1,941 research outputs found
The Origin of (90) Antiope From Component-Resolved Near-Infrared Spectroscopy
The origin of the similary-sized binary asteroid (90) Antiope remains an
unsolved puzzle. To constrain the origin of this unique double system, we
recorded individual spectra of the components using SPIFFI, a near-infrared
integral field spectrograph fed by SINFONI, an adaptive optics module available
on VLT-UT4. Using our previously published orbital model, we requested
telescope time when the separation of the components of (90) Antiope was larger
than 0.087", to minimize the contamination between components, during the
February 2009 opposition. Several multi-spectral data-cubes in J band (SNR=40)
and H+K band (SNR=100) were recorded in three epochs and revealed the two
components of (90) Antiope. After developing a specific photometric extraction
method and running an error analysis by Monte-Carlo simulations, we
successfully extracted reliable spectra of both components from 1.1 to 2.4 um
taken on the night of February 21, 2009. These spectra do not display any
significant absorption features due to mafic mineral, ices, or organics, and
their slopes are in agreement with both components being C- or Cb- type
asteroids. Their constant flux ratio indicates that both components' surface
reflectances are quite similar, with a 1-sigma variation of 7%. By comparison
with 2MASS J, H, K color distribution of observed Themis family members, we
conclude that both bodies were most likely formed at the same time and from the
same material. The similarly-sized system could indeed be the result of the
breakup of a rubble-pile proto-Antiope into two equal-sized bodies, but other
scenarios of formation implying a common origin should also be considered.Comment: 46 pages, 1 table, 11 figures accepted for publication to Icaru
Cherenkov and Scintillation Light Separation in Organic Liquid Scintillators
The CHErenkov / Scintillation Separation experiment (CHESS) has been used to
demonstrate the separation of Cherenkov and scintillation light in both linear
alkylbenzene (LAB) and LAB with 2g/L of PPO as a fluor (LAB/PPO). This is the
first such demonstration for the more challenging LAB/PPO cocktail and improves
on previous results for LAB. A time resolution of 338 +/- 12 ps FWHM results in
an efficiency for identifying Cherenkov photons in LAB/PPO of 70 +/- 3% and 63
+/- 8% for time- and charge-based separation, respectively, with scintillation
contamination of 36 +/- 5% and 38 +/- 4%. LAB/PPO data is consistent with a
rise time of 0.75 +/- 0.25 ns
A spectral comparison of (379) Huenna and its satellite
We present near-infrared spectral measurements of Themis family asteroid
(379) Huenna (D~98 km) and its 6 km satellite using SpeX on the NASA IRTF. The
companion was farther than 1.5" from the primary at the time of observations
and was approximately 5 magnitudes dimmer. We describe a method for separating
and extracting the signal of a companion asteroid when the signal is not
entirely resolved from the primary. The spectrum of (379) Huenna has a broad,
shallow feature near 1 {\mu}m and a low slope, characteristic of C-type
asteroids. The secondary's spectrum is consistent with the taxonomic
classification of C-complex or X-complex. The quality of the data was not
sufficient to identify any subtle feature in the secondary's spectrum.Comment: 6 pages, 4 figures, 2 tables - Accepted for publication in Icaru
Analogy of the slow dynamics between the supercooled liquid and the supercooled plastic crystal states of difluorotetrachloroethane
Slow dynamics of difluorotetrachloroethane in both supercooled plastic
crystal and supercooled liquid states have been investigated from Molecular
Dynamics simulations. The temperature and wave-vector dependence of collective
dynamics in both states are probed using coherent dynamical scattering
functions . Our results confirm the strong analogy between molecular
liquids and plastic crystals for which -relaxation times and
non-ergodicity parameters are controlled by the non trivial static correlations
as predicted by the Mode Coupling Theory. The use of infinitely thin
needles distributed on a lattice as model of plastic crystals is discussed
A Giant Crater on 90 Antiope?
Mutual event observations between the two components of 90 Antiope were
carried out in 2007-2008. The pole position was refined to lambda0 =
199.5+/-0.5 eg and beta0 = 39.8+/-5 deg in J2000 ecliptic coordinates, leaving
intact the physical solution for the components, assimilated to two perfect
Roche ellipsoids, and derived after the 2005 mutual event season (Descamps et
al., 2007). Furthermore, a large-scale geological depression, located on one of
the components, was introduced to better match the observed lightcurves. This
vast geological feature of about 68 km in diameter, which could be postulated
as a bowl-shaped impact crater, is indeed responsible of the photometric
asymmetries seen on the "shoulders" of the lightcurves. The bulk density was
then recomputed to 1.28+/-0.04 gcm-3 to take into account this large-scale
non-convexity. This giant crater could be the aftermath of a tremendous
collision of a 100-km sized proto-Antiope with another Themis family member.
This statement is supported by the fact that Antiope is sufficiently porous
(~50%) to survive such an impact without being wholly destroyed. This violent
shock would have then imparted enough angular momentum for fissioning of
proto-Antiope into two equisized bodies. We calculated that the impactor must
have a diameter greater than ~17 km, for an impact velocity ranging between 1
and 4 km/s. With such a projectile, this event has a substantial 50%
probability to have occurred over the age of the Themis family.Comment: 30 pages, 3 Tables, 8 Figures. Accepted for publication in Icaru
Optimizing dual energy cone beam CT protocols for preclinical imaging and radiation research
Objective: The aim of this work was to investigate whether quantitative dual-energy CT (DECT) imaging is feasible for small animal irradiators with an integrated cone-beam CT (CBCT) system.
Methods: The optimal imaging protocols were determined by analyzing different energy combinations and dose levels. The influence of beam hardening effects and the performance of a beam hardening correction (BHC) were investigated. In addition, two systems from different manufacturers were compared in terms of errors in the extracted effective atomic numbers (Z(eff)) and relative electron densities (rho(e)) for phantom inserts with known elemental compositions and relative electron densities.
Results: The optimal energy combination was determined to be 50 and 90kVp. For this combination, Z(eff) and r rho(e) can be extracted with a mean error of 0.11 and 0.010, respectively, at a dose level of 60cGy.
Conclusion: Quantitative DECT imaging is feasible for small animal irradiators with an integrated CBCT system. To obtain the best results, optimizing the imaging protocols is required. Well-separated X-ray spectra and a sufficient dose level should be used to minimize the error and noise for Z(eff) and rho(e). When no BHC is applied in the image reconstruction, the size of the calibration phantom should match the size of the imaged object to limit the influence of beam hardening effects. No significant differences in Z(eff) and rho(e) errors are observed between the two systems from different manufacturers.
Advances in knowledge: This is the first study that investigates quantitative DECT imaging for small animal irradiators with an integrated CBCT system
Is there something of the MCT in orientationally disordered crystals ?
Molecular Dynamics simulations have been performed on the orientationally
disordered crystal chloroadamantane: a model system where dynamics are almost
completely controlled by rotations. A critical temperature T_c = 225 K as
predicted by the Mode Coupling Theory can be clearly determined both in the
alpha and beta dynamical regimes. This investigation also shows the existence
of a second remarkable dynamical crossover at the temperature T_x > T_c
consistent with a previous NMR and MD study [1]. This allows us to confirm
clearly the existence of a 'landscape-influenced' regime occurring in the
temperature range [T_c-T_x] as recently proposed [2,3].Comment: 4 pages, 5 figures, REVTEX
Onset of slow dynamics in difluorotetrachloroethane glassy crystal
Complementary Neutron Spin Echo and X-ray experiments and Molecular Dynamics
simulations have been performed on difluorotetrachloroethane (CFCl2-CFCl2)
glassy crystal. Static, single-molecule reorientational dynamics and collective
dynamics properties are investigated. The orientational disorder is
characterized at different temperatures and a change in nature of rotational
dynamics is observed. We show that dynamics can be described by some scaling
predictions of the Mode Coupling Theory (MCT) and a critical temperature
is determined. Our results also confirm the strong analogy between
molecular liquids and plastic crystals for which -relaxation times and
non-ergodicity parameters are controlled by the non trivial static correlations
as predicted by MCT
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