In recent years, heightened security concerns have prompted increased interest in radiation portal monitors, instruments capable of detecting, and in some cases identifying, gamma-ray and neutron-emitting radioactive ...
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In recent years, heightened security concerns have prompted increased interest in radiation portal monitors, instruments capable of detecting, and in some cases identifying, gamma-ray and neutron-emitting radioactive material passing through them. Along with efficient and reliable physical detectors, such monitors need software capable of analyzing the resulting signals in order to effectively catch weak radioactive sources. The software should be able to distinguish between the sometimes subtle signals due to weak sources and the varying background. After investigating several possible source-detection algorithms, a program has been developed to locate signals from weakly radioactive sources in real time. By including corrections for background suppression due to trucks shielding the detector and changing background levels, this program is capable of detecting significantly weaker sources than current software. The limitations of this method will also be discussed.
Most of the theory developed to date concerningd discontinuities assumes that production times prior to buildup were long enough for the discontinuity to influence the pressure drawdown behavior of the well. When this...
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In this paper, the design of a deadlock avoidance controller is described. The uncontrolled system is modeled using colored Petri nets. The system controller is based on a restrictive (not maximally permissive) deadlo...
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In this paper, the design of a deadlock avoidance controller is described. The uncontrolled system is modeled using colored Petri nets. The system controller is based on a restrictive (not maximally permissive) deadlock avoidance policy to resolve deadlocks and control the real-time resource allocation decisions in the system. Performance evaluation of systems controlled by not maximally permissive algorithms is essential in determining the applicability and effectiveness of the control algorithms. The performance of the controlled system is compared with performance of optimal control policies to quantify the effects of the restrictiveness of the deadlock avoidance policy on system performance. (C) 2004 Elsevier Ltd. All rights reserved.
THz communication technology has been known as an important enabler for 6G. This paper summarizes the condition of THz research in some countries, and introduces the characters of THz communication, ultrahigh communic...
THz communication technology has been known as an important enabler for 6G. This paper summarizes the condition of THz research in some countries, and introduces the characters of THz communication, ultrahigh communication speed with ultra-high path loss for the same reason–the ultra-high frequency (more than 100 GHz). The THz communication is more applicable in satellite constellation for the ideal channel. This paper analyzes the problem that need to be handled before practical application, such as Doppler shift and tracking problem. We propose a typical LEO satellite constellation and analyze the performance of THz communication in multi-scenarios (intra-orbit, inter-orbit and satellite-ground). The results show that the Doppler shift and tracking problem, which THz communication being used in LEO satellite constellation may face, can be handled easily. Furthermore, the THz communication can also achieve the goal of ultra-high communication speed at a long distance.
a-Se is a well-known material for X-ray imaging. It is a direct x-ray conversion photoconductor-detector material with less image blurring than scintillator-photodiode based imagers, which provides excellent resolutio...
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a-Se is a well-known material for X-ray imaging. It is a direct x-ray conversion photoconductor-detector material with less image blurring than scintillator-photodiode based imagers, which provides excellent resolution. The newly improved imager has a special blocking structure that ensures very low dark current. This allows the electric field to be increased from the standard 10 V/μm up to 20 V/μm. The higher field reduced the energy required to produce an e-h pair from 50 eV/e-h to about 30 eV/e-h increasing the sensitivity by 60%. Special doping processes and blocking layers also improved the a-Se's thermal stability to extend over the whole 0-70 C° temperature range. The high resolution is evident from the Modulation Transfer Function (MTF) value of >40% at the Nyquist Frequency of 10-lp/mm. The Detective Quantum Efficiency (DQE) was determined at different dose rates and as a function of spatial frequency. The higher electrical field increased the signal level at each given dose. The result was a 70% DQE value at low spatial frequencies for a high dose of 157 μGy. However, the low frequency DQE only dropped by about 10% to 60% at a low dose of 23 μGy. When the electric field at low dose was decreased to 10 V/μm the low frequency DQE fell to less than 40%. This new imager shows very low image lag. This is important for high frame rate breast tomosynthesis imaging applications. Ghosting properties of the imager were also tested. At 20 V/μm the ghosting was less than 1/4 th of that measured at 10 V/μm for an array, with relatively higher image lag. However, ghosting was practically 0 for an array having relatively low image lag, a further major advantage. Finally, breast phantom images resolved even the smallest calcifications, embedded in the phantom.
This paper considers the free vibration of two coaxial cylinders with finite lengths immersed in a restricted fluid medium. The general coupled equations of motion are derived. Qualitative study of the governing diffe...
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This paper considers the free vibration of two coaxial cylinders with finite lengths immersed in a restricted fluid medium. The general coupled equations of motion are derived. Qualitative study of the governing differential equation shows that only modes of the cylinders with their axial mode numbers directly proportional to their lengths are coupled. For the uncoupled modes, each cylinder vibrates as if the other one is rigid, and the only effect of fluid loading is an apparent increase in its mass (virtual mass effect). The natural frequencies in the fluid can then be computed by including the virtual mass to its physical mass. Experimental results have been used to verify the analysis. Agreement between experimental and theoretical results is excellent.
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