47,719 research outputs found
A cooled avalanche photodiode with high photon detection probability
An avalanche photodiode has been operated as a photon-counting detector with 2 to 3 times the sensitivity of currently-available photomultiplier tubes. APD (avalanche photodiodes) detection probabilities that exceed 27% and approach 50% have been measured at an optimum operating temperature which minimizes noise. The sources of noise and their dependence on operating temperature and bias voltage are discussed
Analytic Scattering and Refraction Models for Exoplanet Transit Spectra
Observations of exoplanet transit spectra are essential to understanding the
physics and chemistry of distant worlds. The effects of opacity sources and
many physical processes combine to set the shape of a transit spectrum. Two
such key processes - refraction and cloud and/or haze forward scattering - have
seen substantial recent study. However, models of these processes are typically
complex, which prevents their incorporation into observational analyses and
standard transit spectrum tools. In this work, we develop analytic expressions
that allow for the efficient parameterization of forward scattering and
refraction effects in transit spectra. We derive an effective slant optical
depth that includes a correction for forward scattered light, and present an
analytic form of this correction. We validate our correction against a
full-physics transit spectrum model that includes scattering, and we explore
the extent to which the omission of forward scattering effects may bias models.
Also, we verify a common analytic expression for the location of a refractive
boundary, which we express in terms of the maximum pressure probed in a transit
spectrum. This expression is designed to be easily incorporated into existing
tools, and we discuss how the detection of a refractive boundary could help
indicate the background atmospheric composition by constraining the bulk
refractivity of the atmosphere. Finally, we show that opacity from Rayleigh
scattering and collision induced absorption will outweigh the effects of
refraction for Jupiter-like atmospheres whose equilibrium temperatures are
above 400-500 K.Comment: ApJ accepted; submitted Feb. 7, 201
Furthur development of the dynamic gas temperature measurement system
Candidate concepts capable of generating dynamic temperatures were identified and analyzed for use in verifying experimentally the frequency response of the dynamic gas temperature measurement system. A rotating wheel concept and one other concept will be selected for this purpose. Modifications to the data reduction code algorithms developed were identified and evaluated to reduce substantially the data reduction execution time. These modifications will be incorporated in a new data reduction program to be written in FORTRAN IV
Recommended from our members
Multi-Material Ultrasonic Consolidation
Ultrasonic consolidation (UC) is a recently developed direct metal solid freeform
fabrication process. While the process has been well-demonstrated for part fabrication in Al alloy
3003 H18, including with intricate cooling channels, some of the potential strengths of the
process have not been fully exploited. One of them is its flexibility with build materials and the
other is its suitability for fabrication of multi-material and functionally graded material parts with
enhanced functional or mechanical properties. Capitalizing on these capabilities is critical for
broadening the application range and commercial utilization of the process. In the current work,
UC was used to investigate ultrasonic bonding of a broad range of engineering materials, which
included stainless steels, Ni-base alloys, brass, Al alloys, and Al alloy composites. UC multimaterial part fabrication was examined using Al alloy 3003 as the bulk part material and the
above mentioned materials as performance enhancement materials. Studies were focused on
microstructural aspects to evaluate interface characteristics between dissimilar material layers.
The results showed that most of these materials can be successfully bonded to Al alloy 3003 and
vice versa using the ultrasonic consolidation process. Bond formation and interface
characteristics between various material combinations are discussed based on oxide layer
characteristics, material properties, and others.Mechanical Engineerin
Plant root proliferation in nitrogen-rich patches confers competitive advantage
Plants respond to environmental heterogeneity, particularly below ground, where spectacular root proliferations in nutrient-rich patches may occur. Such 'foraging' responses apparently maximize nutrient uptake and are now prominent in plant ecological theory. Proliferations in nitrogen-rich patches are difficult to explain adaptively, however. The high mobility of soil nitrate should limit the contribution of proliferation to N capture. Many experiments on isolated plants show only a weak relation between proliferation and N uptake. We show that N capture is associated strongly with proliferation during interspecific competition for finite, locally available, mixed N sources, precisely the conditions under which N becomes available to plants on generally infertile soils. This explains why N-induced root proliferation is an important resource-capture mechanism in N-limited plant communities and suggests that increasing proliferation by crop breeding or genetic manipulation will have a limited impact on N capture by well-fertilized monocultures
Method and apparatus for supercooling and solidifying substances
An enclosure provides a containerless environment in which a sample specimen is positioned. The specimen is heated in the containerless environment, and the specimen melt is dropped through the tube in which it cools by radiation. The tube is alternatively backfilled with an inert gas whereby the specimen melt cools by both radiation and convection during its free fall. During the free fall, the sample is in a containerless, low-gravity environment which enhances supercooling in the sample and prevents sedimentation and thermal convection influences. The sample continues to supercool until nucleation occurs which is detected by silicon photovoltaic detectors. The sample solidifies after nucleation and becomes completely solid before entering the detachable catcher. The amount of supercooling of the specimen can be measured by knowing the cooling ratio and determining the time for nucleation to occur
Further development of the dynamic gas temperature measurement system
The objective of this effort was to experimentally verify a dynamic gas temperature measurement system in laboratory experiments. The dynamic gas temperature measurement system verification program is described. A brief description of the sensor geometry and construction is followed by a discussion of the probe heat transfer analysis and subsequent compensation method. The laboratory experiments are described and experimental results are discussed. Finally, directions for further investigation are given
The SPAR thermal analyzer: Present and future
The SPAR thermal analyzer, a system of finite-element processors for performing steady-state and transient thermal analyses, is described. The processors communicate with each other through the SPAR random access data base. As each processor is executed, all pertinent source data is extracted from the data base and results are stored in the data base. Steady state temperature distributions are determined by a direct solution method for linear problems and a modified Newton-Raphson method for nonlinear problems. An explicit and several implicit methods are available for the solution of transient heat transfer problems. Finite element plotting capability is available for model checkout and verification
Dynamic gas temperature measurement system
A gas temperature measurement system with compensated frequency response of 1 KHz and capability to operate in the exhaust of a gas turbine combustor was developed. Environmental guidelines for this measurement are presented, followed by a preliminary design of the selected measurement method. Transient thermal conduction effects were identified as important; a preliminary finite-element conduction model quantified the errors expected by neglecting conduction. A compensation method was developed to account for effects of conduction and convection. This method was verified in analog electrical simulations, and used to compensate dynamic temperature data from a laboratory combustor and a gas turbine engine. Detailed data compensations are presented. Analysis of error sources in the method were done to derive confidence levels for the compensated data
- …