X-ray Spectroscopy Experiments Research Articles

A monolithic array of 77 silicon drift detectors for X-ray spectroscopy and gamma-ray imaging applications

Monolithic arrays of Silicon Drift Detectors (SDDs) have been employed successfully in X-ray spectroscopy and /spl gamma/-ray imaging applications. Thanks to the low electronics noise achieved at short shaping time, the SDD is an ideal device for high-resolution and high-rate X-ray spectroscopy experiments at synchrotron sources. Moreover, small monolithic arrays of SDDs have also been used as photodetector of the scintillation light in a first prototype of Anger Camera for /spl gamma/-ray imaging characterized by an intrinsic resolution better than 0.3 mm. In this work we present a new large-area monolithic array of Silicon Drift Detectors. It consists of a single chip composed by 77 single hexagonal units, each one with a front-end JFET integrated in its center, arranged in a honeycomb configuration. Each SDD unit has an active area of 8.7 mm/sup 2/, for a total active area of the device of 6.7 cm/sup 2/. The linear dimensions of the active area are approximately 29/spl times/26 mm/sup 2/. It represents the largest monolithic array of SDDs with on-chip JFETs produced up to now for X-ray and /spl gamma/-ray detection. The results achieved in the experimental characterization of a first prototype of the detector array are presented. They include also the preliminary X-ray spectroscopy characterization of the SDD's units. The energy resolution measured at 6 keV with the single unit of the array is of 142eV at -10/spl deg/C.

Investigation on crystallization and influence of Nd 3+ doping of transparent oxyfluoride glass-ceramics

Nd 3+ doped transparent oxyfluoride glass-ceramics containing LaF 3 nano-crystals were prepared by melt quenching. Differential thermal analysis (DTA) showed that the addition of NdF 3 decreased the LaF 3 crystallization temperature. The kinetics studies indicated that the crystallization was a diffusion-controlled growth process with zero nucleation rate for the whole process. LaF 3 crystallization activation energy and crystalline size as the function of NdF 3 doping lever exhibit the closely related evolutions. For the crystallized samples, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and absorption spectroscopy experiments evidenced the homogeneous distribution of spherical LaF 3 nano-crystals (10–20 nm) with the incorporation of Nd 3+ ions in the lattice among the oxide glassy matrix, which is important for obtaining desirable luminescent performance of the material.

Voxel-based Monte Carlo simulation of X-ray imaging and spectroscopy experiments

A Monte Carlo code for the simulation of X-ray imaging and spectroscopy experiments in heterogeneous samples is presented. The energy spectrum, polarization and profile of the incident beam can be defined so that X-ray tube systems as well as synchrotron sources can be simulated. The sample is modeled as a 3D regular grid. The chemical composition and density is given at each point of the grid. Photoelectric absorption, fluorescent emission, elastic and inelastic scattering are included in the simulation. The core of the simulation is a fast routine for the calculation of the path lengths of the photon trajectory intersections with the grid voxels. The voxel representation is particularly useful for samples that cannot be well described by a small set of polyhedra. This is the case of most naturally occurring samples. In such cases, voxel-based simulations are much less expensive in terms of computational cost than simulations on a polygonal representation. The efficient scheme used for calculating the path lengths in the voxels and the use of variance reduction techniques make the code suitable for the detailed simulation of complex experiments on generic samples in a relatively short time. Examples of applications to X-ray imaging and spectroscopy experiments are discussed.

X-ray-induced fluorescence spectroscopy with highly charged ion beam produced by a laser ion source

We designed and tested a beam line for transport of highly charged ions produced by a laser ion source. The ion beam is prepared for an x-ray spectroscopy experiment based on a laser-induced fluorescence spectroscopy. A C4+ beam which has a peak current of 160 μA and bunch width of 500 ns was obtained. It corresponds to 1.3×108 ions/pulse, and the quantity enables the x-ray spectroscopy experiment.

Dependence of Conductivity on the Interplay of Structure and Polymer Dynamics in a Composite Polymer Electrolyte

The conductivity and transport properties of a composite polymer electrolyte were studied by comparing the structure and dynamics as a function of both salt and organic−inorganic composite content. The system consisted of poly(ethylene oxide) (PEO), an organic−inorganic composite (OIC) prepared from aluminum tri-sec-butoxide and [(3-glycidyloxy)propyl]trimethoxysilane (GLYMO) and lithium triflate (LiCF3SO3). The systems with and without salt yielded strikingly different physical properties when the OIC content exceeded 50%. Through analysis of 29Si NMR spectra, it was found that the lithium ion of LiCF3SO3 (LiTf) promotes the condensation of GLYMO, which peaks near 50% OIC content. Also, short-range structural evidence for PEO−OIC blending at high OIC content was observed in the salt-free system through comparisons of the line shapes of the 27Al NMR spectra. This blending is absent in the ternary system due to prominent PEO−LiTf interactions, as confirmed by X-ray, DSC, and impedance spectroscopy experiments. Furthermore, the glass transition temperature exhibits a linear increase as a function of OIC content, whereas the conductivity over this range first shows a sharp increase followed by a mild decrease. The dielectric constant also was found to vary nonlinearly with OIC content, indicating that ionic screening is modulated by OIC. Because in this system the conductivity and the glass transition temperature do not show a significant correlation, although structurally it is clear that PEO and salt are intimately mixed, a model was developed for the transport that focuses principally on the density of mobile lithium ions. The model predicts relatively constant ion mobilities and diffusion constants but a strongly varying mobile ion number density as a function of OIC content, which then explains the dependence of conductivity on OIC content in this electrolyte.

High stability X-ray spectroscopy system with on-chip front-end in charge amplifier configuration

This paper presents the performance of a two-chip detector system based on a silicon drift detector and on a charge amplifier, part of the latter being integrated in the detector chip. The two-chip system is intended for high-resolution X-ray spectroscopy experiments in which stability of operation is mandatory to avoid on-line calibration procedures. Experimental measurements have been carried out to test the stability of the whole system with a quantitative analysis of the gain stability obtained by comparing X-ray spectra taken at different operating conditions, varying the temperature and/or the detector bias. A gain variation of less than 0.4% has been obtained at extreme operating conditions. The problems and critical aspects of the two-chip charge amplifier solution are discussed.

Determination of the refractive indices of AlN, GaN, and AlxGa1−xN grown on (111)Si substrates

The refractive indices of several AlxGa1−xN alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of AlxGa1−xN, the refractive index is given in form of a Sellmeier law.

Determination of AlxGa1xN refractive indexes at low and room temperature, in the 300–600 nm range, for the optimisation of GaN-based microcavities

AlxGa1–xN alloys are deposited on (111)Si substrate by molecular beam epitaxy. The Al composition is deduced from energy dispersive X-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k, at room temperature, in the 300–600 nm range, are extracted from a two-stage procedure based on spectroscopic ellipsometry and reflectivity measurements. In the transparent region, the refractive index is determined as a function of wavelength, using a Sellmeier law. The accurate knowledge of the later is essential for the fabrication of optimised GaN-based microcavities. For this purpose, two simulations of microcavities with nitride-based distributed Bragg reflectors are presented from the refractive indexes determined in this work at 5 K. The first one is constituted by a (λ/2) GaN cavity embedded between two Al0.19Ga0.81N/Al0.675Ga0.325N distributed Bragg reflectors. In the second one, a lower aluminium composition (0.47) is used in place of (0.675) in order to reduce the strain effects due to the difference between the lattice parameters

Simon short circuit effect in ECRIS

The plasma confinement within an electron cyclotron resonance ion source significantly influences the charge state distribution and hence the performance of the source. The different axial and radial diffusion processes govern the confinement time. In many experiments it has been shown that negatively biasing the end plate in the injection region improves the charge state distribution. In a few x-ray and vacuum ultraviolet spectroscopy experiments to clarify the mechanism it is observed that the biasing improves the confinement of the plasma. It is estimated that the effect cannot be explained solely by secondary electron emission from the plate into the plasma. We propose that by biasing, the overall balance between radial ion losses and axial electron losses will change, resulting in a different diffusional mode of the entire plasma. Hence, the plasma potential and the average charge state of ions in the plasma are significantly influenced. Usually, the ion flux is dominating radial diffusion while the electron flux is dominating axial losses. This is possible due to compensating wall currents in the electrical conducting plasma chamber (“Simon short circuit”). Thus the usual approach of ambipolar diffusion does not hold in this situation. A similar effect takes place if the plasma chamber is coated with electrically insulating materials. The condition of overall flux balance to the walls is no longer fulfilled and has to be replaced by the local ambipolar particle movement. Again the entire diffusion profile of the plasma changes and the confinement improves. We examine the short circuit current as a measure for the diffusion mode in more detail and try to develop an approximate calculation on the influence of plasma potential and average charge ion state Z in the plasma. First results are presented and discussed.

Soft x-ray spectroscopy experiments on the nearK-edge of B inMB2(M=Mg,Al, Ta, and Nb)

Soft X-ray absorption and emission measurements are performed for the K- edge of B in MB$_2$ (M=Mg, Al, Ta and Nb). Unique feature of MgB$_2$ with a high density of B 2$p_{xy}(\sigma)$-state below and above the Fermi edge, which extends to 1 eV above the edge, is confirmed. In contrast, the B 2$p$ density of states in AlB$_2$ and TaB$_2$, both of occupied and unoccupied states, decreased linearly towards the Fermi energy and showed a dip at the Fermi energy. Furthermore, there is a broadening of the peaks with $p\sigma$-character in XES and XAS of AlB$_2$, which is due to the increase of three dimensionality in the $p\sigma$-band in AlB$_2$. The DOS of NbB$_2$ has a dip just below the Fermi energy. The present results indicate that the large DOS of B-2$p\sigma$ states near the Fermi energy are crucial for the superconductivity of MgB$_2$.

Experimental behavior of a two-chip charge amplifier for high-stability spectroscopy systems

We present the first experimental characterization of an innovative two-chip detector system for X-ray spectroscopy experiments in which stability of operation is mandatory to avoid on-line calibration procedures. The system is based on a silicon drift detector and on a charge amplifier partly integrated in the detector chip. The design guidelines of both the "detector chip" produced on a high resistivity substrate and the "amplifier chip" designed in 0.8-/spl mu/m BiCMOS technology are reviewed. The proper functionality of each of the two chips and the performance of the whole system have been experimentally verified.

Embedded front-end for charge amplifier configuration with sub-threshold MOSFET continuous reset

The paper reports the considerations necessary to design state of the art front-end electronic devices integrated on silicon X-ray detectors to perform functional charge amplification. In particular it concentrates on the characteristics of a MOSFET feedback device operated in subthreshold mode to perform a continuous reset of the feedback capacitance with minimum added noise and maximum linearity. Details of the external microelectronic circuitry that complements the front-end devices are also given. The designed two-chip system aims to reach very high stability in charge-to-voltage conversion and to fulfil the requirements of the most advanced X-ray spectroscopy experiments.

First experimental characterization of ROTOR: the new switched-current VLSI amplifier for X-ray spectroscopy with silicon drift detectors

In this paper we present the results of an experimental characterization of ROTOR, a new VLSI shaping amplifier conceived for multi-anode Silicon Drift Detectors, or other multi-element solid-state X-ray detection systems. The readout scheme is suitable for the demanding requirements of new X-ray spectroscopy experiments with synchrotron light, like X-ray holography, where both high count-rates and good energy resolutions are required. The circuit, which is of time-variant nature, features a trapezoidal weighting function obtained by means of a switched-current technique. A first prototype of the circuit, integrated with the 1 /spl mu/m/5 V JFET/CMOS technology of the Fraunhofer-Institut, Duisburg (Germany), has been characterized in spectroscopy measurements with a Silicon Drift Detector. The experimental results are presented and discussed in the work.

Comparative X-ray Photoemission Spectroscopy Study of Au, Ni, and AuNi Clusters Produced by Laser Vaporization of Bulk Metals

Supported Au and Ni clusters as well as bimetallic AuNi clusters have been produced by laser vaporization of both pure bulk metals and alloy. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) experiments show that they have a narrow distribution size centered around 2.5 nm and that bimetallic clusters have the same composition as the vaporized Au0.5Ni0.5 alloy. X-ray photoemission spectroscopy (XPS) has been used to measure core level binding energy shifts of Au 4f7/2 and Ni2p3/2. The shifts observed in the bimetallic system show the combined effects of both alloying and surface local order in relation with strong Au surface segregation as deduced from previous low energy ion scattering (LEIS) experiments.

Theoretical Study on Total-Reflection-Angle Effect of Fluorescent X-Rays Emitted from Atoms Deposited on Surfaces

Theoretical analysis is performed for the angular distribution of fluorescent X-rays in a total-reflection-angle X-ray spectroscopy (TRAXS) experiment. The angular dependence of fluorescent X-ray intensity is caused by the interference effect between the direct beam and that reflected by the substrate surface. The calculation results agree with the experimental results reported by Hasegawa et al. [Jpn. J. Appl. Phys. 24 (1985) L387].

Theoretical and experimental electronic distributions in AL6Mn

The authors report on a comparison between densities of states of crystalline Al6Mn calculated using the LMTO band structure method and electronic distributions obtained by means of soft X-ray spectroscopy experiments. The agreement is found to be rather good. They propose that changes in Al sites are responsible for the modifications of the densities of states observed in quasicrystalline Al-Mn alloys.

Evidence of a wide pseudo-gap in AlCuFe icosahedral alloys

Using X-ray spectroscopy experiments electronic distributions of both valence and conduction bands have been investigated separately in icosahedral Al 63Cu 25Fe 12 and crystalline ω-Al 7Cu 2Fe. Photoemission measurements determine the position of the Fermi level; this allows comparison of the various electronic distributions. Effects of hybridization are found. Both a quite low Al density of states at the Fermi level and a rather wide pseudo-gap are observed in the icosahedral phase; such a pseudo-gap also exists in the crystalline alloy but it is smaller by a factor three. Besides hybridization effects, a possible charge transfer to Fe orbitals is proposed to interpret the empty Fe d electronic distributions of the alloys.

Mosaic crystal X-ray spectrometer to resolve inelastic background from anomalous scattering experiments

We describe a mosaic crystal X-ray energy analyzing spectrometer to separate the inelastic scattering enhanced by radiation near an absorption edge from the elastic diffraction of interest for atomic site determinations. An important consideration is to minimize the sensitivity to changing source profile so that diffracted intensities are comparable over large angular ranges. Energy resolution limited mainly by fractional-mm source sizes is comparable to that with nearly perfect crystals. High spectrometer efficiency for the diffusely distributed X-ray scattering associated with amorphous materials and the weak Laue scattering from chemical order in crystalline solid solutions is achieved with a combination of sagittal and parafocusing from a mosaic graphite monochromator. Because of the mosaic nature of the crystal, the parafocusing geometry achieves meridional focusing for a magnification of 1 : 1, and a broad energy range proportional to the mosaic spread is dispersed along a linear position-sensitive detector for simultaneous detection. Energy resolution ΔE/ E of 1 500 and lower is achieved in the 6–10-keV energy range tested. Though the sagittal radius is fixed, the spectrometer will operate effectively over an X-ray energy range of E ± 0.5 E with fixed focal distances. This mosaic crystal spectrometer offers similar advantages for X-ray spectroscopy experiments.

Consequences of hot gas in the broad-line region of active galactic nuclei

Models for hot gas in the broad line region of active galactic nuclei are discussed. The results of the two phase equilibrium models for confinement of broad line clouds by Compton heated gas are used to show that high luminosity quasars are expected to show Fe XXVI L alpha line absorption which will be observed with spectrometers such as those planned for the future X-ray spectroscopy experiments. Two phase equilibrium models also predict that the gas in the broad line clouds and the confining medium may be Compton thick. It is shown that the combined effects of Comptonization and photoabsorption can suppress both the broad emission lines and X-rays in the Einstein and HEAO-1 energy bands. The observed properties of such Compton thick active galaxies are expected to be similar to those of Seyfert 2 nuclei. The implications for polarization and variability are also discussed.

The broad-band X-ray spectrum of Cygnus X-2

Cygnus X-2 was observed with the broad-band X-ray spectroscopy experiment, HEAO 1 A-2, in the energy range 0.4-18 keV for four intervals of approximately 31 s over the course of 5 days in 1977. The spectra can be adequately represented by single-temperature thermal bremmstrahlung continua with temperatures ranging from 3.7 x 10 to the 7th K to 6.4 x 10 to the 7th K. An examination of the spectra and the spectra-luminosity relationship effectively rules out one degenerate dwarf model for the X-ray emission. The far-UV continuum emission could be dominated by this continuum component during X-ray high states, an effect which would be detected in optical UV line observations. A Comptonized X-ray cloud around a neutron star remains a viable model for the observed X-ray spectra.