8,087 research outputs found
Synthesis of calculational methods for the design and analysis of radiation shields for nuclear rocket systems. Volume 2 - Analysis of radiation measurements in a nuclear rocket propellant tank mockup using simulated liquid hydrogen
Calculational methods for nuclear rocket radiation shield design - analysis of radiation measurements in nuclear rocket propellant tank mockup using simulated liquid hydroge
Gas of self-avoiding loops on the brickwork lattice
An exact calculation of the phase diagram for a loop gas model on the
brickwork lattice is presented. The model includes a bending energy. In the
dense limit, where all the lattice sites are occupied, a phase transition
occuring at an asymmetric Lifshitz tricritical point is observed as the
temperature associated with the bending energy is varied. Various critical
exponents are calculated. At lower densities, two lines of transitions (in the
Ising universality class) are observed, terminated by a tricritical point,
where there is a change in the modulation of the correlation function. To each
tricritical point an associated disorder line is found.Comment: 19 pages, 6 figures. to appear in J. Phys. A : Math. & Ge
Data report for the Siple Coast (Antarctica) project
This report presents data collected during three field seasons of glaciological studies in the Antarctica and describes the methods employed. The region investigated covers the mouths of Ice Streams B and C (the Siple Coast) and Crary Ice Rise on the Ross Ice Shelf. Measurements included in the report are as follows: surface velocity and deformation from repeated satellite geoceiver positions; surface topography from optical levelling; radar sounding of ice thickness; accumulation rates; near-surface densities and temperature profiles; and mapping from aerial photography
QU Carinae: a SNeIa progenitor?
Optical spectra obtained in 2006-07 of the nova-like cataclysmic variable QU
Car are studied for radial velocities, line profiles, and line identifications.
We are not able to confirm the reported 10.9 hr orbital period from 1982,partly
because our sampling is not ideal for this purpose and also, we suspect,
because our radial velocities are distorted by line profile changes due to an
erratic wind. P-Cygni profiles are found in several of the emission lines,
including those of C IV. Carbon lines are abundant in the spectra, suggesting a
carbon enrichment in the doner star. The presence of [O III] 5007\AA and [N II]
6584\AA is likely due to a diffuse nebula in the vicinity of the system.
The wind signatures in the spectra and the presence of nebular lines are in
agreement with the accretion wind evolution scenario that has been suggested to
lead to SNeIa. We argue that QU Car is a member of the V Sge subclass of CVs,
and a possible SNeIa progenitor. It is shown that the recent light curve of QU
Car has ~1 mag low states, similar to the light curve of V Sge, strengthening
the connection of QU Car with V Sge stars, supersoft x-ray sources, and SNeIa
progenitors.Comment: Accepted in the Astronomical Journal. 11 pages, 3 tables, 5 figure
Radio Images of 3C 58: Expansion and Motion of its Wisp
New 1.4 GHz VLA observations of the pulsar-powered supernova remnant 3C 58
have resulted in the highest-quality radio images of this object to date. The
images show filamentary structure over the body of the nebula. The present
observations were combined with earlier ones from 1984 and 1991 to investigate
the variability of the radio emission on a variety of time-scales. No
significant changes are seen over a 110 day interval. In particular, the upper
limit on the apparent projected velocity of the wisp is 0.05c. The expansion
rate of the radio nebula was determined between 1984 and 2004, and is
0.014+/-0.003%/year, corresponding to a velocity of 630+/-70 km/s along the
major axis. If 3C 58 is the remnant of SN 1181, it must have been strongly
decelerated, which is unlikely given the absence of emission from the supernova
shell. Alternatively, the low expansion speed and a number of other arguments
suggest that 3C 58 may be several thousand years old and not be the remnant of
SN 1181.Comment: 12 pages; accepted for publication in the Astrophysical Journa
Frustrated antiferromagnetic quantum spin chains for spin length S > 1
We investigate frustrated antiferromagnetic Heisenberg quantum spin chains at
T=0 for S=3/2 and S=2 using the DMRG method. We localize disorder and Lifshitz
points, confirming that quantum disorder points can be seen as quantum remnants
of classical phase transitions. Both in the S=3/2 and the S=2 chain, we observe
the disappearance of effectively free S=1/2 and S=1 end spins respectively. The
frustrated spin chain is therefore a suitable system for clearly showing the
existence of free end spins S'=[S/2] also in half-integer antiferromagnetic
spin chains with S>1/2. We suggest that the first order transition found for
S=1 in our previous work is present in all frustrated spin chains with S>1/2,
characterized by the disappearance of effectively free end spins with S'=[S/2].Comment: 6 pages, 8 ps figures, uses RevTeX, submitted to PR
Frustrated quantum Heisenberg ferrimagnetic chains
We study the ground-state properties of weakly frustrated Heisenberg
ferrimagnetic chains with nearest and next-nearest neighbor antiferromagnetic
exchange interactions and two types of alternating sublattice spins S_1 > S_2,
using 1/S spin-wave expansions, density-matrix renormalization group, and
exact- diagonalization techniques. It is argued that the zero-point spin
fluctuations completely destroy the classical commensurate- incommensurate
continuous transition. Instead, the long-range ferrimagnetic state disappears
through a discontinuous transition to a singlet state at a larger value of the
frustration parameter. In the ferrimagnetic phase we find a disorder point
marking the onset of incommensurate real-space short-range spin-spin
correlations.Comment: 16 pages (LaTex 2.09), 6 eps figure
RELATIONSHIP OF GROUND AND KNEE JOINT REACTION FORCES IN PLYOMETRIC EXERCISES
The purpose of the current study was to assess the relationship between peak vertical ground reaction force (GRF) and peak knee joint reaction force (KJRF). Eighteen recreationally active college students performed a countermovement jump, single leg jump, and drop jump from a height equal to their vertical jump. Vertical ground reaction forces were assessed with a force plate and KJRF were assessed using a combination of GRF and video data. A Paired samples t-Test revealed GRF to be significantly greater compared to KJRF for all jumps (
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