42,503 research outputs found
An investigation of the optimization of parameters affecting the implementation of fourier transform spectroscopy at 20-500 micron from the C-141 airborne infrared observatory
A program for 20-500 micron spectroscopy from the NASA flying C141 infrared observatory is being carried out with a Michelson interferometer. The parameters affecting the performance of the instrument are studied and an optimal configuration for high performance on the C-141 aircraft is recommended. As each parameter is discussed the relative merits of the two modes of mirror motion (rapid scan or step and integrate) are presented
Relativistic Winds from Compact Gamma-Ray Sources: II. Pair Loading and Radiative Acceleration in Gamma-ray Bursts
We consider the effects of rapid pair creation by an intense pulse of
gamma-rays propagating ahead of a relativistic shock. Side-scattered photons
colliding with the main gamma-ray beam amplify the density of scattering
charges. The acceleration rate of the pair-loaded medium is calculated, and its
limiting bulk Lorentz factor related to the spectrum and compactness of the
photon source. One obtains, as a result, a definite prediction for the relative
inertia in baryons and pairs. The deceleration of a relativistic shock in the
moving medium, and the resulting synchrotron emissivity, are compared with
existing calculations for a static medium. The radiative efficiency is
increased dramatically by pair loading. When the initial ambient density
exceeds a critical value, the scattering depth traversed by the main gamma-ray
pulse rises above unity, and the pulse is broadened. These considerations place
significant constraints on burst progenitors: a pre-burst mass loss rate
exceeding 10^{-5} M_\odot per year is difficult to reconcile with individual
pulses narrower than 10 s, unless the radiative efficiency is low. An
anisotropic gamma-ray flux (on an angular scale \Gamma^{-1} or larger) drives a
large velocity shear that greatly increases the energy in the seed magnetic
field forward of the propagating shock.Comment: 19 pp., LaTeX (aaspp4.sty), revised 12/23/99, Ap. J. in press;
summary section added and several minor improvements in presentatio
Two neutron decay of 16Be
Recently, the first example of two-neutron decay from the ground state of an
unbound nucleus, Be, was seen. Three-body methods are ideal for exactly
treating the degrees of freedom important for these decays. Using a basis
expansion over hyperspherical harmonics and the hyperspherical R-matrix method,
we construct a realistic model of Be in order to investigate its decay
mode and the role of the two-neutron interaction. The neutron-Be
interaction is constrained using shell model predictions. We obtain a ground
state for Be that is over-bound by approximately 1 MeV with a width of
approximately 0.23 MeV. This suggests, that for such systems, the three-body
force needs to be repulsive.Comment: 4 pages, 1 figure, contribution to the proceedings for the 21st
International Conference on Few Body Problems in Physics, Chicago, Illinois,
US
Three-body breakup within the fully discretized Faddeev equations
A novel approach is developed to find the three-body breakup amplitudes and
cross sections within the modified Faddeev equation framework. The method is
based on the lattice-like discretization of the three-body continuum with a
three-body stationary wave-packet basis in momentum space. The approach makes
it possible to simplify drastically all the three- and few-body breakup
calculations due to discrete wave-packet representations for the few-body
continuum and simultaneous lattice representation for all the scattering
operators entering the integral equation kernels. As a result, the few-body
breakup can be treated as a particular case of multi-channel scattering in
which part of the channels represents the true few-body continuum states. As an
illustration for the novel approach, an accurate calculations for the
three-body breakup process with non-local and local
interactions are calculated. The results obtained reproduce nicely the
benchmark calculation results using the traditional Faddeev scheme which
requires much more tedious and time-consuming calculations.Comment: 17 pages, 13 figure
Reaction cross-section predictions for nucleon induced reactions
A microscopic calculation of the optical potential for nucleon-nucleus
scattering has been performed by explicitly coupling the elastic channel to all
the particle-hole (p-h) excitation states in the target and to all relevant
pickup channels. These p-h states may be regarded as doorway states through
which the flux flows to more complicated configurations, and to long-lived
compound nucleus resonances. We calculated the reaction cross sections for the
nucleon induced reactions on the targets Ca, Ni, Zr and
Sm using the QRPA description of target excitations, coupling to all
inelastic open channels, and coupling to all transfer channels corresponding to
the formation of a deuteron. The results of such calculations were compared to
predictions of a well-established optical potential and with experimental data,
reaching very good agreement. The inclusion of couplings to pickup channels
were an important contribution to the absorption. For the first time,
calculations of excitations account for all of the observed reaction
cross-sections, at least for incident energies above 10 MeV.Comment: 6 pages, 6 figures. Submitted to INPC 2010 Conference Proceeding
Spatial Structure of Ion Beams in an Expanding Plasma
We report spatially resolved perpendicular and parallel, to the magnetic field, ion velocity distribution function (IVDF) measurements in an expanding argon helicon plasma. The parallel IVDFs, obtained through laser induced fluorescence (LIF), show an ion beam with v ≈ 8000 m/s flowing downstream and confined to the center of the discharge. The ion beam is measurable for tens of centimeters along the expansion axis before the LIF signal fades, likely a result of metastable quenching of the beam ions. The parallel ion beam velocity slows in agreement with expectations for the measured parallel electric field. The perpendicular IVDFs show an ion population with a radially outward flow that increases with distance from the plasma axis. Structures aligned to the expanding magnetic field appear in the DC electric field, the electron temperature, and the plasma density in the plasma plume. These measurements demonstrate that at least two-dimensional and perhaps fully three-dimensional models are needed to accurately describe the spontaneous acceleration of ion beams in expanding plasmas
Self-gravitating astrophysical mass with singular central density vibrating in fundamental mode
The fluid-dynamical model of a self-gravitating mass of viscous liquid with
singular density at the center vibrating in fundamental mode is considered in
juxtaposition with that for Kelvin fundamental mode in a homogeneous heavy mass
of incompressible inviscid liquid. Particular attention is given to the
difference between spectral formulae for the frequency and lifetime of -mode
in the singular and homogeneous models. The newly obtained results are
discussed in the context of theoretical asteroseismology of pre-white dwarf
stage of red giants and stellar cocoons -- spherical gas-dust clouds with dense
star-forming core at the center.Comment: Mod. Phys. Lett. A, Vol. 24, No. 40 (2009) pp. 3257-327
Charge-ice dynamics in the negative thermal expansion material Cd(CN)
We use variable-temperature (150--300\,K) single-crystal X-ray diffraction to
re-examine the interplay between structure and dynamics in the ambient phase of
the isotropic negative thermal expansion (NTE) material Cd(CN). We find
strong experimental evidence for the existence of low-energy vibrational modes
that involve off-centering of Cd ions. These modes have the effect of
increasing network packing density---suggesting a mechanism for NTE that is
different to the generally-accepted picture of correlated Cd(C/N) rotation
modes. Strong local correlations in the displacement directions of neighbouring
cadmium centres are evident in the existence of highly-structured diffuse
scattering in the experimental X-ray diffraction patterns. Monte Carlo
simulations suggest these patterns might be interpreted in terms of a basic set
of `ice-rules' that establish a mapping between the dynamics of Cd(CN) and
proton ordering in cubic ice VII.Comment: 5 pages, 5 figures, submitted to PR
Polo like kinase 2 tumour suppressor and cancer biomarker: new perspectives on drug sensitivity/resistance in ovarian cancer
The polo-like kinase PLK2 has recently been identified as a potential theranostic marker in the management of chemotherapy sensitive cancers. The methylation status of the PLK2 CpG island varies with sensitivity to paclitaxel and platinum in ovarian cancer cell lines. Importantly, extrapolation of these in vitro data to the clinical setting confirms that the methylation status of the PLK2 CpG island predicts outcomes in patients treated with carboplatin and paclitaxel chemotherapy. A second cell cycle regulator, p57Kip2, is also subject to epigenetic silencing in carboplatin resistance in vitro and in vivo, emphasising that cell cycle regulators are important determinants of sensitivity to chemotherapeutic agents and providing insights into the phenomenon of collateral drug sensitivity in oncology. Understanding the mechanistic basis and identification of robust biomarkers to predict collateral sensitivity may inform optimal use of chemotherapy in patients receiving multiple lines of treatment
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