Results 1 - 10 of 3633
Results 1 - 10 of 3633. Search took: 0.035 seconds
|Sort by: date | relevance|
[en] This paper describes the automation of APD closed cycle helium refrigerator for Low Temperature Positron Annihilation Studies. This involves communication between the temperature controller and a personal computer run by a software developed in visual basic (VB) 6.0. (author)
[en] A key assumption in the usual computations of Doppler broadening is to ignore the dependence of Γ(j) on the state of the compound nucleus. This is an excellent approximation for the γ-emission contribution to Γ(j). It is not so clear that it is as good for the neutron contribution. In this paper, we derive an expression for Γn(j) and show that the effect of the dependence of Γn(j) on the intermediate energy of the target εj is not significant for an ideal gas, so that the absorption line shape is identical to the absorption line shape in which Γn is constant. Therefore, the effects of j dependence Γn(j) on the absorption cross section is not significant. (author). 7 refs., 1 tab., 2 figs
[en] The Boltzmann constant k_B is a fundamental physical constant in thermodynamics. The present CODATA recommended value of k_B is 1.3806488(13) × 10"–"2"3 J/K (relative uncertainty 0.91 ppm), which is mainly determined by acoustic methods. Doppler broadening thermometry (DBT) is an optical method which determines k_B T by measuring the Doppler width of an atomic or molecular transition. The methodology and problems in DBT are reviewed, and DBT measurement using the sensitive cavity ring-down spectroscopy (CRDS) is proposed. Preliminary measurements indicate that CRDS-based DBT measurement can potentially reach an accuracy at the 1 ppm level. (topical review)
[en] Complete text of publication follows. Simultaneous tristatic measurements of thermospheric winds and temperatures were made with three imaging Fabry-Perot interferometers located in central Alaska at Poker Flat, Fort Yukon, and Eagle using the observed Doppler shifts and Doppler broadening of the 630-nm line shape emission profile observed in aurora and airglow. These results have been analyzed for 5 nights during the period of 2006 to 2008 to determine the thermospheric divergence and vorticity temporal variations throughout the night for the region defined by four common volume locations located between the three stations. In addition, zonal and meridional measurements of the background thermospheric wind field were also made. These results are examined relative to the time history of auroral activity to assess the importance of particle and Joule heating forcing functions in modifying the local time variations of these two parameters of the thermospheric neutral wind field.
[en] The Voigt function H(a,v) is defined as the convolution of the Gaussian and Lorentzian functions. Recent papers puplished in different areas of physics emphasize the importance of the fast and accurate calculation of the Voigt function for different orders of magnitude of variables a and v. An alternative analytical formulation for the Voigt function is proposed in this paper. This formulation is based on the solution of the non-homogeneous ordinary differential equation, satisfied by the Voigt function, using the Frobenius and parameter variation methods. The functional form of the Voigt function, as proposed, proved simple and precise. Systematic tests are accomplished demonstrating some advantages with other existent methods in the literature and with the numeric method of reference.
[en] Intrahepatic portosystemic venous shunts are rare. To date, only 28 cases have been reported, mostly in adults with liver cirrhosis, although congenital cases have also been described. We present a case of intrahepatic portosystemic venous shunt in a newborn that was diagnosed by color Doppler ultrasound. We stress the role of color Doppler as the technique of choice in the study of hepatic lesions of vascular origin. (Author) 7 refs
[en] In this study, general expressions for the signals in frequency-modulation spectroscopy (FMS) appear in the literature but are often reduced to simple analytical equations following the assumption of a weak modulation index. This is little help to the experimentalist who wants to predict signals for modulation depths of the order of unity or greater, where strong FMS signals reside. Here, we develop general formulas for FMS signals in the case of an absorber with a Voigt line shape and then link these expressions to an example and existing numerical code for the line shape. The resulting computational recipe is easy to implement and exercised here to show where the larger FMS signals are found over the coordinates of modulation index and modulation frequency. One can also estimate from provided curves the in-phase FMS signal over a wide range of modulation parameters at either the Lorentzian-broadening or Doppler-broadening limit, or anywhere in between by interpolation