988 resultados para Menzies, Robert, Sir, 1894-1978


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"First published, November, 1929."

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The energy spectra of 235U atoms sputtered from a 93% enriched 235U metal foil and a hot pressed 235U02 pellet by an 80 keV 40Ar+ beam have been measured in the range 1 eV to 1 keV. The measurements were made using a mechanical time-of-flight spectrometer in conjunction with the fission track technique for detecting 235U. The design and construction of this spectrometer are discussed in detail, and its operation is mathematically analyzed.

The results of the experiment are discussed in the context of the random collision cascade model of sputtering. The spectrum obtained by the sputtering of the 235U metal target was found to be well described by the functional form E(E+Eb)-2.77, where Eb = 5.4 eV. The 235U02 target produced a spectrum that peaked at a lower energy (~ 2 eV) and decreased somewhat more rapidly for E ≳ 100 eV.

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Theoretical and experimental studies of a gas laser amplifier are presented, assuming the amplifier is operating with a saturating optical frequency signal. The analysis is primarily concerned with the effects of the gas pressure and the presence of an axial magnetic field on the characteristics of the amplifying medium. Semiclassical radiation theory is used, along with a density matrix description of the atomic medium which relates the motion of single atoms to the macroscopic observables. A two-level description of the atom, using phenomenological source rates and decay rates, forms the basis of our analysis of the gas laser medium. Pressure effects are taken into account to a large extent through suitable choices of decay rate parameters.

Two methods for calculating the induced polarization of the atomic medium are used. The first method utilizes a perturbation expansion which is valid for signal intensities which barely reach saturation strength, and it is quite general in applicability. The second method is valid for arbitrarily strong signals, but it yields tractable solutions only for zero magnetic field or for axial magnetic fields large enough such that the Zeeman splitting is much larger than the power broadened homogeneous linewidth of the laser transition. The effects of pressure broadening of the homogeneous spectral linewidth are included in both the weak-signal and strong-signal theories; however the effects of Zeeman sublevel-mixing collisions are taken into account only in the weak-signal theory.

The behavior of a He-Ne gas laser amplifier in the presence of an axial magnetic field has been studied experimentally by measuring gain and Faraday rotation of linearly polarized resonant laser signals for various values of input signal intensity, and by measuring nonlinearity - induced anisotropy for elliptically polarized resonant laser signals of various input intensities. Two high-gain transitions in the 3.39-μ region were used for study: a J = 1 to J = 2 (3s2 → 3p4) transition and a J = 1 to J = 1 (3s2 → 3p2) transition. The input signals were tuned to the centers of their respective resonant gain lines.

The experimental results agree quite well with corresponding theoretical expressions which have been developed to include the nonlinear effects of saturation strength signals. The experimental results clearly show saturation of Faraday rotation, and for the J = 1 t o J = 1 transition a Faraday rotation reversal and a traveling wave gain dip are seen for small values of axial magnetic field. The nonlinearity induced anisotropy shows a marked dependence on the gas pressure in the amplifier tube for the J = 1 to J = 2 transition; this dependence agrees with the predictions of the general perturbational or weak signal theory when allowances are made for the effects of Zeeman sublevel-mixing collisions. The results provide a method for measuring the upper (neon 3s2) level quadrupole moment decay rate, the dipole moment decay rates for the 3s2 → 3p4 and 3s2 → 3p2 transitions, and the effects of various types of collision processes on these decay rates.

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An exploration of the revolutionary period of prehistory that began when humans abandoned the nomadic hunting and gathering existence they had known for millennia to take up a completely new way of life the decisive move to farming and herding the ration of permanent settlements and the discovery of metals setting the stage for the arrival of the worldʼs first civilisation. Stores from the Stone Age ask some intriguing questions. Why did some of our ancestors never become farmers at all? Why do some still continue hunting and gathering despite their contact with farming people and advanced technologies? How and why did our paths become uniquely shaped after emerging as a species from a single genetic family in Africa? Based on extensive research, Stories from the Stone Age takes us on a journey where we get to live alongside our ancestors as they cross between the Old and the New Worlds and into Civilisation. The series utilises detailed re-enactments and short interviews with key archaeological experts.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Mode of access: Internet.

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Corporate contributors include: Genesee Pure Food Company; Jell-O-Co. Inc.

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"Limited autographed edition"--back flap.

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National Highway Traffic Safety Administration, Washington, D.C.

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National Highway Traffic Safety Administration, Washington, D.C.

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Edited by Clements R. Markham.