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排序方式: 共有104条查询结果,搜索用时 31 毫秒
1.
The K-shell X-ray fluorescence cross sections are determined experimentally for 10 elements such as Pb, Hg, Ir, W, Lu, Tm,
Dy, Tb, Gd and Nd at excitation energy of 661.6 keV associated with γ-rays of 137Cs radioisotope. The technique employed involves the measurement of total intensity of fluorescent K X-rays that follow the
photoeffect absorption of a known flux of γ-rays using a well type Nal(Tl) detector. The obtained results are compared with the available theoretical values and other
measured values. 相似文献
2.
In contrast to the high precision with which masses of atoms and molecules can be specified, their external dimensions remain intrinsically fuzzy. Attention is directed to the difference between measurements of interatomic separations within molecules, that in many instances are cited with error bars of +/– 0.002 Å, and the available approximate magnitudes of over-all sizes, needed for estimating packing densities (in solids or liquids). The following account, is a critical examination of the underlying concepts and of the diversity of data in the literature, regarding internal dimensions (bond lengths), overall extensions, cross-sectional areas, and volumes of molecules that may be derived from a variety of experiments. Historical markers of the evolution of the underlying theories and experiments are included. 相似文献
3.
P K Chakraborti 《Pramana》1978,11(3):307-311
By means of refinements in the modulated molecular beam technique the signal-to-noise ratio can be greatly improved, and differential
cross-sections, for collision of molecuies of the same species, can be measured. This was accomplished by combining beam modulation
and phase sensitive detection with very sharp turning on the front end of the lock-in-amplifier and long integration times
on the output. In addition, the signal-to-noise ratio of the Ar-Ar system as a function of integration time was investigated
using two different types of electron bombardment detectors an Aberth ion-source and a quadrupole mass filter. With an integration
time of 40 min the estimated upper limit to the signal-to-noise ratio is 1500 to 1 for the Aberth ion-source. Using quadrupole
mass filter with an integration time of 60 min the estimated upper limit to the signal-to-noise ratio is 5 × 104 to 1. For chemical kinetics studies this ratio may be two orders of magnitude higher.
Measurements were carried out at the Purdue University, Lafayette, Indiana, USA. 相似文献
4.
The spectroscopic amplitudes, form factors, angular distributions and total cross-sections for two nucleon transfer reactions
in Zr-region in the zero range distorted wave Born approximation are calculated using consistent set of shell model wave functions.
A single normalisation factor gives a good fit to all the two neutron transfer reaction data whereas the corresponding fit
for the two-proton transfer reaction data is less satisfactory. 相似文献
5.
6.
Freeman Dyson has questioned whether any conceivable experiment in the real universe can detect a single graviton. If not, is it meaningful to talk about gravitons as physical entities? We attempt to answer Dyson’s question and find it is possible concoct an idealized thought experiment capable of detecting one graviton; however, when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson’s conjecture is very likely true. We also point out several mistakes in the literature dealing with graviton detection and production. 相似文献
7.
8.
An analysis of nonequilibrium phenomena behind a plane shock is presented concerning the vibrational relaxation and the dissociation of a pure diatomic gas. In the first part, the temperature range is 600 K–2500 K and the dissociation processes are neglected. The population of each vibrational level is computed by solving relaxation and conservation equations. The relaxation process is described by the master equations of each vibrational level. The vibrational transition probabilities appearing in the relaxation equations are calculated analytically and take into account the anharmonicity of molecular vibration and the potential angular dependence. The populations obtained are compared to those calculated using a Treanor model and to those calculated with a nonequilibrium Boltzmann distribution. For moderately high levels significant differences may be observed. The importance of the V-V process is found to be weak for the transitions involving the lowest levels. In the second part, the temperature range is 2500 K–5500K and the dissociation process is taken into account as well as the gas dynamic behavior which did not appear in several recent works. The kinetic equations are transformed to obtain a first order differential system and the resolution of such a system coupled with the conservation equations leads to the population of each vibrational level. The vibrational transition probabilities associated with the atom-molecule interaction are deduced from the cross section calculation used in the first part. The bound-free transition probabilities are obtained, following Marrone and Treanor, assuming that dissociation must occur preferentially from the higher vibrational states: the Marrone and Treanor probability model is extended and employed with an anharmonic oscillator. In the present investigation, behind the shock wave, the evolution of the population distribution expressed as a function of the distance is not monotonous: a lag time appears as shown experimentally in previous works for the macroscopic parameters. For moderately high levels the influence of the anharmonicity and those of the V-V processes appear significant and strongly related. In a general way, in both temperature ranges investigated, the V-V processes reduce the effects of the T-V transfer. Finally the influence of thecharacteristic probability temperature U of Marrone and Treanor is analyzed and a method of determination of local varying U is proposed. 相似文献
9.
V. S. Kulhar 《Pramana》2004,63(3):543-551
The muonium/muonic hydrogen atom formation in μ±−H collisions is investigated, using a two-state approximation in a time dependent formalism. It is found that muonium cross-section
results are similar to the cross-section results obtained for positronium formation in e+-H collision. Muonic hydrogen atom formation cross-sections in μ--H collision are found to be significant in a narrow range of energy (5 eV–25 eV). 相似文献
10.
K. F. Canter P. G. Coleman T. C. Griffith G. R. Heyland 《Applied Physics A: Materials Science & Processing》1974,3(3):249
Experimentally determined values of the total scattering cross-sections for positrons in the energy range 2–400 eV on He,
Ne, Ar, Kr and Xe are presented.
Paper F 6 presented at 3rd Internal. Conf. Positron Annihilation, Otaniemi, Finland (August 1973). 相似文献