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21.
We discuss our experiences with exceptionally intense Mössbauer sources (of order 100 Ci) and their use in making precision determinations of line shape parameters. We review our measurements of interference and recoilless fraction using conventional transmission geometry. Fitting simultaneous sets of data with known constraints greatly improves the efficiency of line-shape determination. Measurements of interference have been made using a two-absorber technique first introduced by Mössbauer and coworkers. Our results indicate that the method can be useful for determining interference, but that incorrect line shape parameters are obtained if powder samples are used which do not comply well with the assumption of thickness uniformity in the absorber. 相似文献
22.
We report a crystal structure analysis of the room temperature phase of Cs2PbCu(NO2)6. In the space group Fmmm the Cu2+ ions have a tetragonally compressed octahedral environment of N atoms. The thermal ellipsoids of the nitro-groups in the (001)-plane indicate the presence of a two-dimensionally dynamic Jahn-Teller effect. This result is interpreted in terms of a time-averaged picture of tetragonally elongated octahedra in antiferrodistortive order. 相似文献
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Saavedra HM Barbu CM Dameron AA Mullen TJ Crespi VH Weiss PS 《Journal of the American Chemical Society》2007,129(35):10741-10746
Alkanethiol molecules in solution displace 1-adamantanethiolate self-assembled monolayers on Au{111}, ultimately leading to complete molecular exchange. Specifically, here, fast insertion of n-dodecanethiolate at defects in the original 1-adamantanethiolate monolayer nucleates an island growth phase, which is followed by slow ordering of the n-dodecanethiolate domains into a denser and more crystalline form. Langmuir-based kinetics, which describe alkanethiolate adsorption on bare Au{111}, fail to model this displacement reaction. Instead, a Johnson-Mehl-Avrami-Kolmogorov model of perimeter-dependent island growth yields good agreement with kinetic data obtained by Fourier transform infrared spectrometry over 100-fold variation in n-dodecanethiol concentration. Rescaling the growth rate at each concentration collapses all the data onto a single universal curve, suggesting that displacement is a scale-free process. The rate of displacement varies as the square-root of the n-dodecanethiol concentration across the 0.01-1.0 mM range studied. 相似文献
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Andrew T. Clayman K. Mark Lawrence Gary L. Mullen Harald Niederreiter N.J.A. Sloane 《组合设计杂志》1999,7(5):381-393
We present an updated survey of the known constructions and bounds for (t, m, s)- nets as well as tables of upper and lower bounds on their parameters for various bases. © 1999 John Wiley & Sons, Inc. J. Cobin Designs 7: 381–393, 1999 相似文献
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J. M. Schwantes R. S. Addleman J. D. Davidson M. Douglas D. Meier O. D. Mullen M. Myjak M. E. Jones M. L. Woodring B. Johnson P. H. Santschi 《Journal of Radioanalytical and Nuclear Chemistry》2009,282(3):889-895
We are developing a medium-resolution autonomous in situ gamma detection system for marine and coastal waters. The system is designed to extract and preconcentrate isotopes of interest from natural waters prior to detection in order to eliminate signal attenuation of the gamma rays traveling through water and lower the overall background from the presence of naturally occurring radioactive isotopes (40K and U–Th series radionuclides). Filtration is used to preconcentrate target isotopes residing on suspended particles, while chemosorption is employed to preferentially extract truly dissolved components from the water column. Used filter and chemosorbent media will be counted autonomously using two LaBr3 detectors in a near 4-π configuration around the samples. A compact digital pulse processing system, developed in-house and capable of running in coincidence mode, is used to process the signal from the detectors to a small on-board computer. The entire system is extremely compact (9″ dia. × 30″ len.) and platform independent, but designed for initial deployment on a research buoy. A variety of commercial and in-house nano-porous chemosorbents have been selected, procured or produced, and these and filter and detector components have been tested. 相似文献
30.
We discuss several enumerative results for irreducible polynomials of a given degree and pairs of relatively prime polynomials of given degrees in several variables over finite fields. Two notions of degree, the total degree and the vector degree, are considered. We show that the number of irreducibles can be computed recursively by degree and that the number of relatively prime pairs can be expressed in terms of the number of irreducibles. We also obtain asymptotic formulas for the number of irreducibles and the number of relatively prime pairs. The asymptotic formulas for the number of irreducibles generalize and improve several previous results by Carlitz, Cohen and Bodin. 相似文献