23,808 research outputs found
Preliminary estimates of radiation exposures for manned interplanetary missions from anomalously large solar flare events
Preliminary estimates of radiation exposures for manned interplanetary missions resulting from anomalously large solar flare events are presented. The calculations use integral particle fluences for the February 1956, November 1960, and August 1972 events as inputs into the Langley Research Center nucleon transport code BRYNTRN. This deterministic code transports primary and secondary nucleons (protons and neutrons) through any number of layers of target material of arbitrary thickness and composition. Contributions from target nucleus fragmentation and recoil are also included. Estimates of 5 cm depth doses and dose equivalents in tissue are presented behind various thicknesses of aluminum, water, and composite aluminum/water shields for each of the three solar flare events
Solar-flare shielding with Regolith at a lunar-base site
The Langley high energy nucleon transport computer code BRYNTRN is used to predict time-integrated radiation dose levels at the lunar surface due to high proton flux from solar flares. The study addresses the shielding requirements for candidate lunar habitat configurations necessary to protect crew members from these large and unpredictable radiation fluxes. Three solar proton events have been analyzed, and variations in radiation intensity in a shield medium due to the various primary particle energy distributions are predicted. Radiation dose predictions are made for various slab thicknesses of a lunar soil model. Results are also presented in the form of dose patterns within specific habitat configurations shielded with lunar material
Decoupling Transition I. Flux Lattices in Pure Layered Superconductors
We study the decoupling transition of flux lattices in a layered
superconductors at which the Josephson coupling J is renormalized to zero. We
identify the order parameter and related correlations; the latter are shown to
decay as a power law in the decoupled phase. Within 2nd order renormalization
group we find that the transition is always continuous, in contrast with
results of the self consistent harmonic approximation. The critical temperature
for weak J is ~1/B, where B is the magnetic field, while for strong J it
is~1/sqrt{B} and is strongly enhanced. We show that renormaliztion group can be
used to evaluate the Josephson plasma frequency and find that for weak J it
is~1/BT^2 in the decoupled phase.Comment: 14 pages, 5 figures. New sections III, V. Companion to following
article on "Decoupling and Depinning II: Flux lattices in disordered layered
superconductors
Building capacity for evidence-based public health: Reconciling the pulls of practice and the push of research
Timely implementation of principles of evidence-based public health (EBPH) is critical for bridging the gap between discovery of new knowledge and its application. Public health organizations need sufficient capacity (the availability of resources, structures, and workforce to plan, deliver, and evaluate the preventive dose of an evidence-based intervention) to move science to practice. We review principles of EBPH, the importance of capacity building to advance evidence-based approaches, promising approaches for capacity building, and future areas for research and practice. Although there is general agreement among practitioners and scientists on the importance of EBPH, there is less clarity on the definition of evidence, how to find it, and how, when, and where to use it. Capacity for EBPH is needed among both individuals and organizations. Capacity can be strengthened via training, use of tools, technical assistance, assessment and feedback, peer networking, and incentives. Modest investments in EBPH capacity building will foster more effective public health practice
Radiation exposure for manned Mars surface missions
The Langley cosmic ray transport code and the Langley nucleon transport code (BRYNTRN) are used to quantify the transport and attenuation of galactic cosmic rays (GCR) and solar proton flares through the Martian atmosphere. Surface doses are estimated using both a low density and a high density carbon dioxide model of the atmosphere which, in the vertical direction, provides a total of 16 g/sq cm and 22 g/sq cm of protection, respectively. At the Mars surface during the solar minimum cycle, a blood-forming organ (BFO) dose equivalent of 10.5 to 12 rem/yr due to galactic cosmic ray transport and attenuation is calculated. Estimates of the BFO dose equivalents which would have been incurred from the three large solar flare events of August 1972, November 1960, and February 1956 are also calculated at the surface. Results indicate surface BFO dose equivalents of approximately 2 to 5, 5 to 7, and 8 to 10 rem per event, respectively. Doses are also estimated at altitudes up to 12 km above the Martian surface where the atmosphere will provide less total protection
Estimates of galactic cosmic ray shielding requirements during solar minimum
Estimates of radiation risk from galactic cosmic rays are presented for manned interplanetary missions. The calculations use the Naval Research Laboratory cosmic ray spectrum model as input into the Langley Research Center galactic cosmic ray transport code. This transport code, which transports both heavy ions and nucleons, can be used with any number of layers of target material, consisting of up to five different arbitrary constituents per layer. Calculated galactic cosmic ray fluxes, dose and dose equivalents behind various thicknesses of aluminum, water and liquid hydrogen shielding are presented for the solar minimum period. Estimates of risk to the skin and the blood-forming organs (BFO) are made using 0-cm and 5-cm depth dose/dose equivalent values, respectively, for water. These results indicate that at least 3.5 g/sq cm (3.5 cm) of water, or 6.5 g/sq cm (2.4 cm) of aluminum, or 1.0 g/sq cm (14 cm) of liquid hydrogen shielding is required to reduce the annual exposure below the currently recommended BFO limit of 0.5 Sv. Because of large uncertainties in fragmentation parameters and the input cosmic ray spectrum, these exposure estimates may be uncertain by as much as a factor of 2 or more. The effects of these potential exposure uncertainties or shield thickness requirements are analyzed
Out-of-plane fluctuation conductivity of layered superconductors in strong electric fields
The non-Ohmic effect of a high electric field on the out-of-plane
magneto-conductivity of a layered superconductor near the superconducting
transition is studied in the frame of the Langevin approach to the
time-dependent Ginzburg-Landau equation. The transverse fluctuation
conductivity is computed in the self-consistent Hartree approximation for an
arbitrarily strong electric field and a magnetic field perpendicular to the
layers. Our results indicate that high electric fields can be effectively used
to suppress the out-of-plane fluctuation conductivity in high-temperature
superconductors and a significant broadening of the transition induced by a
strong electric field is predicted. Extensions of the results are provided for
the case when the electric field is applied at an arbitrary angle with respect
to the layers, as well as for the three-dimensional anisotropic regime of a
strong interlayer coupling.Comment: to be published in Phys. Rev.
High Field Studies of Superconducting Fluctuations in High-T_c Cuprates: Evidence for a Small Gap distinct from the Large Pseudogap
We have used pulsed magnetic fields up to 60Tesla to suppress the
contribution of superconducting fluctuations(SCF)to the conductivity above Tc
in a series of YBa2Cu3O6+x from the deep pseudogapped state to slight
overdoping. Accurate determinations of the SCF conductivity versus temperature
and magnetic field have been achieved. Their joint quantitative analyses with
respect to Nernst data allow us to establish that thermal fluctuations
following the Ginzburg-Landau(GL) scheme are dominant for nearly optimally
doped samples. The deduced coherence length xi(T) is in perfect agreement with
a gaussian (Aslamazov-Larkin) contribution for 1.01Tc<T<1.2Tc. A phase
fluctuation contribution might be invoked for the most underdoped samples in a
T range which increases when controlled disorder is introduced by electron
irradiation. For all dopings we evidence that the fluctuations are highly
damped when increasing T or H. The data permits us to define a field Hc^prime
and a temperature Tc^prime above which the SCF are fully suppressed. The
analysis of the fluctuation magnetoconductance in the GL approach allows us to
determine the critical field Hc2(0). The actual values of Hc^prime(0) and
Hc2(0) are found quite similar and both increase with hole doping. These
depairing fields, which are directly connected to the magnitude of the SC gap,
do therefore follow the Tc variation which is at odds with the sharp decrease
of the pseudogap T* with increasing hole doping. This is on line with our
previous evidence that T* is not the onset of pairing. We finally propose a
three dimensional phase diagram including a disorder axis, which allows to
explain most peculiar observations done so far on the diverse cuprate families.Comment: revised version, to be published in Physical Review B. Small
modifications have been done in paragraphs VI.A and VI
Interpolation of the Josephson interaction in highly anisotropic superconductors from a solution of the two dimensional sine-Gordon equation
In this paper we solve numerically the two dimensional elliptic sine-Gordon
equation with appropriate boundary conditions. These boundary conditions are
chosen to correspond to the Josephson interaction between two adjacent pancakes
belonging to the same flux-line in a highly anisotropic superconductor. An
extrapolation is obtained between the regimes of low and high separation of the
pancakes. The resulting formula is a better candidate for use in numerical
simulations than previously derived formulas.Comment: 18 pages, 9 figure
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