389 research outputs found
Local dielectric spectroscopy of near-surface glassy polymer dynamics
A non-contact scanning-probe-microscopy method was used to probe local
near-surface dielectric susceptibility and dielectric relaxation in
poly-vinyl-acetate (PVAc) near the glass transition. Dielectric spectra were
measured from 10-4 Hz to 102 Hz as a function of temperature. The measurements
probed a 20 nm thick layer below the free-surface of a bulk film. A small (4 K)
reduction in glass transition temperature and moderate narrowing of the
distribution of relaxation times was found. In contrast to results for
ultra-thin-films confined on or between metallic electrodes, no reduction in
the dielectric strength was found, inconsistent with the immobilization of
slower modes.Comment: submitte
Spectral Hardness Decay with Respect to Fluence in BATSE Gamma-Ray Bursts
We have analyzed the evolution of the spectral hardness parameter Epk as a
function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with
the trend reported by Liang & Kargatis (1996) which found that Epk decays
exponentially with respect to photon fluence. We also fit these pulses with a
slight modification of this trend, where Epk decays linearly with energy
fluence. In both cases, we found the set of 41 pulses to be consistent with the
trend. For the latter trend, which we believe to be more physical, the
distribution of the decay constant is roughly log-normal, with a mean of 1.75
+/- 0.07 and a FWHM of 1.0 +/- 0.1. Regarding an earlier reported invariance in
the decay constant among different pulses in a single burst, we found
probabilities of 0.49 to 0.84 (depending on the test used) that such invariance
would occur by coincidence, most likely due to the narrow distribution of decay
constant values among pulses.Comment: 17 pages, 7 figure pages, 2 table pages, submitted to The
Astrophysical Journa
Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts
We report evidence that the asymptotic low-energy power law slope alpha
(below the spectral break) of BATSE gamma-ray burst photon spectra evolves with
time rather than remaining constant. We find a high degree of positive
correlation exists between the time-resolved spectral break energy E_pk and
alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of
alpha was found to correlate with that of the spectral break energy E_pk at the
99.7% and 98% confidence levels respectively. We also find that in the flux
rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive
and in some bursts the maximum value of alpha is consistent with a value > +1.
BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These
findings challenge GRB spectral models in which alpha must be negative of
remain constant.Comment: 12 pages (including 6 figures), accepted to Ap
Multiwavelength Observations of GX 339-4 in 1996. I. Daily Light Curves and X-ray and Gamma-Ray Spectroscopy
As part of our multiwavelength campaign of GX 339-4 observations in 1996 we
present our radio, X-ray, and gamma-ray observations made in July, when the
source was in a hard state (= soft X-ray low state). The radio observations
were made at the time when there was a possible radio jet. We show that the
radio spectrum was flat and significantly variable, and that the radio spectral
shape and amplitude at this time were not anomalous for this source. Daily
light curves from our pointed observation July 9-23 using OSSE, from BATSE, and
from the ASM on RXTE also show that there was no significant change in the X-
and gamma-ray flux or hardness during the time the possible radio jet-like
feature was seen. The higher energy portion of our pointed RXTE observation
made July 26 can be equally well fit using simple power law times exponential
(PLE) and Sunyaev-Titarchuk (ST) functions. An additional soft component is
required, as well as a broad emission feature centered on 6.4 keV. This may be
an iron line that is broadened by orbital Doppler motions and/or scattering off
a hot medium. Its equivalent width is 600 eV. Our simplistic continuum fitting
does not require an extra reflection component. Both a PLE and a ST model also
fit our OSSE spectrum on its own. Although the observations are not quite
simultaneous, combining the RXTE and CGRO spectra we find that the PLE model
easily fits the joint spectrum. However, the ST model drops off too rapidly
with increasing energies to give an acceptable joint fit.Comment: Submitted to Astrophysical Journal. 25 pages. 11 figure
Enhanced low-energy -decay strength of Ni and its robustness within the shell model
Neutron-capture reactions on very neutron-rich nuclei are essential for
heavy-element nucleosynthesis through the rapid neutron-capture process, now
shown to take place in neutron-star merger events. For these exotic nuclei,
radiative neutron capture is extremely sensitive to their -emission
probability at very low energies. In this work, we present
measurements of the -decay strength of Ni over the wide range
MeV. A significant enhancement is found in the
-decay strength for transitions with MeV. At present,
this is the most neutron-rich nucleus displaying this feature, proving that
this phenomenon is not restricted to stable nuclei. We have performed
-strength calculations within the quasiparticle time-blocking
approximation, which describe our data above MeV very well.
Moreover, large-scale shell-model calculations indicate an nature of the
low-energy strength. This turns out to be remarkably robust with
respect to the choice of interaction, truncation and model space, and we
predict its presence in the whole isotopic chain, in particular the
neutron-rich .Comment: 9 pages, 9 figure
Coastal Upwelling Enhances Abundance of a Symbiotic Diazotroph (UCYN-A) and Its Haptophyte Host in the Arctic Ocean
The apparently obligate symbiosis between the diazotroph Candidatus Atelocyanobacterium thalassa (UCYN-A) and its haptophyte host, Braarudosphaera bigelowii, has recently been found to fix dinitrogen (N2) in polar waters at rates (per cell) comparable to those observed in the tropical/subtropical oligotrophic ocean basins. This study presents the novel observation that this symbiosis increased in abundance during a wind-driven upwelling event along the Alaskan Beaufort shelfbreak. As upwelling relaxed, the relative abundance of B. bigelowii among eukaryotic phytoplankton increased most significantly in waters over the upper slope. As the host’s nitrogen demands are believed to be supplied primarily by UCYN-A, this response suggests that upwelling may enhance N2 fixation as displaced coastal waters are advected offshore, potentially extending the duration of upwelling-induced phytoplankton blooms. Given that such events are projected to increase in intensity and number with ocean warming, upwelling-driven N2 fixation as a feedback on climate merits investigation
Emerging collectivity from the nuclear structure of Xe 132: Inelastic neutron scattering studies and shell-model calculations
Inelastic neutron scattering was used to study the low-lying nuclear structure of 132Xe. A comprehensive level scheme is presented, as well as new level lifetimes, multipole mixing ratios, branching ratios, and transition probabilities. Comparisons of these data as well as previously measured E2 strengths and g factors are made with new shell-model calculations for 132, 134, 136 Xe
to explore the emergence of collectivity in the Xe isotopes with N<82 near the closed shell.This material is based upon work supported by the US
National Science Foundation under Grant No. PHY-1606890.
This research was also sponsored in part by the Australian
Research Council under Grant No. DP17010167
Inelastic Neutron Scattering Studies of \u3csup\u3e132,134\u3c/sup\u3eXe: Elucidating Structure in a Transitional Region and Possible Interferences for 0vββ Searches
Highly enriched (\u3e 99.9%) 132Xe and 134Xe gases were converted to solid 132XeF2 and 134XeF2 and were used as scattering samples for inelastic neutron scattering measurements at the University of Kentucky Accelerator Laboratory (UKAL). Lifetimes of levels up to 3.5MeV in excitation energy in these xenon isotopes were measured using the Doppler-shift attenuation method, allowing the determination of reduced transition probabilities. Gamma rays corresponding to new transitions and levels have been observed. In particular, tentative new excited 0+ states and associated decays have been examined in an effort to elucidate the structure of these nuclei in a transitional region, and comparisons have been drawn with models which seek to describe such nuclei, e.g., the E(5) critical-point symmetry of the IBM. Newly identified potential interferences for neutrinoless double-beta decay searches involving 136Xe are also discussed
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