48 research outputs found
Millimeter Wave Scattering from Neutral and Charged Water Droplets
We investigated 94GHz millimeter wave (MMW) scattering from neutral and
charged water mist produced in the laboratory with an ultrasonic atomizer.
Diffusion charging of the mist was accomplished with a negative ion generator
(NIG). We observed increased forward and backscattering of MMW from charged
mist, as compared to MMW scattering from an uncharged mist. In order to
interpret the experimental results, we developed a model based on classical
electrodynamics theory of scattering from a dielectric sphere with
diffusion-deposited mobile surface charge. In this approach, scattering and
extinction cross-sections are calculated for a charged Rayleigh particle with
effective dielectric constant consisting of the volume dielectric function of
the neutral sphere and surface dielectric function due to the oscillation of
the surface charge in the presence of applied electric field. For small
droplets with (radius smaller than 100nm), this model predicts increased MMW
scattering from charged mist, which is qualitatively consistent with the
experimental observations. The objective of this work is to develop indirect
remote sensing of radioactive gases via their charging action on atmospheric
humid air.Comment: 18 pages, 8 figure
Neutron experiments at Portsmouth for measuring flow and {sup 235}U content in UF{sub 6} gas
The Portsmouth Gaseous Diffusion Plant produces enriched uranium for use in commercial power reactors. The plant also aids disposal of excess high-enrichment uranium (HEU) by blending it with lower-enrichment material. Experiments were conducted to test two neutron-based methods for monitoring the down-blending of HEU. Results of the initial experiments showed that gas (on-off) could be detected, but that additional tests and data are needed to quantify the flow velocity and {sup 235}U content. The experiments used a {sup 252}Cf neutron source to induce fission in a small fraction of the {sup 235}U contained in the UF{sub 6} gas. The first method measured the attenuation of neutrons passing through the low-pressure UF{sub 6} gas in a 7.6-cm diameter pipe. The concept was based on the fact that some of the thermal neutrons are absorbed by {sup 235}U, thus changing the observed count rate. The second method, tested on a 20-cm diameter pipe where gas pressure was higher, used a modulated neutron flux to induce fission in the {sup 235}U. Modulation was achieved by moving a neutron source. During both experiments, plant monitoring equipment showed that light gases (freon, oxygen, and nitrogen) were present in widely varying amounts, along with the UF{sub 6} gas. These gases may have affected the experimental results, at least to the extent that they replaced UF{sub 6}. This report also contains results of computer simulations and tests performed on the electronics after the experiments were completed at Portsmouth. Recommendations are made for follow-on work to measure the flow velocity and {sup 235}U content
The Continued Need for Modeling and Scaled Testing to Advance the Hanford Tank Waste Mission
Hanford tank wastes are chemically complex slurries of liquids and solids that can exhibit changes in rheological behavior during retrieval and processing. The Hanford Waste Treatment and Immobilization Plant (WTP) recently abandoned its planned approach to use computational fluid dynamics (CFD) supported by testing at less than full scale to verify the design of vessels that process these wastes within the plant. The commercial CFD tool selected was deemed too difficult to validate to the degree necessary for use in the design of a nuclear facility. Alternative, but somewhat immature, CFD tools are available that can simulate multiphase flow of non-Newtonian fluids. Yet both CFD and scaled testing can play an important role in advancing the Hanford tank waste mission—in supporting the new verification approach, which is to conduct testing in actual plant vessels; in supporting waste feed delivery, where scaled testing is ongoing; as a fallback approach to design verification if the Full Scale Vessel Testing Program is deemed too costly and time-consuming; to troubleshoot problems during commissioning and operation of the plant; and to evaluate the effects of any proposed changes in operating conditions in the future to optimize plant performance
Methods and Instruments for Fast Neutron Detection
Pacific Northwest National Laboratory evaluated the performance of a large-area (~0.7 m2) plastic scintillator time-of-flight (TOF) sensor for direct detection of fast neutrons. This type of sensor is a readily area-scalable technology that provides broad-area geometrical coverage at a reasonably low cost. It can yield intrinsic detection efficiencies that compare favorably with moderator-based detection methods. The timing resolution achievable should permit substantially more precise time windowing of return neutron flux than would otherwise be possible with moderated detectors. The energy-deposition threshold imposed on each scintillator contributing to the event-definition trigger in a TOF system can be set to blind the sensor to direct emission from the neutron generator. The primary technical challenge addressed in the project was to understand the capabilities of a neutron TOF sensor in the limit of large scintillator area and small scintillator separation, a size regime in which the neutral particle’s flight path between the two scintillators is not tightly constrained
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A portable neutron coincidence counter
Pacific Northwest National Laboratory has designed and constructed a prototype portable neutron coincidence counter intended for use in a variety of applications, such as the verification and inspection of weapons components, safety measurements for novel and challenging situations, portable portal deployment to prevent the transportation of fissile materials, uranium enrichment measurements in hard-to-reach locations, waste assays for objects that cannot be measured by existing measurement systems, and decontamination and decommissioning. The counting system weighs less than 40 kg and is composed of parts each weighing no more than 5 kg. In addition, the counter`s design is sufficiently flexible to allow rapid, reliable assembly around containers of nearly arbitrary size and shape. The counter is able to discern the presence of 1 kg of weapons-grade plutonium within an ALR-8 (30-gal drum) in roughly 100 seconds and 10 g in roughly 1000 seconds. The counter`s electronics are also designed for maximum adaptability, allowing operation under a wide variety of circumstances, including exposure to gamma-ray fields of 1 R/h. This report provides a detailed review of the design and construction process. Finally, preliminary experimental measurements that confirm the performance capabilities of this counter are discussed. 6 refs., 18 figs., 3 tabs
Atmospheric pressure ionization permanent magnet fourier transform ion cyclotron resonance mass spectrometry
Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential
Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation
detection, but their application is limited by the high cost of detector-grade
materials. High crystal costs primarily result from property non-uniformity
that causes low manufacturing yield. While tremendous efforts have been made in
the past to reduce Te inclusions / precipitates in CZT, this has not resulted
in an anticipated improvement in material property uniformity. Moreover, it is
recognized that in addition to Te particles, dislocation cells can also cause
electric field perturbation and the associated property non-uniformity. Further
improvement of the material, therefore, requires that dislocations in CZT
crystals be understood and controlled. Here we use a recently developed CZT
bond order potential to perform representative molecular dynamics simulations
to study configurations, energies, and mobilities of 29 different types of
possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to
derive activation free energies and activation volumes of thermally activated
dislocation motion will be explored. Our focus gives insight into understanding
important dislocations in the material, and gives guidance toward experimental
efforts for improving dislocation network structures in CZT crystals
Submicrometer Pattern Fabrication by Intensification of Instability in Ultrathin Polymer Films under a Water-Solvent Mix
Dewetting of ultrathin (< 100 nm) polymer films, by heating above the glass
transition, produces droplets of sizes of the order of microns and mean
separations between droplets of the order of tens of microns. These relatively
large length scales are because of the weak destabilizing van der Waals forces
and the high surface energy penalty required for deformations on small scales.
We show a simple, one-step versatile method to fabricate sub-micron (>~100 nm)
droplets and their ordered arrays by room temperature dewetting of ultrathin
polystyrene (PS) films by minimizing these limitations. This is achieved by
controlled room temperature dewetting under an optimal mixture of water,
acetone and methyl-ethyl ketone (MEK). Diffusion of organic solvents in the
film greatly reduces its glass transition temperature and the interfacial
tension, but enhances the destabilizing field by introduction of electrostatic
force. The latter is reflected in a change in the exponent, n of the
instability length scale, {\lambda} ~h^n, where h is the film thickness and n =
1.51 \pm 0.06 in the case of water-solvent mix, as opposed to its value of 2.19
\pm 0.07 for dewetting in air. The net outcome is more than one order of
magnitude reduction in the droplet size as well as their mean separation and
also a much faster dynamics of dewetting. We also demonstrate the use of this
technique for controlled dewetting on topographically patterned substrates with
submicrometer features where dewetting in air is either arrested, incomplete or
unable to produce ordered patterns