Low-energy X-ray Emission Research Articles

Design and optimization of a proportional counter for the absolute determination of low-energy x-ray emission rates.

We recently presented the construction of a proportional counter for the absolute measurement of low-energy x-ray emission rates. Its main features were presented in a previous study, along with the testing of its performance as an absolute technique. Full details on its design, construction, and characterization processes are given in this paper. First, the analytical calculations and Monte Carlo simulations that led to the choice of optimal dimensions will be presented, followed by the study of the charge creation and the modeling of the electric field that confirmed that the criteria for a reproducible charge multiplication are met. Afterward, details on the electronics and gas control system will be provided. The dimensional measurements for the accurate determination of the solid angle will be described in detail. Special emphasis will be placed on the production of the detector window, which was made of reduced graphene oxide instead of beryllium. Some tests on the operation of the counter will be presented, namely, the measurement of spectra of several radionuclides that emit x-rays between 3 keV and 9 keV, as well as the fluorescence spectrum of chlorine, at 2.6 keV. A brief explanation of the determination of emission rates will be given, including the approach adopted for the treatment of the escape-peak area. The emission rate measured with the proportional counter was converted into activity concentration using the emission probabilities from the bibliography, in order to compare the results with those obtained by liquid scintillation counting, and both results were found to be in good agreement.

Absolute determination of low-energy X-ray emission rates with a proportional counter

A defined-solid-angle proportional counter has been designed for the absolute determination of X-ray emission rates between 2 keV and 10 keV. The main features of the counter are described in this work, and spectra of two radionuclides with low-energy X-ray emissions are presented. The activity concentration of an 55Fe solution was determined by measuring the emission rate of three samples and using tabulated emission probabilities. This activity concentration is compared with the value obtained by liquid scintillation counting, and an excellent agreement has been found.

Swift, NuStar and XMM-Newton observations of the NLS1 galaxy RX J2317.8–4422 in an extreme X-ray low flux state

We report the discovery of RX J2317.8-4422 in an extremely low X-ray flux state by the Neil Gehrels Swift observatory in 2014 April/May. In total, the low-energy X-ray emission dropped by a factor 100. We have carried out multi-wavelength follow-up observations of this Narrow-Line Seyfert 1 galaxy. Here we present observations with Swift, XMM-Newton, and NuSTAR in October and November 2014 and further monitoring observations by Swift from 2015 to 2018. Compared with the beginning of the Swift observations in 2005, in the November 2014 XMM and NuSTAR observation RX J2317--4422.8 dropped by a factor of about 80 in the 0.3-10 keV band. While the high-state Swift observations can be interpreted by a partial covering absorption model with a moderate absorption column density of $N_H=5.4\times 10^{22}$ cm$^{-2}$ or blurred reflection, due to dominating background at energies above 2 keV the low-state XMM data can not distinguish between different multi-component models and were adequately fit with a single power-law model. We discuss various scenarios like a long-term change of the accretion rate or absorption as the cause for the strong variability seen in RX J2317.8--4422.

Search for Low-energy X-ray and Particle Emissions from an Electrochemical Cell

Several theories to explain anomalous heat production predict the emission of low-energy X-rays and/or MeV alpha particles from PdD cathodes in electrochemical cells. Such radiation, however, is not detectable from outside of a standard electrochemical cell due to absorption in the electrolyte and cell walls. A custom cell was therefore assembled which permits X-rays of energy ≥ 1 keV to pass through a thin cathodic membrane and enter into an X-ray detector with minimal attenuation. This test cell geometry also potentially allows any emitted MeV alpha particles to be detected when they impact a Pd cathode and cause fluorescent emission of Pd-Kα (21.2, 23.8 keV) X-rays. The detection of X-ray emissions from a membrane electrolytic cell potentially permits the mechanism(s) for anomalous heat production to be investigated with great sensitivity. As an example, a typical X-ray detector allows 1 keV X-rays to be detected at emission rates of less than one per second and this level of sensitivity corresponds to a thermal resolution of < 0.2 fW. Time resolved X-ray spectral data ranging from 1 to 30 keV was collected for over a year using various types of membranes and different electrolytic solutions. None of these test cells, however, yielded any X-rays which were above ambient background levels.

Sulfur X-ray absorption fine structure in porous Li–S cathode films measured under argon atmospheric conditions

In this paper we present the first results for the characterization of highly porous cathode materials with pore sizes below 1μm for Lithium Sulfur (Li–S) batteries by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. A novel cathode material of porous carbon films fabricated with colloidal array templates has been investigated. In addition, an electrochemical characterization has been performed aiming on an improved correlation of physical and chemical parameters with the electrochemical performance. The performed NEXAFS measurements of cathode materials allowed for a chemical speciation of the sulfur content inside the cathode material. The aim of the presented investigation was to evaluate the potential of the NEXAFS technique to characterize sulfur in novel battery material. The long term goal for the characterization of the battery materials is the sensitive identification of undesired side reactions, such as the polysulfide shuttle, which takes place during charging and discharging of the battery.The main drawback associated with the investigation of these materials is the fact that NEXAFS measurements can usually only be performed ex situ due to the limited in situ instrumentation being available. For Li–S batteries this problem is more pronounced because of the low photon energies needed to study the sulfur K absorption edge at 2472eV.We employed 1μm thick Si3N4 windows to construct sealed argon cells for NEXAFS measurements under ultra high vacuum (UHV) conditions as a first step towards in situ measurements. The cells keep the sample under argon atmosphere at any time and the X-ray beam passes mainly through vacuum which enables the detection of the low energy X-ray emission of sulfur. Using these argon cells we found indications for the presence of lithium polysulfides in the cathode films whereas the correlations to the offline electrochemical results remain somewhat ambiguous. As a consequence of these findings one may state that measurements for battery material characterization are preferably to be performed completely in situ with the electrochemical measurements taking place simultaneously in order to obtain clear and unambiguous results.

GRB INVESTIGATIONS BY ESA GAIA AND LOFT

The possibility of studying GRBs with the ESA Gaia and LOFT missions is briefly addressed. The ESA Gaia satellite to be launched in November 2013 will focus on high precision astrometry of stars and all objects down to limiting magnitude 20. The satellite will also provide photometric and spectral information and hence important inputs for various branches of astrophysics, including the study of GRBs and related optical afterglows (OAs) and optical transients (OTs). The strength of Gaia in GRB analyses will be the fine spectral resolution (spectro-photometry and ultra-low dispersion spectroscopy), which will allow the correct classication of related triggers. An interesting feature of Gaia BP and RP instruments will be the study of highly redshifted triggers. Similarly, the low dispersion spectroscopy provided by various plate surveys can also supply valuable data for investigations of high-energy sources. The ESA LOFT candidate mission, now in the assessment study phase, will also be able to detect and be used in the study of GRBs, with emphasis on low-energy (X-ray) emission.

GRB INVESTIGATIONS BY ESA GAIA AND LOFT

The possibility of studying GRBs with the ESA Gaia and LOFT missions is briefly addressed. The ESA Gaia satellite to be launched in November 2013 will focus on high precision astrometry of stars and all objects down to limiting magnitude 20. The satellite will also provide photometric and spectral information and hence important inputs for various branches of astrophysics, including the study of GRBs and related optical afterglows (OAs) and optical transients (OTs). The strength of Gaia in GRB analyses will be the fine spectral resolution (spectro-photometry and ultra-low dispersion spectroscopy), which will allow the correct classication of related triggers. An interesting feature of Gaia BP and RP instruments will be the study of highly redshifted triggers. Similarly, the low dispersion spectroscopy provided by various plate surveys can also supply valuable data for investigations of high-energy sources. The ESA LOFT candidate mission, now in the assessment study phase, will also be able to detect and be used in the study of GRBs, with emphasis on low-energy (X-ray) emission.

Development of a low-energy PIXE analysis system based on an ion implanter

A low-energy particle-induced X-ray emission (LE-PIXE) analysis system with precise energy control was developed based on a 200kV ion implanter. The energy dependence of LE-PIXE was evaluated using a millimeter-sized low-energy proton beam by varying the incident energy with precision better than 5keV. LE-PIXE spectra were obtained for protons with an energy range of 40–120keV. The LE-PIXE sensitivity was approximately proportional to the sixth power of the incident proton energy. The characteristic X-ray intensity strongly depended on the depth distribution of elements in the target. Experimental evaluation with multilayered thin film samples demonstrated that LE-PIXE was effective for depth distribution analysis to 1000nm deep. Multilayered structures containing four different elements were characterized by LE-PIXE analysis using multiple probe energies.

X-ray variability patterns and radio/X-ray correlations in Cyg X-1

We have studied the X-ray variability patterns and correlations of the radio and X-ray fluxes in all spectral states of Cyg X-1 using X-ray data from RXTE/ASM, CGRO/BATSE, and Swift/BAT. In the hard state, the dominant spectral variability is a changing of normalisation with fixed spectral shape, while in the intermediate state the slope changes, with a pivot point around 10 keV. In the soft state, the low energy X-ray emission dominates the bolometric flux which is only loosely correlated with the high energy emission. In black hole binaries in the hard state, the radio flux is generally found to depend on a power of the X-ray flux, F_R ~ F_X^p. We confirm this for Cyg X-1. Our new finding is that this correlation extends to the intermediate and soft states provided the broad-band X-ray flux in the Comptonization part of the spectrum (excluding the blackbody component) is considered instead of a narrow-band medium-energy X-ray flux. We find an index p=1.7+-0.1 for 15 GHz radio emission, decreasing to p=1.5+-0. at 2.25 GHz. We conclude that the higher value at 15 GHz is due to the effect of free-free absorption in the wind from the companion. The intrinsic correlation index remains uncertain. However, based on a theoretical model of the wind in Cyg X-1, it appears to be close to 1.3, which, in the framework of accretion/jet models, implies that the accretion flow in Cyg X-1 is radiatively efficient. The correlation with the flux due to Comptonization emission indicates that the radio jet is launched by the hot electrons in the accretion flow in all spectral states of Cyg X-1. On the other hand, we rule out the X-ray jet model. Finally, we find that the index of the correlation, when measured using the X-ray flux in a narrow energy band, strongly depends on the band chosen and is, in general, different from that for either the bolometric flux or the flux in the hot-electron emission.

Plasma doping two-dimensional characterization using low energy x-ray emission spectroscopy and full wafer secondary ion mass spectrometry/angle-resolved x-ray electron spectroscopy techniques

Several metrologies may be used to analyze plasma doping (PLAD) [S. Qin and A. McTeer, IEEE Trans. Electron Devices 54, 2497 (2007)] results, each with its own advantages and disadvantages. An investigation into the available characterization methods has been performed in order to reduce costs associated with the current qualification process, such as additional clean and annealing steps. This article will introduce an improved analytical method developed at Micron and demonstrate results of a correlation study between characterization techniques for B doped PLAD. The combined secondary ion mass spectrometry/angle-resolved x-ray electron spectroscopy two-dimensional (2D) dose mapping data provide excellent confirmation of the low energy x-ray emission spectroscopy (LEXES) 2D dose mapping data, and the nonuniformity from annealing can be deconvoluted from nonuniformities due to the implantation process. The data demonstrate that LEXES may replace the thermally processed RS measurement for tool and/or process qualifications.

CONSTRAINTS ON THE LOW-ENERGY CUTOFF IN THE ELECTRON DISTRIBUTION OF THE PKS 0637-752 JET

We reanalyze the Chandra ACIS spectrum of the kpc-scale jet in PKS 0637-752 to investigate the possible low-energy cutoff in the relativistic electron spectrum producing the nonthermal radiation in the scenario of inverse Compton (IC) emission off the cosmic microwave background. This was among the first objects targeted by the Chandra Observatory and gives a unique opportunity to study the low-energy X-ray emission free of detector contamination. As previously reported in the literature, the spectrum can be fit by a power law, with the slope predicted by the radio spectrum, modified by low energy absorption through the Galaxy as determined from the spectrum of the quasar core and by HI 21 cm observations. We obtain a marginally better fit with a model of IC emission produced by an electron population that exhibits a cutoff at γmin δ10 between about 50 and 80 (assuming Γ = δ). This range for γmin is higher than has previously been assumed in broadband spectral fits to the jet emission. The observed optical flux can be used to place a lower limit on γmin; the constraint is not very strong, but does suggest that γmin must be higher than 1 to avoid overproducing the optical emission. We investigate the effect of uncertainties in the column density for galactic absorption as well as the calibration of Chandra for these early observations. Finally, we discuss the implication of these limits on the jet luminosity in this source.

Two-Stage Atomic Layer Deposition of Aluminum Oxide on Alkanethiolate Self-Assembled Monolayers Using n-Propanol and Water as Oxygen Sources

Aluminum oxide (AlOx) was deposited onto strong hydrophobic surfaces of alkanethiolate self-assembled monolayers (SAMs) by a two-stage atomic layer deposition (ALD) process; the first stage utilized n-propanol as the oxygen source, and the second stage proceeded with water. The resulting AlOx layers were characterized with spectroscopic ellipsometry, reflection−absorption infrared spectroscopy, low-energy X-ray emission spectroscopy, and atomic force microscopy. The optimized two-step ALD process significantly improved the surface morphology of AlOx layers and effectively protected the structural integrities of underlying SAMs.

State-of-the-Art Characterization for 65 nm CMOS Processes and Beyond

Characterization of advanced CMOS devices and control of front- end of line (FEOL) processes require state-of-the-art analytical techniques and specific measurement protocols. Dynamic Secondary Ion Mass Spectrometry (D-SIMS) is well suited for in-depth quantitative analysis of both bulk- and dopant concentrations in GexSi1-x binary alloys. A similar D-SIMS protocol -in combination with Auger Electron Spectroscopy (AES) - allows monitoring nickel-silicide phase transitions upon silicidation of (doped) Ni Totally Silicided (TOSI) gates. Combination of Low-Energy Electron X-ray Emission Spectroscopy (LEXES) and high-precision DSIMS provides accurate control of dose, junction depth, and wafer uniformity for Ultra-Shallow Junctions (USJ) at high analytical throughput, opening the way to USJ metrology. Similar techniques (LEXES, SIMS) are employed to control wafer uniformity of deposited NiPt metal films as Source/Drain (S/D) contact metal, to monitor NixSi1-x phase transformations and Pt-redistribution upon silicidation, and to determine root causes of defectivity at the device level (micro- AES). The given examples demonstrate the importance of having available (novel) state-of-the-art characterization techniques - pushing the analytical limits in terms of resolution, sensitivity, and accuracy while simultaneously increasing throughput and reproducibility- to support process development, improve yield, and to speed up yield learning of advanced CMOS processes.

Relationship between X-ray Intensity and Electric Bias on Al<SUB>2</SUB>O<SUB>3</SUB> Surface during Low Energy Ga<SUP>+</SUP> Irradiation

Low-energy characteristic X-ray emission is detected during bombardment of positive low energy ions onto insulator materials. The phenomenon is considered to be related to surface charge-up. To study further the mechanism, the characteristic X-rays was studied during 30keV Ga + ions bombardment onto Al 2 O 3 monocrystalline specimens applied with a direct current (DC) bias in the present work. The applied DC voltage builds an electric field parallel to the surface of the specimen. The results show that the characteristic X-rays of O-Kα and Al-Ka increased with the increasing of the applied DC voltages.

AnXMM‐NewtonView of the Soft Gamma Repeater SGR 1806−20: Long‐Term Variability in the Pre–Giant Flare Epoch

The low-energy (<10 keV) X-ray emission of the soft gamma repeater SGR 1806-20 has been studied by means of four XMM-Newton observations carried out in the last two years, the latest performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5. The source was caught in different states of activity; over the 2003-2004 period, the 2-10 keV flux doubled with respect to the historical level observed previously. The long-term rise in luminosity was accompanied by a gradual hardening of the spectrum, with the power-law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations are consistent with an average spin-down rate of 5.5 × 10-10 s s-1, higher than the values observed in the previous years. The long-term behavior of SGR 1806-20 exhibits the correlation between spectral hardness and spin-down rate previously found only by comparing the properties of different sources (both SGRs and anomalous X-ray pulsars [AXPs]). The best-quality spectrum (obtained on 2004 September 6) cannot be fitted by a single power law but requires an additional blackbody component [kTBB = 0.79 keV, RBB = 1.9 (d/15 kpc)2 km], similar to the spectra observed in other SGRs and AXPs. No spectral lines were found in the persistent emission, with equivalent width upper limits in the range 30-110 eV. Marginal evidence for an absorption feature at 4.2 keV is present in the cumulative spectrum of 69 bursts detected in 2004 September-October.

Variability of the Hα emission of Cygnus X-1 and its connection with the soft X-ray radiation

High-resolution Halpha monitoring of Cyg X-1, HD226868 was carried out during 1996-2002 and the resultant spectra analysed in conjunction with 1.5-12 keV X-ray monitoring. We demonstrate that the Halpha line-profiles have complex variability on different timescales, controlled in particular by the orbital period and the focused wind model of mass loss. We find that long-term variability of the mass loss by the supergiant and short-term variability due to clumpy structure of the stellar wind dominate during the low/hard X-ray state and that X-ray photoionization has a relatively small influence on the line-profile shape and EW variability. During the high/soft X-ray state and flaring the effect of photoionization the line-profile and EW of Halpha increases but is still unable to describe the loose anti-correlation between EW and the low energy X-ray emission. We propose that variability of the mass loss by the supergiant can change wind velocities in the Stromgren zone around the accretion disc of the secondary, leading to an increase in accretion rate through the disc.

Chemical Interactions by Low-Energy Electron-Induced X-Ray Emission Spectroscopy, LEXES

The possibilities presented by low-energy electron-induced X-ray emission spectroscopy to study chemical interactions in solids are discussed. Examples of change observed for the emissions between core levels as a function of the chemical environment of the emitting atoms are given. By comparing the partial densities of the valence states associated to each type of atoms in the compound, it is shown that the strength of the metal-ligand interactions can be obtained. Information on the charge densities around each type of atoms can be deduced. Application to the study of the interactions at the atomic scale to solid–solid interfaces is presented.

Quantitative determination of dopant dose in shallow implants using the low energy x-ray emission spectroscopy technique

A technique using low energy x-ray emission spectroscopy (LEXES) is used to determine the dose of shallow implanted dopants. This technique is nondestructive and dopant selective. This technique can provide absolute dose values. CAMECA has developed specific instrumentation called the shallow probe which adapts the LEXES technique to the requirements of the semiconductor industry in terms of sensitivity and reproducibility of dose measurements. This work reports the performance of the shallow probe applied to a wide variety of boron, phosphorus, and arsenic implants into silicon wafers. Measured doses are compared to both ultralow energy secondary ion mass spectrometry and implanter tool values.

High- and Low-Energy Nonthermal X-Ray Emission from the Abell 2199 Cluster of Galaxies

We report the detection of both soft and hard excess X-ray emission in the cluster of galaxies A2199, based upon spatially resolved spectroscopy with data from the BeppoSAX, Extreme Ultraviolet Explorer, and ROSAT missions. The excess emission is visible at radii larger than 300 kpc and increases in strength relative to the isothermal component. The total 0.1-100 keV luminosity of this component is 15% of the cluster luminosity, but it dominates the cluster luminosity at high and low energies. We argue that the most plausible interpretation of the excess emission is an inverse Compton interaction between the cosmic microwave background and relativistic electrons in the cluster. The observed spatial distribution of the nonthermal component implies that there is a large halo of cosmic-ray electrons between 0.5 and 1.5 Mpc surrounding the cluster core. The prominent existence of this component has cosmological implications, since it is significantly changing our picture of clusters' particle acceleration history, dynamics between the thermal and relativistic media, and total mass budgets.

EPMA using low-energy (0.1-1.0 keV) X-rays-an historical perspective

Studies with the electron-probe microanalyser using low-energy (<1 keV) X-rays are reviewed with particular reference to analysis of the ultra-light elements (atomic number Z<11). The paper begins by describing the historical development of methods for measuring low-energy X-ray spectra and goes on to show that minimum detection levels achieved for ultra-light elements are now comparable to those for heavier elements, in ideal circumstances about 100 ppm using wavelength-dispersive spectrometry and about 1000 ppm with energy-dispersive spectrometry. Regarding quantitative analysis, the correction models developed during the 1960s worked fairly well in most cases but, because they were based upon an X-ray depth distribution function of limited validity, were unsatisfactory for the ultra-light elements. Since then, advances made using empirical curve-fitting methods to derive this function have led to accuracies of a few per cent. relative, close to figures for heavier elements, any limitations being most likely associated with the accuracy of the mass absorption coefficients used rather than the models themselves. The merit of utilising low-energy radiation for surface analysis is discussed and it is shown that, by employing a low-voltage electron probe to restrict X-ray excitation to surface layers and recording low-energy X-ray emission, surface films down to about 1 nm thickness may be investigated. Finally, examples are given to show the value of studying the shape of low-energy spectra as a means of identifying the phase of microstructural features.