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排序方式: 共有892条查询结果,搜索用时 15 毫秒
1.
B. M. Forster J. M. Bailey G. A. Beer J. L. Beveridge J. H. Brewer W. N. Hardy T. M. Huber K. R. Kendall A. R. Kunselman J. A. Macdonald G. M. Marshall G. R. Mason A. Olin M. Senba J. B. Warren 《Hyperfine Interactions》1991,65(1-4):1007-1013
Muonic hydrogen isotopes (μ− p, μ− d, and μ−t) are simple quantum mechanical systems ideally suited for studies of numerous fundamental phenomena in electroweak and strong
interactions as well as in applied areas such as muon chemistry or muon catalyzed fusion.
Emission of muonic hydrogen isotopes into vacuum helps to overcome the limitations which are normally imposed on conventional
investigations with gaseous and liquid targets. A proof of principle experiment for this new technique was performed at TRIUMF
last year. Negative muons with 30 MeV/c momentum were stopped in a thin film of solid hydrogen and produced very low energy μ−d in vacuum. The distribution center of the normal velocity components of emitted μ−d atoms was measured to be ∼1 cm/μs. The yield of μ−d in vacuum is an increasing function of H2 film thickness δ up to a value of δ≥1 mm. 相似文献
2.
K. D. Duch M. Heel H. Kalinowsky F. Kayser E. Klempt B. May O. Schreiber P. Weidenauer M. Ziegler D. Bailey S. Barlag J. M. Butler U. Gastaldi R. Landua C. Sabev W. Dahme F. Feld-Dahme U. Schaefer W. R. Wodrich J. C. Bizot B. Delcourt J. Jeanjean H. Nguyen E. G. Auld D. A. Axen K. L. Erdman B. Howard R. Howard B. L. White S. Ahmad M. Comyn G. M. Marshall G. Beer L. P. Robertson M. Botlo C. Laa H. Vonach C. Amsler M. Doser J. Riedlberger U. Straumann P. Truöl ASTERIX Collaboration 《Zeitschrift fur Physik C Particles and Fields》1989,45(2):223-234
Antiproton-proton annihilation at rest in a gaseous H2 target at NTP into the final state π+ π? K ± π? (K 0) with an undetectedK 0 or \(\bar K^0 \) has been investigated. We observe theE(1420) resonance in the invariant mass spectrum (K 0)miss K ± π? with massM E =1413±8 MeV/c2 and widthГ E =62 ± 16MeV/c2 and find evidence for the production of thef 1(1285). The absolute branching ratio of \(\bar p\) p → π+ π? E 0,E 0 →K 0 L K ± π ? at (61±6)%P wave annihilation is (3.0±0.9)·10?4 of all annihilations. The observed suppression of theE production fromP wave with respect to theS wave together with some simple selection rules suggest that the quantum numbers of theE(1420) areJ pc=0?+ and not I++. 相似文献
3.
Indications for the occurrence of nitric oxide synthases in Dictyostelium, Neurospora, Phycomyces and the leguminous plant Mucuna hassjoo as well as a physiological role of nitric oxide in Neurospora crassa are demonstrated. An exogenous nitric oxide donor, sodium nitroprus-side, inhibited light-stimulated conidiation in N. crassa. Specific inhibitors of nitric oxide synthase, like the arginine derivatives NG-nitro-L-arginine (L-NA) and NG-nitro-L-arginine-methyl ester (L-NAME), enhanced conidiation in darkness and in the light, whereas the stereo-isomer D-NAME was inactive. This communication reports to our knowledge the first time the presence of enzymatic activity of nitric oxide synthase in fungi and a higher plant and an effect of nitric oxide in fungal photophysiology. 相似文献
4.
Janusz Z. Beer Kathleen M. Olvey Wah LEE Barbara Z. Zmudzka 《Photochemistry and photobiology》1994,59(6):643-649
Abstract: Effects of different radiation treatments on the human immunodeficiency virus-1 (HIV) promoter were reassessed for exposures comparable to those encountered in clinical or cosmetic practice, using survival of the host cell as a basis for comparisons. The exposures were performed with two ultraviolet radiation sources commonly used as medical or cosmetic devices (UVASUN 2000 and FS20 lamps), a germicidal (G15T8) lamp and an X-ray machine. The UVC component of the FS20 lamp was filtered out. The emission spectra of the lamps were determined. The characteristics of these sources allowed us to discriminate among effects of UVA1 (340–400 nm), UVB + UVA2 (280–340 nm) and UVC (254 nm) radiations. Effects of irradiation were ascertained using cultures of HeLa cells stably transfected with the HIV promoter linked to a reporter—chloramphenicol acetyl transferase—gene. The exposures used caused at least two logs of cell killing. In this cytotoxicity range, UVA1 or X radiations had no effect on the HIV promoter, whereas UVB + UVA2 or UVC radiations activated the HIV promoter in a fluence-dependent manner. Survivals following exposure to UVB + UVA2 or UVC radiation were (1) at the lowest measurable HIV promoter activation, 30 and 20%, respectively, (2) at one-half maximal activation, 6 and 3%, respectively and (3) at the maximal activation, 0.5 and 0.2%, respectively. The results suggest that, among the radiations studied, UVB is the most important modality from the viewpoint of its potential effects on HIV-infected individuals, since (1) UVA1 or X radiations have no effects on the HIV promoter, (2) human exposure to UVC radiation is infrequent and (3) human UVB exposure is very common. 相似文献
5.
T. de Beer G. Ph. Hoornweg G. J. Grootendorst N. H. Velthorst C. Gooijer 《Analytica chimica acta》1996,330(2-3):189-197
An explorative study on the compatibility of liquid separation systems, such as (micro) liquid chromatography (LC) and capillary electrophoresis (CE), and forward-scattering degenerate four-wave mixing (F-D4WM) as a detection method is presented. F-D4WM is a laser-based technique showing some analogy with holographic spectroscopy: a signal on a theoretical dark background is observed as a result of light absorption by an analyte. Parameters considered are solvent composition focussing on acetonitrile, methanol and water; mobile phases in LC and CE), detector cell construction, and influences of laser beam powers. A specially designed detector cell has been developed to meet the Brewster condition, both at the air-quartz and the quartz-liquid boundaries. For practical reasons, the tested cell has an optical pathlength of 1 mm; reduction to 100 μm is required to apply the cell in microseparations. The F-D4WM technique has been involved for detection in a conventional-size, reversed-phase LC separation of 1- and 2-aminoanthraquinones. The detection limit obtained (for the 1 mm cell) is 2 × 10−5 absorbance units. The experiments indicate that further reduction of background deserves explicit attention. 相似文献
6.
Paul D. Beer Zheng Chen Philip A. Gale Jennifer A. Heath Rachel J. Knubley Mark I. Ogden Michael G. B. Drew 《Journal of inclusion phenomena and macrocyclic chemistry》1994,19(1-4):343-359
The synthesis, metal, ammonium and alkyl ammonium cation coordination chemistry and electrochemical recognition studies of new diester-and diamide-calix[4]arenediquinone receptors are described. In addition the synthesis and coordination properties of a novel diamide benzo-15-crown-5-calix[4]arene molecule is reported.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes. 相似文献
7.
On-line high-performance liquid chromatographic diode array spectrophotometric analysis of steroidal hormones in illegal preparations 总被引:1,自引:0,他引:1
J O De Beer 《Journal of chromatography. A》1989,489(1):139-155
An on-line high-performance liquid chromatographic diode array spectroscopic analytical method for the identification of more than 60 different steroidal compounds is described. For the chromatographic separation, a gradient elution that could distinguish the esterified and non-esterified steroids in the same run on a reversed-phase C18 column, using methanol as modifier, was developed. For both types of compound an internal standard was chosen to establish reproducible relative retention times that could be used as one element of the identification; the second element is the UV spectrum, which is recorded on-line during the separation. The combination of chromatographic and UV spectroscopic recordings selects only a few probable steroids, which could be the unknown. This approach has been applied to forensic analysis of illicit preparations used in cattle-breeding, some examples of which are shown. For those steroids that are very difficult to distinguish using this procedure, because of their chromatographic and spectroscopic similarity on this system, other solvent mixtures are used in place of methanol as modifier, namely acetonitrile or tetrahydrofuran, or both, with the same solvent strength, as proposed by Snyder. In this way totally different elution patterns and separations are obtained, providing complementary information for identification, as shown by the systematic analysis on two other isoeluotropic solvent systems. 相似文献
8.
Zusammenfassung Beim Erhitzen von Betain mit Phenylisocyanat (12) auf 140–160° entsteht Diphenylimidazolidin-dion-(2,4) (I).
Diphenylimidazolidine-dion-2,4 (I) was obtained by the reaction of betaine with phenylisocyanate (12) at 140–160°.相似文献
9.
Zusammenfassung Die Weiterführung der Spaltungsversuche an methylsubstituierten 4-Hydroxystyryl-alkylketonen mit Chinonimidchlorid hat gezeigt, daß auch Systeme mit ungesättigter Kohlenstoffkette, die nur einen aromatischen Ring aufweisen, mittels Kupplungsreagentien unter Eliminierung der Seitenkette reagieren.
Herrn Prof. Dr.A. Wacek, Vorstand des Institutes für Organische Chemie und Organisch-chemische Technologie der Technischen Hochschule Graz mit freundlichen Wünschen zum 70. Geburtstag gewidmet. 相似文献
The cleavage reaction of methyl 4-hydroxystyryl-alkylketones with p-benzoquinone-monochloroimine shows, that also compounds with an unsaturated carbon chain and with only one aromatic ring react under elimination of the side chain.
Herrn Prof. Dr.A. Wacek, Vorstand des Institutes für Organische Chemie und Organisch-chemische Technologie der Technischen Hochschule Graz mit freundlichen Wünschen zum 70. Geburtstag gewidmet. 相似文献
10.
H. Nowotny Helga Auer-Welsbach J. Bruss A. Kohl 《Monatshefte für Chemie / Chemical Monthly》1959,90(1):15-23
Zusammenfassung Die Parameter des D 88-Struktur werden mit Hilfe gut ausgebildeter Einkristalle von Ti5Si3 neu bestimmt. Die dazu isotype Kristallart Zr5Al3 wird im System: Zr–Al nachgewiesen. Im Schnitt: Ti5Sn3–Zr5Sn3 bzw. Zr5Pb3–Ti (5) Pb (3) tritt ein lückenloser bzw. sehr ausgedehnter Bereich von Mischkristallen mit D88-Typ auf.Mit 1 Abbildung 相似文献