Volume 80, Issue 2, February 2009
Index of content:
- OPTICS; ATOMS AND MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
80(2009); http://dx.doi.org/10.1063/1.3077144View Description Hide Description
An integral part of the National Institute of Standards and Technology Center for Neutron Research (NCNR) expansion project is the addition of five cold neutron guide tubes serving multiple experimental stations in an expanded guide hall. The guides have curved-straight arrangements in the horizontal plane, employing horizontally or vertically defocusing and focusing sections in some cases to improve transmission efficiency or for beam reshaping. The horizontally curved sections eliminate direct lines of sight between the source and the experimental stations, and the outer (concave) surfaces generally have higher critical angles than the inner (convex) surfaces. These features result in well-filtered cold neutron beams with no intensity losses at shorter wavelengths with respect to curved guides having the higher critical angle coatings on both surfaces. For all guides the critical angle of the outer coating of the curved section is selected to achieve a desirable characteristic wavelength, consistent with the instrument requirements. On guides where the scattering-plane beam divergence must be strictly limited, the inner radial coatings of the curved sections and the side coatings and lengths of the final straight sections are chosen to produce the desired beam divergence while the outer radial coating is selected so as to obtain a spatial-angular uniformity of the transmitted beam that is not achievable using a curved guide alone. The long-wavelength transmission of such guides tends to exceed that of equivalent straight guides using crystal filters.
A versatile electron-ion coincidence spectrometer for photoelectron momentum imaging and threshold spectroscopy on mass selected ions using synchrotron radiation80(2009); http://dx.doi.org/10.1063/1.3079331View Description Hide Description
We present a photoelectron-photoion coincidence (PEPICO) spectrometer named DELICIOUS II which combines a velocity map imaging apparatus with a Wiley–McLaren time of flight analyzer for the study of gas phase samples in interaction with the synchrotron radiation (SR). This versatile system is capable of providing photoelectron images on mass-selected compounds with kinetic energy resolutions of and a 17 eV bandwidth, as well as threshold photoelectron spectra with a measured resolution of 0.8 meV, as demonstrated on the ionization of argon. This instrument is also employed for threshold PEPICO experiments, allowing the selection of the parent ion’s internal state with sub-meV resolution for light masses and with typically 2 meV resolution for a mass of 100 amu and with a mass resolving power above 200. The continuous operation of the extraction fields and the independence from the electron’s time of flight are well adapted to the quasicontinuous multibunch mode of the SR. This, together with the high transmission of both the electron and ion detection, allows a high coincidence counting rate and facilitates the subtraction of false coincidences. We illustrate the spectrometer’s coincidence principle of operation with examples from the valence photoionization of an mixture and of .
80(2009); http://dx.doi.org/10.1063/1.3077284View Description Hide Description
Although reliable models may predict the detection efficiency of semiconductordetectors, measurements are needed to check the parameters supplied by the manufacturers, namely, the thicknesses of dead layer, beryllium window, and crystal active area. The efficiency of three silicon detectors has been precisely investigated in their entire photon energy range of detection. In the zero to a few keV range, we developed a new method based on the detection of the decay of the metastable transition. Very good theoretical knowledge of the energetic distribution of the decay mode enables precise characterization of the absorbing layers in front of the detectors. In the high-energy range , the detector crystal thickness plays a major role in the detection efficiency and has been determined using a source.
80(2009); http://dx.doi.org/10.1063/1.3081015View Description Hide Description
The extended use of biomass for heat and power production has caused increased operational problems with fouling and high-temperature corrosion in boilers. These problems are mainly related to the presence of alkali chlorides (KCl and NaCl) at high concentrations in the flue gas. The in situ alkali chloride monitor (IACM) was developed by Vattenfall Research and Development AB for measuring the alkali chloride concentration in hot flue gases . The measurement technique is based on molecular differential absorption spectroscopy in the UV range. Simultaneous measurement of concentration is also possible. The measuring range is 1–50 ppm for the sum of KCl and NaCl concentrations and 4–750 ppm for . This paper describes the principle of the IACM as well as its calibration. Furthermore, an example of its application in an industrial boiler is given.
80(2009); http://dx.doi.org/10.1063/1.3079379View Description Hide Description
A chromatic-free spatially resolved diagnostic system for microplasma measurement is proposed and demonstrated, which consists of an optical chromatic-free microscopemirror system, an electron multiplying charge coupled device (EMCCD), and bandpass filters. The diagnostic system free of chromatic aberrations with a spatial resolution of about is achieved. The factors that limit the resolution of this diagnostic system have been analyzed, which are optical diffraction, the pixel size of the EMCCD, and the thickness of the microplasma. In this paper, the optimal condition for achieving a maximum resolution power has been analyzed. With this diagnostic system, we revealed the spatial nonuniformity of a microwave atmospheric-pressure argon microplasma. Furthermore, the spatial distribution of the time-averaged effective electron temperature has been estimated from the intensity distributions of 750.4 and 415.8 nm emissions.