Emission data for radionuclide X-ray intensity standards are revised, and an extensive set of standards is used to determine a very precise efficiency curve between 3 and 40 keV energy for a Si(Li) detector. The curve...
Emission data for radionuclide X-ray intensity standards are revised, and an extensive set of standards is used to determine a very precise efficiency curve between 3 and 40 keV energy for a Si(Li) detector. The curve is compared to predictions of a physical model; it is used to assess the internal consistency of the standard set and to determine the minimum subset of standards necessary to maintain the accuracy afforded by the entire set. The curve is fully consistent with the detector's properties as measured by independent experiments, provided that a surface layer of ice is invoked, as was recently suggested by Cohen. The efficiency values that it generates have accuracy under 1% in the 5–30 keV region. The need for new work on radionuclide standards in the 1–5 keV region is discussed.
A force constant model is developed and applied to the vibrations of the hydrogen halide crystals, HBr, HCl, and HF, in their ordered phases. Fundamental frequencies are calculated and compared to those observed by in...
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A force constant model is developed and applied to the vibrations of the hydrogen halide crystals, HBr, HCl, and HF, in their ordered phases. Fundamental frequencies are calculated and compared to those observed by infrared and Raman spectroscopy. It is found that two bending forces, representing electrostatic interactions, are necessary, one of which partially cancels elastic forces for certain modes. Variations of force constants with interatomic distances and comparisons of their values for the three crystals are briefly discussed. Es wird ein Kraftkonstantenmodell entwickelt und auf die Schwingungen der Wasserstoffhalogenkristalle, HBr, HC1 und HF in ihren geordneten Phasen angewendet. Die Grundfrequenzen werden berechnet und mit denen verglichen, die bei Infrarot- und Ramanspektroskopie beobachtet werden. Es wird gefunden, daß zwei Biegekräfte, die die elektrostatischen Wechselwirkungen darstellen, notwendig sind, von denen eine teilweise die elastischen Kräfte fur bestimmte Moden kompensiert. Die Ánderungen der Kraftkonstanten mit den interatomaren Abständen und ein Vergleich ihrer Werte für die drei Kristalle werden kurz diskutiert.
The generalised model potential theory, developed recently for liquid sp-type metals, is extended to the variational calculation of the Helmholtz free energy for the alloys of these metals. Hence, model potentials, wh...
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The generalised model potential theory, developed recently for liquid sp-type metals, is extended to the variational calculation of the Helmholtz free energy for the alloys of these metals. Hence, model potentials, which include the usual high-order pseudopotential perturbation (HOPP) effects, are obtained for use in alloy calculations. Thesese model potentials are applied to first-principles calculations of thermodynamic quantities and electrical resistivities for some typical liquid metallic binary alloys. The results thus obtained are found to be in reasonable agreement with observation. When compared with the results obtained using the usual low-order pseudopotential perturbation theory, they indicate that HOPP effects become extremely important in the case of the heat of mixing for any metallic binary alloy, whereas for some other properties these effects are essential only for those metallic alloys in which charge transfer is significant.
The authors' recently suggested approach (1984) for calculating the Helmholtz free energy, including the usual high-order pseudopotential perturbation corrections for the nearly-free-electron-like liquid metals, i...
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The authors' recently suggested approach (1984) for calculating the Helmholtz free energy, including the usual high-order pseudopotential perturbation corrections for the nearly-free-electron-like liquid metals, is critically examined and is now improved so as to be applicable in principle to any sp-type disordered metal in its liquid phase. It appears that the mean atomic volume, the packing density parameter and the related thermal quantities can now be predicted from first principles with reasonable accuracy by the presently described variational thermodynamic calculation for nearly all the sp-type liquid metals.
A method of calculating the pair correlation function from liquid structure factor measurements is described which attempts to maximize the entropy of the calculated distribution. Known constraints are applied to the ...
A method of calculating the pair correlation function from liquid structure factor measurements is described which attempts to maximize the entropy of the calculated distribution. Known constraints are applied to the pair correlation function, and the solution is forced to lie as close as possible to the measured data by means of a feedback procedure in which the difference between the calculated distribution and the measured data is used to make successive estimates of the trial function. In this way distortion which may be present in the original data is kept to a minimum in the calculated function. Previously published neutron diffraction data on liquid argon and liquid water are analysed with this new technique, and a substantial improvement in the Fourier transforms of the water diffraction data is seen.
The results of a new neutron diffraction experiment to measure the structure of water are presented. The data, measured at the McMaster Nuclear Reactor, are of a high quality and are analysed to yield the hydrogen-hyd...
The results of a new neutron diffraction experiment to measure the structure of water are presented. The data, measured at the McMaster Nuclear Reactor, are of a high quality and are analysed to yield the hydrogen-hydrogen pair correlation function using a subtraction procedure which has been used in previous experiments of this kind. This procedure circumvents the necessity of applying inelasticity corrections. The results are in good agreement with earlier work and serve to establish the general correctness of the subtraction procedure when used to determine hydrogen correlations. The data are further analysed to yield separate oxygen-hydrogen and oxygen-oxygen partial structure factors for liquid water. For the second part of the analysis an effective mass model of the dynamic scattering law is used, with the model parameter, the effective mass of the scattering particle, chosen by a least-squares fit to the measured differential cross sections. The final pair correlation functions are obtained using a maximum entropy analysis of the structure functions.
Previous experiments on the neutron structure factor S ( q ) for heavy water near 11.2°C have been extended so that the next term in the Taylor expansion may be examined. An improved result for the isochoric temp...
Previous experiments on the neutron structure factor S ( q ) for heavy water near 11.2°C have been extended so that the next term in the Taylor expansion may be examined. An improved result for the isochoric temperature derivative of S ( q ) at 11.2°C is obtained. These data and those for the (Δ T ) 2 coefficient in the Taylor expansion may be explained on a simple basis: namely that OH and HH distances between neighbouring molecules vary as ( T t 0 ) −n where ( T T 0 ) ≈ 1200 n for T ≈ 300 K.
We study the stability criterion for the critical point of a transition of the second kind in the T-H Q plane of an antiferromagnetic superconductor AFS (T is temperature and H Q is the staggered molecular field). The...
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We study the stability criterion for the critical point of a transition of the second kind in the T-H Q plane of an antiferromagnetic superconductor AFS (T is temperature and H Q is the staggered molecular field). The AFS is described by using the mean-field model given by Nass, Levin and Grest and we assume a one dimensional electron band. Some properties of the system near the critical point are also investigated.
The Green function with an insertion of the gauge-invariant twist-four operator O(x) = − 1 4 (F μν u ) 2 is calculated to one-loop order in the light-cone gauge. We show that the renormalization requires gauge-nonin...
The Green function with an insertion of the gauge-invariant twist-four operator O(x) = − 1 4 (F μν u ) 2 is calculated to one-loop order in the light-cone gauge. We show that the renormalization requires gauge-noninvariant counterterms satisfying the Slavnov-Taylor identity.
Gamma rays from a radioisotope source, 241 Am, are scattered from samples of light water (H 2 O) and heavy water (D 2 O) at temperature 297 K and density 0.0334 molecules/Å 3 to determine what differences occur i...
Gamma rays from a radioisotope source, 241 Am, are scattered from samples of light water (H 2 O) and heavy water (D 2 O) at temperature 297 K and density 0.0334 molecules/Å 3 to determine what differences occur in their structure factors. The apparatus, method of data collection and data analysis are described in detail. The results are compared to the predictions of a computer simulation, in which ground state librational motion of rigid molecules causes significant quantum effects in the site-site correlation functions of the two liquids. Differences between our data and the simulation suggest that coupling occurs between intramolecular and intermolecular modes, and influences the structure of water. It is noted that the structural changes due to quantum effects are similar to those that occur in water upon formation of a 0.55 M solution of LiOH.
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