X-ray Continuum Emission Research Articles

Continuum x-ray emission from the AlcatorAtokamak

X rays from 1 to 25 keV emitted by the Alcator A device have been collected with a Si(Li) detector and pulse-height-analysis system. Under normal operating conditions, spectra are thermal, indicating clean (Z/sub eff/approx.1), Maxwellian distributions. Temperature profiles are provided. When v/sub d//v/sub th/> or approx. =0.03, the x-ray spectra become nonthermal, reflective of non-Maxwellian distributions. The observed nonthermal behavior increases with minor radius and poloidal symmetry, is correlated with poor energy confinement, and cannot be accounted for by classical electric-field-driven perturbation theory. Radial electron diffusion is discussed.

On the equilibrium distribution of the elements in the gas in the Coma cluster

The equilibrium distribution of the elements for the case of an isothermal plasma in a gravitational potential well is given governing equations and a rapidly converging iterative procedure for their solution. Results are applied to the intracluster gas in clusters of galaxies, and implications are identified for the controversy as to whether iron nuclei settle out in the cluster core, and with respect to the interpretation of distribution size and shape for continuum X-ray emission from clusters. A model for the Coma cluster is considered in which the intracluster gas is in equilibrium and partially processed. It is found that this model can account for the angular dependence of the continuum X-ray emission from Coma, as determined by Einstein Observatory observations, at least as well as the uniform composition model.

Spatial Characteristics of Continuum X-Ray Emission from Lateral Energy Transport in CO2-Laser-Produced Plasmas

A modified layered-target technique is used to provide spatial information about continuum x-ray emission in the plane of an infinite target irradiated by a short-pulse C${\mathrm{O}}_{2}$ laser. Hard x-rays are only observed near or within the laser interaction region. Softer x rays are emitted over a large but shallow area of the target surface. These results are used to discuss lateral energy transport mechanisms.

Anisotropic X-ray generation in thin and bulk single crystals (of gold)

Measurements of the anisotropy of characteristic and continuum X-ray emission excited by 20-100 keV electrons are reported for single crystal films and bulk single crystals of gold. A modified theory of the X-ray emission can explain the observed independence of the anisotropy on the X-ray quantum energy, the decrease of anisotropy with increasing film thickness and the increase of anisotropy with increasing electron energy. The anisotropy of X-ray emission of bulk single crystals is of the order of 1%; however, values up to 50% have been observed with thin epitaxially grown gold films. The anisotropy can be neglected, therefore, in the microanalysis of bulk single crystal specimens. A quantitative microanalysis of epitaxial films in single crystal substrates however, has to consider this anisotropy.

X-ray emission from plasma produced by a picosecond ruby laser

The spectrum and angular distribution of the X-ray continuum emission from a picosecond ruby laser produced plasma has been investigated. Massive plane targets made of 20 different elements were used. The dependence of the electron temperature and the effective temperature of suprathermal electrons on the atomic number of the target was determined as well as the dependence of the suprathermal electron temperature on the laser intensity. The most probable mechanism of suprathermal electron generation is resonance absorption.

On the X-ray continuum and iron-line emission of the low state of Hercules X-1

The hard X-ray continuum observed during low-state periods of Hercules X-1 is interpreted as being due to Compton scattering, by the coronal gases above and below the accretion disk, of the primary hard X-rays from the neutron star, which is hidden from our view during these periods. A similar interpretation holds for the low-state soft-X-rays. Because of photoionization, most iron ions in the corona are fully stripped and only a small fraction of them retain one electron. Because of such a high degree of iron ionization, the iron K photons from the corona, even if there are some, will make a line feature at 6.9 keV, and no line feature has been detected at this energy. The low-state iron line at 6.4 keV therefore, cannot be due to the coronal gases. The main production mechanism of this line is most likely fluorescence from the accretion disk.

Fast-electron production in laser-heated plasmas

The continuum X-ray emission from laser-heated targets gives a measure of the energy deposited in superthermal electrons and an estimate of the spectrum and the average electron energy. The validity of the analysis is, however, limited by possible non-collisional energy-loss processes in the pellet corona. If this effect is ignored, fits to experimental X-ray spectra from glass microspheres at 1.06-µm laser wavelength can be readily obtained showing 15%–48% of the absorbed energy in fast electrons.

X-ray emission characteristics of plasmas created by a high-intensity CO2 laser

The spectrum of the x-ray continuum and line emission emanating from Al, Mg, and (CH2)n plasmas created with a nanosecond CO2 laser pulse has been investigated at irradiance levels up to 2×1014 W/cm2.

X-ray spectrometry of laser-produced aluminum plasmas

X-ray lines and continuum emission from a laser-produced aluminum plasma have been studied. The electron temperature variations during the laser pulse (30 ns) were deduced from time-dependent measurements of intensities of resonance lines (2.5 ns time-resolution). Comparison is made with electron temperature values deduced from ion emission measurements.

Laser target model

A numerical simulation model is described for the dense hot plasma produced in the interaction of a focused laser pulse with a target of moderate atomic weight. Specific results are given for the line and continuum x-ray emission, together with other parameters of the plasma, for the case of an aluminum target.

The X-ray line and continuum emission from a solar active region

The X-ray spectrum of the quiet sun in the energy range 2.3 - 6.9 keV was observed from an Aerobee rocket using an uncollimated graphite crystal spectrometer. These results and spatial measurements made with an onboard modulation collimator are analyzed using solar models.

Electron temperature and ionization state in laser produced plasmas

Neodymium glass laser pulses of up to 2 GW power, 10 ns duration and focused flux densities up to 2*1013 W cm-2 have been used to form plasmas at temperatures up to 450 eV. Plane solid plastic targets with atoms of hydrogen, nitrogen, carbon, oxygen and fluorine were used. The relation between laser flux density and electron temperature was studied and found to be of the form temperature varies as (flux density)x where x=0.40+or-0.05. Both the power law and absolute temperature are consistent with steady state hydrodynamical theories predicting x=4/9. Temperatures were deduced from soft X-ray continuum emission using both relative and absolute intensities. The spatial features of the X-ray emission were observed with a pinhole camera. Grazing incidence XUV photographic spectra were used to identify the highest degree of ionization in the plasma. The results were correlated with steady state and transient ionization theories and with the measured electron temperature. Satisfactory agreement was found when self-absorption of resonance radiation was taken into account.

Applications of the Scanning Electron Probe Microanalyzer

The electron probe micoranalyzer, which was initially introduced by Castaing, has proven to be a versatile and important instrument in the fields of ceramics, metallurgy, mineralogy, biology, and solid-state electronics. A review is presented, beginning with the various interaction effects which occur when 5–50 keV electrons impinge on solids. These effects include characteristic and continuum x-ray emission, primary and secondary backscattered electron emission, and production of excess electron-hole pair in semiconductors. The characteristic or line x-ray emission is produced by direct ionization of the target atoms with subsequent radiative decay, and is used for nondestructive qualitative and quantitative analysis with micron spatial resolution. For trace analysis, the limit of detectability is approximately 100 ppm at best. With a typical analysis volume of 1 μm3 and a mass density of 10 g/cm3, this corresponds to a weight sensitivity of approximately 10−15 g. Experimental results are given for the quantitative analysis of sputtered EuO films with thicknesses between 0.3 and 0.5 μm. Scanning electron microscopy can be performed with primary and secondary electrons, as well as x-ray emission. A 200 mesh Cu grid superimposed on a 1000 mesh Ag grid is used as a target to illustrate the use of secondary electron emission intensity for topology micrographs, primary backscattered electron intensity for mass-density micrographs, and characteristic x-ray intensity for elemental micrographs. In addition to composition, the electron probe can be used to determine the thickness of thin films. By increasing the accelerating voltage of the electron beam until penetration to the substrate is accomplished, the thickness of phospho-silicate glass films in the range 700–4000 Å can be determined with an accuracy of ±150 Å. The chemical valence state of atoms can be studied with the long wavelength x-ray spectra. For example, the wavelength position of the peak intensity from Si has been used to reveal the presence of both metallic Si and Si4+ in silicon oxide thin films. In addition, the intensity ratio of two different x-ray lines from the first transition series elements varies with chemical state. Particular attention is devoted to the use of cathodoluminescence for characterizing direct band-gap semiconductors such as GaAs. Band-edge luminescence can be used in a scanning electron microscope mode to display inhomogeneities such as edge dislocations and p-n junctions, as well as bulk dopant inhomogeneities. The voltage dependence of cathodoluminescence can be used to determine transport properties such as minority carrier diffusion length and surface recombination velocity. Transient cathodoluminescence can be used to determine minority carrier lifetime. The quantitative analysis of rare earth materials by cathodoluminescence has also been used to detect trace concentrations. These methods are applicable to epitaxial thin films in general, and to polycrystalline films in some particular cases.

Observation of Two-Photon Continuum Emission from Neon IX

A prominent peak observed in the soft x-ray continuum emission of a deuterium-neon plasma is associated with radiation from the two-photon decay of the 2S1 "metastable" state of heliumlike Ne IX.Received 4 March 1968DOI:https://doi.org/10.1103/PhysRevLett.20.783©1968 American Physical Society