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21.
K. Irmer 《Isotopes in environmental and health studies》2013,49(10):417-418
Bei der kontinuierlichen Durchstrahlungsprüfung von Werkstücken werden überwiegend Meßeinrichtungen, bestehend aus 60Co-Strahlungsquelle, Szintillationsdetektor und schreibendem Meßgerät, verwendet 1. 相似文献
22.
This paper investigates the noise sources in a pneumatic nail gun. The study combines two complementary experimental approaches. The first uses simultaneous data observation, with sound, acceleration and air pressure signals simultaneously recorded in conjunction with a nail gun motion high speed video. This strategy allows the identification of the physical processes involved in the operation of the machine at different time instants, as well as the associated noise generation mechanisms. However, since multiple noise sources radiate at the same time, this observation technique is not sufficient for noise source identification and ranking. Thus, a second approach introduces a selective wrapping procedure, and the strategy assures a reliable classification of the noise sources. The investigation considers the following noise origins: the air exhaust, the machine body and the workpiece/worktable. 相似文献
23.
《Wave Motion》2020
We consider a point-like observer that moves in a medium illuminated by noise sources with Lorentz-invariant spectrum. We show that the autocorrelation function of the signal recorded by the observer allows it to perceive its environment. More precisely, we consider an observer with constant acceleration (along a Rindler trajectory) and we exploit the recent work on the emergence of the Green’s function from the cross correlation of signals transmitted by noise sources. First we recover the result that the signal recorded by the observer has a constant Wigner transform, i.e. a constant local spectrum, when the medium is homogeneous (this is the classical analogue of the Unruh effect). We complete that result by showing that the Rindler trajectory is the only straight-line trajectory that satisfies this property. We also show that, in the presence of an obstacle in the form of an infinite perfect mirror, the Wigner transform is perturbed when the observer comes into the neighborhood of the obstacle. The perturbation makes it possible for the observer to determine its position relative to the obstacle once the entire trajectory has been traversed. 相似文献
24.
《Wave Motion》2015
This paper presents a detailed analysis of time-reversal experiments involving a moving point source that emits a pulse. Different configurations are addressed with full-aperture or partial-aperture time-reversal mirrors and with subsonic or supersonic sources. Doppler effects and lack of source-receiver reciprocity significantly affect the time-reversal refocusing when the velocity of the source becomes comparable as or larger than the speed of propagation. The main result is that refocusing can be enhanced when the velocity of the source becomes close to the speed of propagation compared to the classical diffraction-limited refocusing properties when the source does not move, and this super-resolution effect can be quantified by simple and explicit formulas. 相似文献
25.
G. Brunner 《Isotopes in environmental and health studies》2013,49(9-10):328-333
Mit Berücksichtigung eines die horizontale Probeausdehnung umfassenden Geometriefaktors werden unter praktisch brauchbaren Bedingungen absolute Fluoreszens- und Streustrahlungsausbeuten für sättigungsdicke und für dünne Proben berechnet. Die aus ihnen herleitbaren Meβempfindlichkeiten zeigen, daβ sich geringe Konzentrationen von relativ leichten bis mittelschweren Elementen besonders gut messen lassen, wenn man entweder dicht oberhalb der Absorptionskanten liegenda Energien anwendel oder bei relativ hoher Anregungsenergie die Streustahlung abtrennt. Bei groβen Konzentrationen solcher Elemente kann auch ohne Streustrahlungsabtrennung mit relativ hohen Anregungsenergien gemessen werden, wobei sogar unter gewissen Bedingungen mit Zunahme der Anregungsenergie eine leichte Empfindlichkeitsverbesserung eintritt. Bei schweren Elementen ist der Gesichtspunkt der Probendicke vielfach wichtiger als die Wahl der Anregungsenergie. Für dūnne Proben lassen sich relativ recht hohe Empfindlichkeiten erzielen, was jedoch zu einem Problem der absoluten Intensitāten angesichts des Detektor-Strahlungsuntergrundes fūhren kann. 相似文献
26.
Biodiesel is biodegradable and nontoxic, and it significantly reduces toxic and other emissions when burned as a fuel. The advantages of biodiesel as diesel fuel are its portability, ready availability, renewability, higher combustion efficiency, non-toxicity, higher flash point, and lower sulfur and aromatic content, higher cetane number, and higher biodegradability. The major disadvantages of biodiesel are its higher viscosity, lower energy content, higher cloud point and pour point, higher nitrogen oxide (NOx) emissions, lower engine speed and power, injector coking, engine compatibility, high price, and greater engine wear. The technical disadvantages of biodiesel/fossil diesel blends include problems with fuel freezing in cold weather, reduced energy density, and degradation of fuel under storage for prolonged periods. The sources of biodiesel are vegetable oils and fats. The direct use of vegetable oils and/or oil blends is generally considered to be unsatisfactory and impractical for both direct injection and indirect type diesel engines because of their high viscosities and low volatilities injector coking and trumpet formation on the injectors, higher level of carbon deposits, oil ring sticking, and thickening and gelling of the engine lubricant oil, acid composition. Biodiesel is obtained by transesterifying triglycerides with methanol. A popular variation of the batch transesterification process which needs high alcohol/acid ratio (several separation problems and high corrosivity and toxicity) is the use of continuous stirred tank reactors in series. This continuous process is heterogeneous and is based on reactive distillation. The key factor is the selection of the right and effective solid catalyst which leads to reduction of energy consumption and investments at all. 相似文献
27.
随着能源短缺和环境问题日益突出, 寻找清洁和可再生能源来替代化石燃料是本世纪科学家面临的最紧迫的任务之一. 为了实现我国“双碳”战略目标, 利用太阳能将二氧化碳(CO2)转化为清洁燃料和化学品是实现社会可持续发展的途径之一. 催化剂是CO2光还原技术的核心组成部分, 其可以吸附气态CO2分子, 在可见光照射下将CO2还原为一氧化碳(CO)、 甲酸(HCOOH)、 甲醇(CH3OH)或甲烷(CH4)等能源小分子. 目前, 新型CO2还原光催化体系的开发取得了很好的进展. 本文综合评述了近年来均相及非均相丰产金属卟啉类催化剂在光催化CO2还原中的研究进展, 并对在金属卟啉均相催化剂作用下, CO2光还原为CO或CH4的反应机理分别进行了介绍, 还讨论了金属卟啉基多孔有机聚合物与卟啉有机金属框架在光催化CO2方面的重要应用. 最后, 对可见光驱动卟啉类金属配合物催化的CO2还原的发展前景进行了展望. 相似文献
28.
É. Biémont A. Ellmann P. Lundin S. Mannervik L.-O. Norlin P. Palmeri P. Quinet D. Rostohar P. Royen P. Schef 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(2):211-219
The difficulty associated with an accurate determination of transition rates for forbidden lines in lowly ionized heavy elements
is illustrated in the case of Nd II. We have investigated the radiative decay of the low-lying metastable levels in Nd+ including the two levels
K11/2 and
I13/2. In these two particular cases, using different theoretical approaches, we find that the decay is dominated by the M1 channels
but that the E2 contributions are of the same order of magnitude. These levels have also been studied experimentally by lifetime
measurements with the heavy ion storage ring CRYRING of Stockholm University. The difficulties encountered when performing
such experiments are underlined and discussed. 相似文献
29.
30.
The behaviour of a dislocation pileup with a finite-strength source is investigated in the presence of various stress gradients within a continuum model where a free-dislocation region exists around the source. Expressions for dislocation density and stress field within the pileup are derived for the situation where there are first and second spatial gradients in applied stress. For a pileup configuration under an applied stress, yielding occurs when the force acting on the leading dislocations at the pileup tips reaches the obstacle strength, and at the same time, it is required that the source be at the threshold stress for dislocation production. A numerical methodology is presented to solve the underlying equations that represent the yielding conditions. The yield stress calculated for a pileup configuration is found to depend on stress gradients, obstacle spacing and source/obstacle strengths. It increases with increasing the first stress gradient, yet dependent on the second stress gradient. Furthermore, while the dependency of yield stress on the obstacle spacing intensifies with increasing the first stress gradient, it diminishes with an increase of second stress gradient. Therefore, the second stress gradient, as a newly introduced parameter, can provide a new physical insight into the size-dependent plasticity phenomena at small length scales. 相似文献