Double Crystal Spectrometer Research Articles

Xenon L emission spectra and many-electron effects in core levels.

Nine L x-ray emission lines of xenon (Z=54) in the gas phase were measured in fluorescence. The spectra were obtained on a double-crystal spectrometer using a conventional x-ray tube for primary excitation. The binding energies of the L subshells were obtained using the experimental x-ray photoelectron-spectroscopy (XPS) ionization energies of the final states and found to be in agreement with earlier XPS measurement. The K-shell binding energy was also determined. The transition energies, level energies, and widths were analyzed with respect to available experimental and theoretical data

A fundamental parameters approach to X-ray line-profile fitting

A convolution approach to X-ray powder line-profile fitting is developed in which the line shape is synthesized from the Cu Kα emission profile, the dimensions of the diffractometer and the physical variables of the specimen. In addition to the integrated intensities and 2θ positions of the line profiles, the parameters that may be fitted include the receiving-slit width, the receiving-slit length, the X-ray-source size, the angle of divergence of the incident beam, the X-ray attenuation coefficient of the specimen and the crystallite size. This is a self-consistent approach to fitting as the instrumental parameters are usually known by direct measurement. To minimize correlation between refined instrumental parameters, profiles at high and low 2θ values should be fitted simultaneously. The Cu Kα emission profile used in this work is based on recent monolithic double-crystal spectrometer measurements that have identified a doublet structure in both the Kα 1 and Kα 2 components. Fast and accurate convolution procedures have been developed and a mixture of multilinear regression and Gauss–Newton non-linear least squares with numerical differentials is used for fitting the profiles. The method is evaluated by fitting powder diffraction data from well crystallized specimens of MgO and Y3Al5O12 (YAG). Testing has also been carried out by examining the changes in the fitted values after altering various instrumental parameters (e.g. receiving-slit width, detector defocus, receiving-slit length and inclusion of a monochromator).

A double focusing crystal spectrometer for low energy X-rays

A high resolution double focusing crystal spectrometer is described. Thirteen single silicon crystals are used to reflect the X-ray beam onto three CCD detectors. An energy resolution of 4 eV FWHM was reached in a measurement of the 2.9 keV K α1 and K α2 lines in argon.

Relationship between Self-Texture and Interfacial Restriction on the Epitaxy of ZNO Thinfilms I

ABSTRACTWe investigated relationships between self-texture and interfacial restriction on the epitaxy of directed, bonded, thin films. Defect structures in ZnO films grown epitaxially on (0001)sapphire mere evaluated by 3—dimensional maping of k—spacing using an X—ray double crystal spectrometer. The orientation distribution and variation in lattice spacing of the films wre individually measured to elucidate the mechanism of growth. The growth mode was found to change at a film thickness of approximately 100 Å. The layer within 100Å of the interface had a small orientation distribution (1.8'), a large lattice spacing distribution, and a large cumpressive stress.The layer that was more than 100Å from the interface had a larger orientation distribution, a smaller lattice spacing distribution, and a smaller compressive stress.

Influence of defects on the thickness dependence of integral reflectivity of dislocation-free silicon crystals

The influence of large dislocation loops and clusters (with radius rd of the order of extinction length Λ) in dislocation-free silicon crystals grown by the Czochralsky-method on the thickness dependence of the total integral reflectivity Ri as well as on its Bragg's RB and diffuse RD components is investigated by variation of the X-ray wavelength (MoK, CuK-radiation) and of the order of Laue diffraction. For such defects diffuse scattering is concentrated near the reciprocal lattice point (RLP). Integral intensities are measured by means of the Bragg-Laue double crystal spectrometer in the positions (n, −n) and (n, −2n). The components RB and RD are determined using the formula of the dynamical theory of X-ray scattering in crystals with homogeneously distributed defects and the values of diffraction parameters i.e. the static Debye-Waller's factor L, and coefficient absorption due to diffuse scattering μds measured under conditions of weak absorption where the interfere influence of dynamical effects is negligible. The sensitivity of Ri to large defects is shown to increase at the expence of growing of the relative contribution of the RD component by transition to reflection of higher orders as well as by using of harder short-wavelength radiation. The ratio RD/Ri does not depend on the crystal thickness under conditions of strong absorption what conforms the arising of the dynamical regime for the diffuse component. This effect is absent for small (rd ≫ Λ) defects. The parameter L is shown to be more sensitive to the presence of large defects than μds. [Russian Text Ignored].

Coherent production of light scalar or pseudoscalar particles in Bragg scattering

X-rays penetrating crystals can produce light scalar or pseudoscalar particles via the Primakoff effect. We compute the intensity of scalar and pseudoscalar particles in the reflected beam of a Bragg reflection. We then estimate the sensitivity of a realistic experiment, using synchrotron radiation and a double crystal spectrometer, which can test for the existence of light scalar and pseudoscalar particles.

Beam-foil spectroscopy of n=3 to n=2 transitions in highly stripped bismuth.

The {ital n}=3 to 2 transitions in Ne-like, Na-like, and Mg-like bismuth were measured on the UNILAC accelerator at the Gesellschaft fuer Schwerionenforschung in Darmstadt, West Germany, using the Lawrence Livermore National Laboratory high-resolution double-crystal spectrometer. The measurements cover the region 10--14 keV with precisions of the order of 100 ppm and include electric-dipole and quadrupole transitions. Theoretical multiconfigurational Dirac-Fock calculations were made for the Ne-like system and compared to the experiment. The data exhibit energy shifts of 1.5--2.5 eV from the theory, confirming trends previously observed in tokamak experiments. The Na-like and Mg-like results are compared with unresolved transition array calculations.

X-ray diffuse scattering identification of interstitial atom aggregates in Si-doped GaAs single crystals

Microdefects of different types in highly Si-doped GaAs single crystals grown by the horizontal Bridgman (HB) method were studied by means of X-ray diffuse scattering intensity measurements carried out with the help of a dispersion-free Bragg-type double-crystal spectrometer. It was shown that the predominant microdefects in GaAs crystals, Si-doped to the level of n=3 ×10 18 cm -3, were the so-called interstitial point defect clusters consisting of single interstitials gathered in “clouds” whose mean radius varied from 0.1 to 0.4 μm. A not earlier observed proportionality law I∞( Δθ) -3 is seen on the linearized curve of angular dependence of X-ray diffuse intensity ( I) scattered by the crystal doped to the level determined by the inequality n > 3 × 10 18 cm -3. It was also shown that the angular dependence I ∞ (Δθ) -3 was due to the predominance of extrinsic dislocation loops with mean radius of ∼0.5 μm. Burgers vector determination was based on measuring of the angle corresponding to the X-ray intensity angular dependence transition from I∞( Δθ) -2 to I∞( Δθ) -3, where Δθ is the deviation of the angular position of the second crystal from the accurate Bragg angle.

Compensation effect of lattice constant in silicon n +-n - junctions by liquid phase epitaxy

Investigations have been made of the compensation effect of lattice constant on LPE grown silicon layer by the simultaneous doping of tin and phosphorus. The difference in the effective lattice constants between the epitaxial layers and the substrates is measured by an X-ray double-crystal spectrometer using asymmetric Bragg reflection. It is possible to eliminate the difference by proper doping of tin, and highly doped layer can be grown on the high resistive substrate under lattice matching condition.

Beamline 14B at Photon Factory: Precision x-ray optics experiment facility (abstract)

The beamline 14B at Photon Factory is one of three branches of the 5T vertical superconducting wiggler beamline (BL14) at Photon Factory. This branch is used mainly for precision x-ray optics experiments and partly for commissioning a low-angle double-crystal spectrometer and a gas-phase scattering apparatus. A double-crystal fixed-exit monochromator was installed on this beamline; it has two offset positions, 200 and 500 mm. In order to avoid the leak of high-energy x rays from the monochromator chamber, the first crystal is shielded by lead with a rotatable 2θ window inside the vacuum chamber. A precision x-ray optics experiment facility consists of two vertical axis goniometers designed for the use of vertical polarization. Rough rotation (360° with the finest step of 3.24 arcsec) and fine rotation (within 6°, the finest step of 0.01 arcsec) are changed with a clutch. A rotary encoder with precision of 1.0 arcsec is equipped. A double-crystal aligner mounted on one of the precision goniometers is constructed to make another double-crystal arrangement. Combining the beamline double-crystal monochromator with this aligner, we can make a fixed-exit four-crystal monochromator of (+n,−n,−m,+m) setting, known as the high resolution x-ray optics in energy as well as momentum. Two independent microcomputer control systems are used for beamline monochromator and experimental facility. These two systems are connected by a serial communication (RS-232C) line via a software controlled line selector. This makes it quite easy for user-made equipment to control the beamline monochromator synchronously by its own control system. A low-angle double-crystal spectrometer was further constructed to characterize synthetic multilayers. This spectrometer consists of two ω-2θ goniometers mounted on x- (used for vertical axis) or xz- (used for horizontal axis) translation stage. These goniometers are mounted on independent carriers of a y-translation stage along the x-ray beam.

A double crystal spectrometer for laboratory EXAFS spectroscopy

A double-crystal x-ray spectrometer designed for in-laboratory EXAFS (extended x-ray absorption fine structure) study is described. By the use of two curved crystals with different structure factors such as Ge (220) and Ge ( 111 ), reflections other than the desired order are virtually eliminated and monochromatic as weB as intense x-ray flux can be obtained by applying higher voltage to the x-ray generator. As a result, data ofvery high quality can be collected in a short period by measuring the incident and the transmitted beam intensity simultaneously. It can cover a wide spectral range (7-26 ke V) and is especially useful in the high-energy region, where the laboratory EXAFS system previously reported has some difficulties.

Many-electron effects in L gamma 2,3 x-ray emission spectroscopy spectrum of BaO.

The L${\ensuremath{\gamma}}_{2}$,3 (2${s}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\rightarrow}4${p}^{\mathrm{\ensuremath{-}}1}$) x-ray emission spectrum of BaO was measured in fluorescence with a high-resolution vacuum double-crystal spectrometer. The spectrum was also calculated by the Green's function method. It is shown that the spectrum can be interpreted essentially in terms of a two-level system in the final state, e.g., the 4p hole level interacting with the product of the 4${p}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\leftrightarrows}4${d}^{\mathrm{\ensuremath{-}}2}$4f super Coster-Kronig process. It is also shown that it is necessary to take into account the 4${d}^{\mathrm{\ensuremath{-}}2}$4f multiplet.

Acceleration, deceleration and monochromatization of neutrons in time dependent magnetic fields

A novel technique using time-dependent magnetic fields to accelerate and decelerate neutrons, is demonstrated using a small prototype system. The energy-shift in a two-stage system with a maximum field amplitude of 0.4 T is measured with a high-resolution double-crystal spectrometer. From the broadening of the rocking curves, the related energy shift was determined to be ±(58.4±1.6) neV. The results of various calculations demonstrate the potential of this technique for an active beam tailoring of slow neutrons. For possible applications multi-stage systems will be required, in which the problem of unwanted adiabatic spin turns will have to be minimized.

Recent developments at the ILL (n,γ)-facility

The (n, gamma )-facilities at the HFR of the ILL Grenoble have been used intensively for several years by a large group of experimentalists. The bent crystal spectrometers GAMS1 and GAMS2/3 serve to investigate secondary gamma -ray transitions after thermal neutron capture. The double flat crystal spectrometer GAMS4 is used for high accuracy, absolute wavelength measurements and for direct measurements of Doppler broadened capture lines. Furthermore a pair spectrometer using a three crystal arrangement, and conventional HPGe-detectors, serve to investigate the high energy part of the gamma -ray spectrum after thermal neutron capture. The HFR was shut down for general renovation and improvement from October 1984 until September 1985. During this time the tangential beam tube for the bent crystal spectrometers had to be replaced. This shut down was used to improve the in-pile target facility and the instruments and the authors report on these developments and give some results.

High accuracy, absolute wavelength determination of capture gamma-ray energies for E⩽5 MeV and the direct determination of binding energies in light nuclei

A substantially improved double flat crystal gamma ray spectrometer (GAMS4) has been put into operation as a joint NBS/ILL facility at the HFR, Grenoble. This facility has the capability of determining gamma ray wavelengths, in absolute units, with uncertainties on the order of 0.1 ppm in the energy range from 0.1 to approximately=5 MeV. This new facility incorporates a number of major improvements over the earlier double flat crystal spectrometer used to determine the n-p 2.2 MeV line with an error of 1 ppm (2 eV). The operation of the instrument and the nature of the improvements is discussed. Experimental results related to the direct determination of the binding energy of several light nuclei (for example D, 13C, 15N) are given. The implications of this work for the atomic mass scale and for the determination of fundamental constants is reviewed.

Methods and new techniques in neutron capture gamma-ray spectroscopy

Neutron capture gamma -ray spectroscopy now employs the most sophisticated techniques to study nuclear structure, yet efforts are continuing to improve still further sensitivity and resolving power of the spectrometers used. The combination of precise measurements of primary gamma -transitions with the precision obtained to date with curved and flat crystal and conversion electron spectrometers has considerably extended the possibilities in the study of nuclear structure. Techniques are also available for the investigation of multiple neutron capture leading to nuclei which are up to three neutrons beyond stability. The nearly perfect resolution obtained with the most modern double flat crystal spectrometers allow the measurement of lifetimes of excited nuclear states in the range <10-12 s via the observation of the Doppler-broadening of gamma -rays due to the recoil of the nucleus following emission of a primary gamma -ray.

Dopper shift attenuation lifetime measurement in 54Cr following thermal neutron capture

The double crystal spectrometer GAMS4 in combination with the ILL high flux reactor has been used to determine the lifetimes of the 2620 KeV 2 + 2, 3074 KeV 2 + 3 and 3720 KeV (1, 2) + states in 54Cr. The initial recoil energy of about 0.5 KeV imparted by the primary γ-radiation after thermal neutron capture in 53Cr produces Doppler broadened line shapes of the secondary transitions. The large 2 + 3→2 + 1 M1 strength of B(M1)=0.39(6) μ 2 N suggests the 2 + 3 state to be mixed symmetry character within the interacting boson model IBM-2.

Dependence of source gas mole ratio on crystalline quality of ZnSe layers grown on (100) ZnSe substrates by atmospheric pressure metalorganic vapor phase epitaxy

Abstract Single crystalline layers of undoped ZnSe have been grown on (100) ZnSe substrates at 250°C by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) using dimethylzinc (DMZ) and hydrogen selenide (H 2 Se). The surface morphologies, observed by a Nomarski phase contrast interference microscope, depend on the growth conditions, particularly the source gas mole ratio η M ( η M =[Se]/[Zn]). The surface morphologies of homoepilayers grown with η M of more than 15 are very rough like polycrystals. However, these layers have the best crystalline quality as determined by a double crystal X-ray diffraction spectrometer (the full width at half maximum (FWHM) of (400) diffraction is 33 arc sec). The surface morphologies of layers grown at values of η M between 3 and 15 are excellent as well as those of heteroepilayers (GaAs substrates are used). On the other hand, the crystalline quality becomes worse by reducing η M . The photoluminescence (PL) spectra at 4.2 K exhibit strong and well-resolved band-edge emissions from the donor bound exciton (hereafter called I x ) and the free exciton (hereafter called E x ) only for the low mole ratio (η M x is 0.8 meV which is sharper than that of heteroepilayers. However, the intensity of emissions from deep levels (deep emissions) of homoepilayers measured at 77 K is several times stronger than that of heteroepilayers. In the region of high mole ratio (η M >10), the PL spectra at 4.2 K show two broad lines: I x and the unknown I v (probably donor bound exciton). And the intensity of I x reduces two orders by decreasing the mole ratio from 20 to 3. This indicates that the concentration of donor decreases for the Se-rich condition. From the investigation of PL measurements, the origin of deep emissions is considered to be caused by complexes of Zn vacancies and impurities from the source gases. The dependences of η M on the quality of homoepilayers observed in this work are quite different from those in heteroepilayers reported previously.

Anomalous L gamma 2,3 x-ray emission spectrum of Xe.

The L${\ensuremath{\gamma}}_{2}$,3(2${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\rightarrow}4${\mathrm{p}}^{\mathrm{\ensuremath{-}}1}$) x-ray emission spectrum of gaseous xenon was measured in fluorescence with a high-resolution vacuum double-crystal spectrometer. The spectrum was also calculated by the Green's-function method. The agreement is excellent. It is shown that the spectrum can be interpreted essentially in terms of the spectral function of the 4p hole. The effects of the initial 2s-hole state on the final 4p-hole state are small. The spectrum shows that the quasiparticle picture of a 4p hole in xenon breaks down because of strong 4${p}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\leftrightarrows}4${d}^{\mathrm{\ensuremath{-}}2}$4(n,\ensuremath{\varepsilon})f super-Coster-Kronig processes.

Low-temperature diffusion of phosphorus in silicon

Nontrivial mechanisms of low-temperature diffusion of impurities in solids have been discovered recently. We studied accelerated diffusion of phosphorus in a single crystal of silicon, which takes place during decomposition of an over-saturated inhomogeneous solid solution. A thin (1.5–2 μm) doped layer was formed by low-temperature diffusion from the gas phase. The distribution profile, which was determined by the method of differential conductivity, shows that heat treatment at 550°C leads to a decrease in the donor concentration in the subsurface region and to the impurity movement into the bulk of the sample. The diffusion coefficient has been calculated on the computer using experimental data, and in the range between 400°C and 700°C has been represented by the Arrhenius equation. The value of the frequency term is equal to 3.7·10−6 cm2/sec, and the value of the activation energy is 1.81 eV. The presented results have been analyzed, as well as the data obtained from the measurements on carrier mobility (using the Hall effect) and the parameters of the crystal lattice (using the method of asymmetric double crystal x-ray spectrometer). A physical model for the dissociation diffusion is proposed, which is controlled by the decay, and analytical expressions are obtained which describe the main characteristics of the process.