Digital spectrometer for coincidence measurement of Doppler broadening of positron annihilation radiation

High-resolution digital coincidence Doppler broadening spectrometer equipped with two high-purity Ge detectors and two-channel 12-bit fast digitizer was developed and tested in this work. Two configurations were compared: (i) semi-digital setup which uses active analogue shaping of detector pulses prior to digitization to improve signal-to-noise ratio, and (ii) pure-digital setup which samples detector pulses directly. Software procedure developed for analysis of sampled waveforms, i.e. precise determination of energy of detected photon and rejection of distorted pulses, is described. Performance of digital coincidence spectrometer was compared with traditional analogue setup connected to the same detectors. It was found that digital spectrometer enables to achieve better energy resolution than in traditional analogue setup. Moreover, in digital configuration one has better control over shape of the signal. This allows efficient elimination of undesired distorted or damaged waveforms and to obtain spectrum of better clarity. The superior parameters of new digital coincidence Doppler broadening spectrometer are demonstrated by benchmark measurements of well defined Fe and Al specimens and also by the detection of rare annihilation in flight events.     

Statistical analysis of the Doppler broadening coincidence spectrum of electron–positron annihilation radiation in silicon

We report a statistical analysis of Doppler broadening coincidence data of electron–positron annihilation radiation in silicon using a ²²Na source. The Doppler broadening coincidence spectrum was fit using a model function that included positron annihilation at rest with 1s, 2s, 2p, and valence band electrons. In-flight positron annihilation was also fit. The response functions of the detectors accounted for backscattering, combinations of Compton effects, pileup, ballistic deficit, and pulse-shaping problems. The procedure allows the quantitative determination of positron annihilation with core and valence electron intensities as well as their standard deviations directly from the experimental spectrum. The results obtained for the core and valence band electron annihilation intensities were 2.56(9)% and 97.44(9)%, respectively. These intensities are consistent with published experimental data treated by conventional analysis methods. This new procedure has the advantage of allowing one to distinguish additional effects from those associated with the detection system response function.     

Measurements of collisional broadening coefficients by infrared polarization spectroscopy

We present measurements of collisional broadening coefficients, obtained at atmospheric pressure, by polarization spectroscopy. Using tunable single mode laser radiation at approximately 2 microm, high-resolution infrared polarization spectra were recorded for CO2-Ar and CO2-He binary mixtures. The recorded polarization spectra were fitted with a Lorentzian cubed function form to obtain the broadening coefficients. The full-width at half-maxima (FWHM) collisional broadening rates of CO2 by Ar and He, for the R14 (12 degrees1<--00 degrees0) line, have been determined to be 0.161+/-0.018 cm-1 atm-1 and 0.1823+/-0.0032 cm-1 atm-1, respectively.     

Deconvolution of 2D coincident Doppler broadening spectroscopy using the Richardson–Lucy algorithm

Coincident Doppler Broadening Spectroscopy (CDBS) measurements are popular in positron solid-state studies of materials. By utilizing the instrumental resolution function obtained from a gamma line close in energy to the 511 keV annihilation line, it is possible to significantly enhance the quality of the CDBS spectra using deconvolution algorithms. In this paper, we compare two algorithms, namely the Non-Negativity Least Squares (NNLS) regularized method and the Richardson–Lucy (RL) algorithm. The latter, which is based on the method of maximum likelihood, is found to give superior results to the regularized least-squares algorithm and with significantly less computer processing time.     

Study of Zn diffusion in n-type GaSb by cathodoluminescence and scanning tunneling spectroscopy

We report here the studies carried out in zinc diffused n-type GaSb by cathodoluminescence (CL) microscopy and by scanning tunneling spectroscopy. Samples with different diffusion profiles measured by secondary ion mass spectrometry (SIMS) were obtained. CL plan-view observations show high homogeneity in the diffused layers. Cross-sectional measurements of the Zn diffused layers were performed by current imaging tunneling spectroscopy (CITS). The junction border was revealed clearly in the CITS images and conductance spectra recorded at differents points of the layers provided information on the local surface band gaps and the conductive behaviour. The results were related to the diffusion profiles and were found to agree with diffusion models suggested previously.     

Electromagnetically induced self-imaging in the Doppler broadening medium

Under the condition of electromagnetically induced transparency, self-imaging in three-level Lambda-type atoms at normal temperature is studied. The influences of the temperature on the position of the self-imaging and the corresponding imaging quality are discussed in detail. Numerical results show that, with the increase of the temperature, the location of the self-imaging linearly moves away from the original object, and the self-imaging quality decreases.     

Raman spectroscopy of n-type and p-type GaSb with multiple excitation wavelengths

The interpretation of Raman spectra of GaSb can be complicated by the presence of a so-called surface space-charge region (SSCR), resulting in an inhomogeneous near-surface Raman scattering environment. To fully interpret Raman spectra, it is important to have an understanding of the SSCR profile relative to the Raman probe depth. However, a priori determination of even the actual SSCR width is not always possible for GaSb under a wide range of doping levels. The primary objective of this report is to provide a convenient reference to aid in the determination of relative contributions to an observed GaSb Raman spectrum of SSCR scattering and bulk scattering for a range of excitation wavelengths, doping levels, and SSCR widths and types. Raman spectra of both n-type and p-type doped GaSb epilayers were obtained using 488 nm, 514.5 nm, 647.1 nm, and 752.55 nm excitation radiation. Both n-type and p-type doped GaSb epilayers were selected for investigation because these layers exhibit the two different SSCR types that are typically encountered with as-grown GaSb and related materials. A range of doping levels were examined for each doping type so as to examine the effects of a varying SSCR width on the observed spectra. A secondary objective of this report is to demonstrate the performance of a spectroscopic system based on 752.55 nm excitation that is sensitive to bulk carrier properties in n-type and p-type doped GaSb epilayers over a wide doping range, unlike visible-wavelength-based optical systems.     

Nickel carbide: Its formation and characterization by transmission electron diffraction,Auger electron spectroscopy and electron spectroscopy for chemical analysis

Nickel carbide (NiC₃) films are formed by the carburization of nickel films in CO at 350°C. The presence of Ni₃C is demonstrated by transmission electron diffraction. The carbon Auger electron signal of Ni₃C is identical with the carbon Auger spectra attributed to Ni₃C by previous authors. The position of the C 1s electron spectroscopy for chemical analysis peak is within 1 eV of the C 1s peak produced by graphite. The grain size of polycrystalline Ni₃C is significantly larger than the grain size of the nickel film from which it is grown.     

Transmission electron microscope studies of tantalum irradiated by MeV energy α particles

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Scattering of phonons by vacancies

The scattering of phonons by vacancies is estimated by a perturbation technique in terms of the missing mass and the missing linkages. An argument is given why distortion effects can be disregarded. The resonance frequency of the defect is sufficiently high so that resonance effects can be disregarded for phonons in the important frequency range for thermal conduction. The theory is applied to the thermal resistance by vacancies in cases where the vacancy concentration is known: potassium chloride with divalent cations, nonstoichiometric zirconium carbide, and tin telluride.     

Oxide formation on the silicon (111) surface studied by Auger electron spectroscopy and by low energy electron loss spectroscopy

Core electron and valence electron excitation spectra measured using low energy electron loss spectroscopy in combination with Auger electron spectroscopy were used to study oxide formation on clean crystalline silicon. The chemical bonds formed in the various oxidation stages are described by localized molecular states. SiO double bonds, Si—O bonds of the type found in SiO₄ tetrahedra, Si—Si bonds and broken Si—O bonds were detected.