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281.
A fiber-ribbon-based coupling method is presented as a practical way to enhance light sensitivity in spectroscopic and similar systems employing a multialkali photomultiplier tube as a detector. The sensitivity enhancement is achieved by an increase in absorption in the photocathode through multiple total internal reflection in the photomultiplier entrance window. Improvements by factors of ~2 (blue-green) to more than 10 (near infrared) have been measured. Implications for spectroscopic applications are discussed. 相似文献
282.
For field theories that include the abelian gauge field Aμν? the field equations allow an arbitrary integration constant, which does not appear in the lagrangian but which does affect the physics. We present two applications: (i) the θ parameter of effective lagrangians for chiral symmetry breaking in QCD, and (ii) the cosmological constant in N = 8 supergravity, which does not require a gauging of the O(8) symmetry, but is rather due to a spontaneous breakdown of supersymmetry. 相似文献
283.
284.
The preparation of 4-deazaformycin A has been achieved. The synthesis features the condensation of a suitably substituted, lithiated 4-picoline with 2,3,5-tri-O-benzyl-d-ribonolactone, dehydration of the resulting hemiacetal, and ionic hydrogenation, followed by manipulation of the protecting groups and subsequent ring closure with the formation of 7-amino-3-(beta-d-ribofuranosyl)pyrazolo[3,4-c]pyridine. 相似文献
285.
Becchi-Rouet-Stora transformations are obtained for the following systems: (i) Pure Einstein gravity in first order form with vierbein and spin connection as independent fields. (ii) First order Einstein gravity coupled to Yang-Mills fields. (iii) Pure supergravity. For the first two systems the results are as in Yang-Mills theory. But for conventional supergravity the BRS transformations leave the effective action invariant only if the classical equations of motion are satisfied. New transformations of the gauge fields of supergravity have been proposed under which the supersymmetry algebra closes. The corresponding BRS transformations do leave the effective action invariant without the need to use the classical equation of motion; moreover, as in Yang-Mills theories, they are nilpotent and have unit Jacobian. 相似文献
286.
287.
Ji-Wang Chern Dean S. Wise Leroy B. Townsend 《Journal of heterocyclic chemistry》1984,21(4):1245-1246
A facile synthesis of methyl oxazolo[5,4-d]pyrimidine-2-carbamic acids by the cyclodesulfurization of a methoxycarbonyl thiourea with dicyclohexylcarbodiimide is described. 相似文献
288.
We investigate the flow of a generalised Newtonian liquid between two contra-rotating cylinders of equal size — the so called two-roll mill problem. A finite element method is used to obtain solutions for the case of a Carreau model and for a power law model. Consideration is given to the influence of shearthinning on the flow pattern around the cylinders. Results are presented for different speeds of rotation of the cylinders and for various values of fluid parameters. A comparison is made with the analytical solution of Jeffrey. 相似文献
289.
Eleven-dimensional supergravity can compactify spontaneously on either S7 or S4. We study the latter case, with attention to its connection with a possible gauged N = 4 supergravity in d = 7. We derive the linearized field equations and supersymmetry transformation rules for the effective d = 7 supergravity multiplet. There are five third rank antisymmetric tensors in this multiplet which are in the 5 representation of the gauge group USp(4) and they are propagated with a self-duality condition in 7 dimensions. There is also a 14 of scalar fields and they are found to propagate with a non-conformal wave operator. 相似文献
290.
Mössbauer spectroscopy has been used to identify the zinc-iron intermetallic phases present in the coating of three galvannealed steels, two of which were produced on commercial galvanizing lines and one in the laboratory. Both CEMS and XMS have been used in order to determine the depth dependence of each phase. Three main zinc-iron phases have been identified with the proportion of each dependent on the preparation conditions. In the commercially produced coatings, CEMS, probing near the top of the coating, indicates that the zinc rich χ-FeZn13 and δ-FeZn10 alloys are present. XMS indicates the presence of some Π-Fe3Zn10 closer to the steel. In contrast, the laboratory produced sample contains nearly pure χ-FeZn13 through the entire coating thickness. Subphases of the delta and gamma alloys were also identified. These subphases appear to be mixed with a preference for the higher zinc subphase of each to form closer to the surface of the coating. 相似文献