott elastic cross sections and experimentally determined stopping powers. Function pointers are used for the most essential routine so that different physical models can easily be implemented. CASINO can be used to generate all of the recorded signals (x-rays, secondary, and backscattered) in a scanning electron microscope either as a point analysis, as a linescan, or as an image format, for all the accelerated voltages (0.1–30 kV). As an example of application, it was found that a 20 nm Guinier-Preston Mg₂Si in a light aluminum matrix can, theoretically, be imaged with a microchannel backscattered detector at 5 keV with a beam spot size of 5 nm.     

Non-monotonic material contrast in scanning ion and scanning electron images

30 keV Ga⁺ focused ion beam induced secondary electron (iSE) imaging was used to determine the relative contrast between several materials. The iSE signal compared from C, Si, Al, Ti, Cr, Ni, Cu, Mo, Ag, and W metal layers does not decrease with an increase in target atomic number Z₂, and shows a non-monotonic relationship between contrast and Z₂. The non-monotonic relationship is attributed to periodic fluctuations of the stopping power and sputter yield inherent to the ion–solid interactions. In addition, material contrast from electron-induced secondary electron (eSE) and backscattered electron (BSE) images using scanning electron microscopy (SEM) also shows non-monotonic contrast as a function of Z₂, following the periodic behavior of the stopping power for electron–solid interactions. A comparison of the iSE and eSE results shows similar relative contrast between the metal layers, and not complementary contrast as conventionally understood. These similarities in the contrast behavior can be attributed to similarities in the periodic and non-monotonic function defined by incident particle–solid interaction theory.     

Experimental ionization cross-sections of phosphorus and calcium by electron spectroscopic imaging

The absolute partial electron scattering cross-section for the phosphorus L_2,3-shell ionization was measured by electron spectroscopic imaging using poliovirus as a primary standard. The equivalent calcium cross-section was obtained in relation to phosphorus using the stoichiometric ratio for these two elements in hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. At 80kcV, the partial cross-section of phosphorus was 2.26 times 10^?20 and 2.68 × 10^?20cm²/atom for poliovirus and hydroxyapatite, respectively, at 150eV loss for a 15-cV energy window and an acceptance angle of 15 mrad. Under the same conditions the calcium cross-section was 0.49 × 10^?20 cm²/atom at 360 eV loss. The experimental values are slightly higher than the theoretical cross-sections calculated either by hydrogenic or Hartree—Slater approaches.     

A sectioned spectrometer of fast neutrons

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched ⁶Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ～10–15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10^?4 to 10² cm^?2 s^?1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 10² s^?1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (～1.2 l); for 3-MeV neutrons, its value is 0.2–0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ～600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.