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81.
Alan Owens Sebastian M. Pascarelle Neil Gehrels Bonnard J. Teegarden 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》1991,310(3):681-684
We have extended the empirical work of Vano et al.[1] relating the slope of the detector efficiency curve to the active volume for Ge detectors. The analysis was carried out using Monte Carlo techniques and covered a wide range of incident energies (200 keV-20 MeV) and active volumes (19.6 cm3–396 cm3). It is shown that the expression of Vano et al.[1] is only valid over the energy range 200 keV-3 MeV for active volumes <50 cm3. The upper bound decreases to 2 MeV for volumes of a few hundred cm3. The usable energy range can, however, be extended to 6 MeV by introducing higher order terms into the polynomial. Above this energy, the shape of the efficiency curve is better described by a non-linear function since linear forms fail simultaneously to fit large active volumes and high energies. We therefore propose a composite function which reduces to the form given in Vano et al. in the low energy/active volume limit. By comparison with the Monte Carlo results, it is estimated that relative efficiencies can be calculated to within 6% over the energy range 200 keV-20 MeV and active volumes 20 cm3–400 cm3. Since the largest errors occur for the smallest volumes, we recommend that for energies <3 MeV a two-fold approach be followed, i.e. using the expression of Vano et al.[1] for active volumes less than 50 cm3 and the proposed non-linear form for larger volumes. For high energy work (E > 3 MeV), we advocate the non-linear form. In this way, average errors can be kept 3%. Finally, we point out that the real power of the expression of Vano et al. lies not in predicting efficiencies, but active volumes. 相似文献
82.
Droplet formation on Si surfaces bombarded by 10 keV beams from a liquid metal indium ion source has been observed. It has been verified that excess indium is transported in the ion beam, probably in the form of charged droplets. SIMS, SEM and RBS techniques have been used to study the distribution of droplets at the surface. At the specified ion source operating conditions, equilibrium droplet coverage is of the order of 25%. Individual droplets have an average diameter of 0.35 ωm and an average height of 2.2 nm. At a bombardment current density of 0.1 mA cm?2, equilibrium coverage is reached in about 1 s. 相似文献
83.
Imageability of places, cities and countries is strongly influenced by tourist and political portraits in the media. As human cognition relies on inferential perception and contrasting categories, marketing strategies exploit partial truths and conflicting city images to direct public perceptions. This article addresses the relationship between international perception, image building and urban design in the case of Brazil and the cities of Rio de Janeiro and Curitiba. Their contrasting images as exploited by the international media expose only partial truths. In fact, these images represent complementary development contradictions that co-exist in any major city today. 相似文献
84.
C. Schönbauer Dipl.-Ing. 《e & i Elektrotechnik und Informationstechnik》2003,120(10):304-306
The new law for the support of electricity produced by renewable sources in Austria (Ökostromgesetz) which came into force with the beginning of 2003, shows its impacts already after the first months. Investments into wind power have increased considerably. These will be accompanied by activities for electricity production by biomass and biogas. The 4%-target quota will probably be reached earlier than the 2008 target period of the law. 相似文献
85.
Zusammenfassung Aufgrund von Eingangs- und Ausgangsst?rungen des Me?systems sowie einer endlichen Frequenzaufl?sung treten stets systematische
(Bias-) und zuf?llige Fehler bei gemessenen Frequenzg?ngen und deren Varianzen auf. Auf solchen Frequenzg?ngen und Varianzen
beruhende Sch?tzungen der modalen Parameter sind damit ebenfalls systematisch verzerrt. Um die Genauigkeit und Sicherheit
der modalen Parameter zu erh?hen, wird ein Sch?tzverfahren vorgestellt, in dem der Einflu? der Biasfehler auf die Sch?tzung
n?herungsweise in einer diagonalen Wichtungsmatrix berücksichtigt wird. Es wird gezeigt, da? die systematischen Fehler Einflu?
auf die Absch?tzung der statistischen Sicherheit der modalen Parameter haben. Anhand zweier Beispiele wird die Wirksamkeit
des Sch?tzverfahrens demonstriert.
Prof. Dr.-Ing. Dr.-Ing. E.h.H.K. T?nshoff: Dipl.-Ing.Z. Liu, Institut für Fertigungstechnik und Spanende Werkzeugmaschinen (IFW), Universit?t Hannover. 相似文献
86.
87.
Y. Romanyshyn S. Guimond H. Kuhlenbeck S. Kaya R. P. Blum H. Niehus S. Shaikhutdinov V. Simic-Milosevic N. Nilius H.-J. Freund M. V. Ganduglia-Pirovano R. Fortrie J. Döbler J. Sauer 《Topics in Catalysis》2008,50(1-4):106-115
Oxidation catalysts are modeled by oxide single crystals, thin oxide films, as well as supported oxide nanoparticles. We characterize the surface of those materials using a variety of surface sensitive techniques including scanning tunneling microscopy and spectroscopy, photoelectron spectroscopy, infrared spectroscopy, and thermal desorption spectroscopy. We find temperature dependent structural transformations from V2O5(001) to V2O3(0001) via V6O13(001). V2O3(0001) is found to be vanadyl terminated in an oxygen ambient and it loses the vanadyl termination after electron bombardment. It is shown that the concentration of vanadyl groups controls the selectivity of the methanol oxy-dehydrogenation towards formaldehyde. A proposal for the mechanism is made. The results on single crystalline thin films are compared with similar measurements on deposited vanadia nanoparticles. The experimental results are correlated with theoretical calculations and models. 相似文献
88.
89.
A. Celaya Sanfiz T. W. Hansen F. Girgsdies O. Timpe E. Rödel T. Ressler A. Trunschke R. Schlögl 《Topics in Catalysis》2008,50(1-4):19-32
This work presents a detailed investigation of the preparation of MoVTeNbO x catalysts by hydrothermal synthesis. Phase-pure synthesis of M1 has been achieved applying the metals in a molar ratio Mo/V/Te/Nb = 1/0.25/0.23/0.12. Raman, UV/Vis spectroscopy, and SEM/EDX analysis show that the elements are inhomogeneously distributed in the initial suspension that is formed after mixing the metal salts in an aqueous medium. Iso- and heteropoly anions of molybdenum, free telluric acid as well as supra-molecular polyoxometalate clusters are observed in the solution, whereas all metals have been found in the precipitate. Complete rearrangement of molecular building blocks under hydrothermal conditions is essential for formation of phase-pure materials. Optimized synthesis conditions with respect to temperature and time result in the formation of a precursor consisting of nano-structured M1 characterized by an extended periodic organization in the [001] direction and a fairly homogeneous distribution of the elements. Residual ammonium containing supra-molecular species in the precursor result in the formation of phase mixtures during the subsequent crystallization by heat treatment in inert gas. Phase-pure M1 exhibits a distinct degree of flexibility with respect to the chemical composition that becomes obvious by incorporating Nb not exclusively into pentagonal bi-pyramidal units, but also into octahedral coordinated positions as shown by EXAFS. Anisotropic growth of the needle-like M1 crystals has been observed during the final heat treatment performed at 873–923 K in inert atmosphere disclosing a potential method to control the catalytic properties of MoVTeNbO x catalysts. 相似文献
90.