884 research outputs found
CT beam dosimetric characterization procedure for personalized dosimetry
Personalized dosimetry in computed tomography (CT) can be realized by a full Monte Carlo (MC) simulation of the scan procedure. Essential input data needed for the simulation are appropriate CT x-ray source models and a model of the patient's body which is based on the CT image. The purpose of this work is to develop comprehensive procedures for the determination of CT x-ray source models and their verification by comparison of calculated and measured dose distributions in physical phantoms. Mobile equipment together with customized software was developed and used for non-invasive determination of equivalent source models of CT scanners under clinical conditions. Standard and physical anthropomorphic CT dose phantoms equipped with real-time CT dose probes at five representative positions were scanned. The accumulated dose was measured during the scan at the five positions. ImpactMC, an MC-based CT dose software program, was used to simulate the scan. The necessary inputs were obtained from the scan parameters, from the equivalent source models and from the material-segmented CT images of the phantoms. 3D dose distributions in the phantoms were simulated and the dose values calculated at the five positions inside the phantom were compared to measured dose values. Initial results were obtained by means of a General Electric Optima CT 660 and a Toshiba (Canon) Aquilion ONE. In general, the measured and calculated dose values were within relative uncertainties that had been estimated to be less than 10%. The procedures developed were found to be viable and rapid. The procedures are applicable to any scanner type under clinical conditions without making use of the service mode with stationary x-ray tube position. Results show that the procedures are well suited for determining and verifying the equivalent source models needed for personalized CT dosimetry based on post-scan MC calculations.Peer reviewe
Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized -Pu
The effects on the local structure due to self-irradiation damage of Ga
stabilized -Pu stored at cryogenic temperatures have been examined
using extended x-ray absorption fine structure (EXAFS) experiments. Extensive
damage, seen as a loss of local order, was evident after 72 days of storage
below 15 K. The effect was observed from both the Pu and Ga sites, although
less pronounced around Ga. Isochronal annealing was performed on this sample to
study the annealing processes that occur between cryogenic and room temperature
storage conditions, where damage is mostly reversed. Damage fractions at
various points along the annealing curve have been determined using an
amplitude-ratio method, standard EXAFS fitting, and a spherical crystallite
model, and provide information complementary to previous electrical
resistivity- and susceptibility-based isochronal annealing studies. The use of
a spherical crystallite model accounts for the changes in EXAFS spectra using
just two parameters, namely, the crystalline fraction and the particle radius.
Together, these results are discussed in terms of changes to the local
structure around Ga and Pu throughout the annealing process and highlight the
unusual role of Ga in the behavior of the lowest temperature anneals.Comment: 13 pages, 10 figure
Reaction physics and mission capabilities of the magnetically insulated inertial confinement fusion reactor
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76048/1/AIAA-25451-882.pd
Antimatter-driven fusion propulsion scheme for solar system exploration
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76531/1/AIAA-23527-891.pd
Gasdynamic fusion propulsion system for space exploration
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76953/1/AIAA-23876-408.pd
Nuclear Cryogenic Propulsion Stage
The fundamental capability of Nuclear Thermal Propulsion (NTP) is game changing for space exploration. A first generation Nuclear Cryogenic Propulsion Stage (NCPS) based on NTP could provide high thrust at a specific impulse above 900 s, roughly double that of state of the art chemical engines. Characteristics of fission and NTP indicate that useful first generation systems will provide a foundation for future systems with extremely high performance. The role of the NCPS in the development of advanced nuclear propulsion systems could be analogous to the role of the DC-3 in the development of advanced aviation. Progress made under the NCPS project could help enable both advanced NTP and advanced NEP
Nuclear Cryogenic Propulsion Stage for Mars Exploration
The fundamental capability of Nuclear Thermal Propulsion (NTP) is game changing for space exploration. A first generation Nuclear Cryogenic Propulsion Stage (NCPS) based on NTP could provide high thrust at a specific impulse above 900 s, roughly double that of state of the art chemical engines. Characteristics of fission and NTP indicate that useful first generation systems will provide a foundation for future systems with extremely high performance. The role of the NCPS in the development of advanced nuclear propulsion systems could be analogous to the role of the DC-3 in the development of advanced aviation. Progress made under the NCPS project could help enable both advanced NTP and advanced Nuclear Electric Propulsion (NEP)
A new Late Pliocene large provannid gastropod associated with hydrothermal venting at Kane Megamullion, Mid-Atlantic Ridge
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Taylor & Francis for personal use, not for redistribution. The definitive version was published in Journal of Systematic Palaeontology 10 (2012): 423-433, doi:10.1080/14772019.2011.607193.A new gastropod, Kaneconcha knorri gen et sp. nov., was found in marlstone dredged
from the surface of Adam Dome at Kane Megamullion on the flank of the Mid-Atlantic
Ridge in an area of former hydrothermal activity. The snail is interpreted as a large
provannid similar to the chemosymbiotic genera Ifremeria and Alviniconcha. This is the
first record of presumably chemosymbiotic provannids from the Atlantic Ocean and also
the first fossil record of such large provannids associated with hydrothermal venting.
Extant Alviniconcha and Ifremeria are endemic to hydrothermal vents in the Pacific and
Indian oceans. Kaneconcha differs from Ifremeria in having no umbilicus and a posterior
notch, and it differs from Alviniconcha in having the profile of the whorl slightly
flattened and having no callus on the inner lip. A dark layer covering the Kaneconcha
shell is interpreted here as a fossilized periostracum. The shell/periostracum interface
shows fungal traces attributed to the ichnospecies Saccomorpha clava. We hypothesize
that large chemosymbiotic provannids (i.e., Kaneconcha, Ifremeria, and Alviniconcha)
form a clade that possibly diverged from remaining provannids in the Late Jurassic, with
the Late Jurassic/Early Cretaceous Paskentana being an early member.R/V Knorr Cruise 180-
2 to Kane Megamullion was supported by National Science Foundation grant OCE-
0118445. A. Kaim acknowledges support from the Alexander von Humboldt Foundation.
B. Tucholke acknowledges support from an Andrew W. Mellon Foundation Award for
Innovative Research and from the Deep Ocean Exploration Institute at Woods Hole
Oceanographic Institution
Study on Alternative Cargo Launch Options from the Lunar Surface
In the future, there will be a need for constant cargo launches from Earth to Mars in order to build, and then sustain, a Martian base. Currently, chemical rockets are used for space launches. These are expensive and heavy due to the amount of necessary propellant. Nuclear thermal rockets (NTRs) are the next step in rocket design. Another alternative is to create a launcher on the lunar surface that uses magnetic levitation to launch cargo to Mars in order to minimize the amount of necessary propellant per mission. This paper investigates using nuclear power for six different cargo launching alternatives, as well as the orbital mechanics involved in launching cargo to a Martian base from the moon. Each alternative is compared to the other alternative launchers, as well as compared to using an NTR instead. This comparison is done on the basis of mass that must be shipped from Earth, the amount of necessary propellant, and the number of equivalent NTR launches. Of the options, a lunar coil launcher had a ship mass that is 12.7% less than the next best option and 17 NTR equivalent launches, making it the best of the presented six options
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