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121.
A gamma-ray laser would stimulate coherent emission of radiation at wavelengths below 1 Å from excited states of nuclei. However, the difficulties in realizing such a device were considered insurmountable when the first cycle of study ended in1981. Since then, research on the feasibility of a gamma-ray laser has taken on a completely new character. A nuclear analog of the ruby laser has been proposed and many of the component steps for pumping the nuclei have been demonstrated experimentally. A quantitative model based upon the new data and the concepts of this decade shows the gamma-ray laser to be feasible if some real isotope has its properties sufficiently close to the ideals. The greatest positive impact has come from the discovery of giant resonances for pumping nuclei with photons that greatly reduce the levels of input power needed. Most recently, attention has been focused upon efforts to demonstrate prelasing levels of fluorescence from simulation nuclides and actual gamma-ray laser candidates. Problems being addressed are the acquisition of macroscopic samples of the best nuclei for testing and the demonstration of appropriate instrumentation. 相似文献
122.
John Leslie Britton died in a climbing accident on the Isleof Skye on 13 June 1994 at the age of 66. He became a memberof the London Mathematical Society in 1957, served the societyas Meetings and Membership Secretary from 1973 to 1976, andat the same time was Editor of the Society's Newsletter. 相似文献
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The perturbation function of a Cd probe atom bound in a hydrogen-decorated vacancy complex in Pt is found to exhibit dynamical
relaxation at 294 K, as measured by the technique of perturbed γ-γ angular correlations. Based on other characteristics of
the complex, a model is proposed for the structure of the complex. In the undecorated state, it consists of a probe atom at
the center of a tetrahedron of vacancies in the fcc structure. In the decorated state at low temperature, the probe atom returns
to one of the four vacant lattice sites. The relaxation at room temperature is attributed to motion of the probe atom in a
cage formed by the four vacant sites. 相似文献
126.
David Kinnear Mark Atherton Michael Collins Jason Dokhan Tassos Karayiannis 《Optics & Laser Technology》2006,38(4-6):286-291
Colour is used in computational fluid dynamic (CFD) simulations in two key ways. First it is used to visualise the geometry and allow the engineer to be confident that the model constructed is a good representation of the engineering situation. Once an analysis has been completed, colour is used in post-processing the data from the simulations to illustrate the complex fluid mechanic phenomena under investigation. This paper describes these two uses of colour and provides some examples to illustrate the key visualisation approaches used in CFD. 相似文献
127.
V.I. Zoran M. Ganciu A.M. Pointu C.B. Collins I.-I. Popescu 《Hyperfine Interactions》1997,107(1-4):415-430
An inexpensive, tunable, filamentary X-ray generator, based on the bremsstrahlung of a pulsed, high-power-density electron
beam cruising along a dielectric fiber is presented. At small scale, it encodes some of the features of an X-ray machine for
triggering the energy stored in the long-lived 178Hfm2 nuclear isomer, in particular an optimum coupling to the active medium
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
128.
Christopher M. Collins Shizhe Li Qing X. Yang Michael B. Smith 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,125(2):233-241
A method for calculatingB1field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. CalculatedB1field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, bothB1field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm,B1homogeneity is greater than in an unshielded coil.B1field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement. 相似文献
129.
Beck S.E. Collins S.M. O'Hanlon J.F. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1994,22(2):128-135
Methods for moving charged particles in RF processing plasmas are investigated. These methods include varying RF power, varying chamber pressure, attraction and repulsion by an electrostatic probe, and movement with magnetic fields. Varying RF power changes the depth of the potential wells where particles are trapped. The RF power affects shape and location of the traps and the bulk plasma potential. Increasing the chamber pressure moves the sheath edge closer to the wafer being processed. Since particle traps are found at the plasma sheath edge increasing the chamber pressure will move the particle traps (and any trapped particles) closer to the wafer being processed. The Langmuir probe can repel particles when under negative bias and attract them when positively biased. This probe can also distort the sheath edge when the tip resides within the sheath. Applying a magnetic field can change the characteristics of the particle traps and produce a force on the charged dust particles 相似文献
130.
Karen L. Collins 《Order》1991,8(4):375-381
The special properties of planar posets have been studied, particularly in the 1970's by I. Rival and others. More recently, the connection between posets, their corresponding polynomial rings and corresponding simplicial complexes has been studied by Stanley and others. This paper, using work of Björner, provides a connection between the two bodies of work, by characterizing when planar posets are Cohen-Macaulay. Planar posets are lattices when they contain a greatest and a least element. We show that a finite planar lattice is lexicographically shellable and therefore Cohen-Macaulay iff it is rank-connected. 相似文献