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851.
852.
S. Withington J. A. Murphy A. Egan R. E. Hills 《International Journal of Infrared and Millimeter Waves》1992,13(10):1515-1537
In this paper, we summarize the issues that should be considered when designing broadband quasioptical systems for submillimetre-wave radioastronomy receivers. We cover topics such as bandwidth, cross-talk, truncation, and aberrations, and we argue that it should be possible to manufacture high-efficiency systems that have several octaves of bandwidth. A key feature of the paper is that we use the language of multimode Gaussian optics throughout, and in this way, we emphasize that a receiver is a diffraction-limited imaging system rather than just a collection of components for guiding Gaussian beams. The whole discussion is conducted in terms of a particular system we are constructing for the James Clerk Maxwell Telescope in Hawaii. 相似文献
853.
R. P. Spielmann Jin Zhen H. J. Triebel V. Nicolas M. Heller E. Bü cheler 《Magnetic resonance imaging》1992,10(6):893-901
To correlate the appearance of poststenotic jets on gradient echo images with features of localized Doppler spectra of the jets, we studied an in vitro model of steady flow-through stenoses of 86, 96, and 99% area reduction. As fluids, water and a 40% glycerol solution in water were used. MRI was performed with a 1.5 T whole body imager and gradient echo images were obtained in planes parallel to the direction of flow. Doppler spectra were acquired separately from the MR measurements at 1 cm intervals for a distance of 10 cm downstream from the stenosis. Poststenotic signal void was observed for water and for the 40% glycerol solution only if the mean velocity within the stenosis exceeded a limit of 50–60 cm/sec. On the MR images, the jets could be divided into two segments: A proximal jet segment of uniform width equal to the diameter of the stenosis, followed by a distal jet segment which was characterized by broadening and then dissipating signal void. Except for the 99% stenosis, a high signal intensity core was present within the proximal jet segment. In the proximal jet segment, the Doppler measurements showed a low temporal fluctuation of the maximal flow velocity and only little flow opposite to the main flow direction. In the distal jet segment, the velocity fluctuation and the intensity of reverse flow increased sharply. The high signal intensity core of the jet was associated with a poststenotic zone of constant maximal flow velocity. The results demonstrate a close relationship between characteristic features of poststenotic jets in MRI and pulsed Doppler sonography. 相似文献
854.
855.
描述了一种延长EPR波谱仪中速调管使用寿命的方法.根据这种方法,只要正确地调谐微波桥的工作状态和适当地调整功率电平器的功率校正电平.即使对于已经严重老化的速调管仍然可以继续使用一段时间,维持仪器的正常运行. 相似文献
856.
Summary We discuss a null experiment to search for the fifth force at ranges around 10 km. It is proposed to use a three-axis gravity
gradiometer to measure the trace of the Earth gravity tensor from a freely falling elevator dropped by a balloon. The main
error sources are considered to some extent.
Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990. 相似文献
857.
塞曼石墨炉原子吸收光谱法测定粉尘降尘中痕量锡 总被引:5,自引:0,他引:5
以KOH熔样,又以KOH作基体改进剂,研究了测定锡的灰化温度、原子化行为和消除干扰及试样前处理的方法,以KOH熔解的粉尘及降尘样液,特别适用于塞曼石墨炉的测定,背景可完全扣除。方法简便、快速、准确,灵敏度和精度高。相对标准偏差为4.0%,回收牢在96.4~110%范围内。 相似文献
858.
Summary The radial evolution of Alfvénic correlation is such that its value decreases with increasing heliocentric distance. So far
this behaviour has been interpreted as an increase in the local production of ?inward? modes interacting destructively with
the ?outward? modes. This work, which deals with largescale turbulence, shows that local generation phenomena are not commonly
found in the solar wind and that the Alfvénic character of the fluctuations mainly depend on the ?outward? modes alone. The
interaction of these modes with density and/or magnetic-field structures convected by the wind causes their destruction and
a consequent depletion of the Alfvénic correlation. The same effect would be obtained if ?inward? modes were really present.
Our conclusions are that large-scale ?inward? modes are the spectral counterpart of non-propagating field and plasma structures
convected by the solar wind and identified as both compression regions and pressure balance structures.
Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990. 相似文献
859.
860.
We investigate low-temperature behaviors of a system with chirality-pair interaction on a one-dimensional lattice. In the course of the investigation, we evaluate asymptotic forms of the characteristic values of the integral equation satisfied by the Mathieu functions. It turns out that the low-temperature behavior of correlation length of the chirality-pair correlation function is different from the one for the Ising model of spin ±1 but akin to the one for the Ising model of infinite spin. 相似文献