343 research outputs found
Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts
The blast-wave model for gamma-ray bursts (GRBs) has been called into
question by observations of spectra from GRBs that are harder than can be
produced through optically thin synchrotron emission. If GRBs originate from
the collapse of massive stars, then circumstellar clouds near burst sources
will be illuminated by intense gamma radiation, and the electrons in these
clouds will be rapidly scattered to energies as large as several hundred keV.
Low-energy photons that subsequently pass through the hot plasma will be
scattered to higher energies, hardening the intrisic spectrum. This effect
resolves the "line-of-death" objection to the synchrotron shock model.
Illuminated clouds near GRBs will form relativistic plasmas containing large
numbers of electron-positron pairs that can be detected within ~ 1-2 days of
the explosion before expanding and dissipating. Localized regions of pair
annihilation radiation in the Galaxy would reveal past GRB explosions.Comment: 9 pages, 1 figure, submitted to ApJ Letter
The electric dipole response of Se above 4 MeV
The dipole response of Se in the energy range 4 to 9 MeV has been
analyzed using a polarized photon scattering
technique, performed at the High Intensity -Ray Source facility, to
complement previous work performed using unpolarized photons. The results of
this work offer both an enhanced sensitivity scan of the dipole response and an
unambiguous determination of the parities of the observed J=1 states. The
dipole response is found to be dominated by excitations, and can
reasonably be attributed to a pygmy dipole resonance. Evidence is presented to
suggest that a significant amount of directly unobserved excitation strength is
present in the region, due to unobserved branching transitions in the decays of
resonantly excited states. The dipole response of the region is underestimated
when considering only ground state decay branches. We investigate the electric
dipole response theoretically, performing calculations in a 3D cartesian-basis
time-dependent Skyrme-Hartree-Fock framework.Comment: 20 pages, 18 figures, to be submitted to PR
Pneumonia in Bighorn Sheep: Testing the Super-Spreader Hypothesis
Following introduction of pneumonia, disease can persist in bighorn sheep (Ovis canadensis) populations for decades as annual or sporadic pneumonia epidemics in lambs. Recurring years of depressed recruitment due to high rates of pneumonia-induced mortality in juveniles is a major obstacle to population recovery. Management strategies for resolving this problem have so far been elusive. We are investigating the feasibility of removing individual âsuper-spreadersâ to improve lamb survival. Individual variation in infection and transmission is well documented in human diseases (e.g. âTyphoid Maryâ). We are testing the hypothesis that pneumonia epidemics in lambs are initiated by transmission of pathogens from a few âchronic-shedderâ ewes. We have completed the first year of a 5-year project in the Hells Canyon region of Idaho, Oregon, and Washington, and in a captive population at South Dakota State University. Through repeated testing of free-ranging individuals in Hells Canyon, we have identified individual differences in shedding of Mycoplasma ovipneumoniae, a primary pathogen in the bighorn sheep respiratory disease complex. We also found that when penned separately in captivity, lambs of ewes that consistently tested positive (chronic shedders) were infected and died of pneumonia, whereas lambs born to ewes from an infected population that tested negative (non-shedders), were not infected and survived. Over the next 4 years we plan to 1) continue and expand testing of free-ranging and captive animals, 2) determine whether removal of chronic-shedder ewes improves lamb survival in free-ranging populations, 3) expand and replicate chronic-shedder commingling experiments in captivity, and 4) establish and monitor a new population founded with non-shedders from an infected population
Swift multi-wavelength observations of the bright flaring burst GRB051117A
We report on the temporal and spectral characteristics of the early X-ray
emission from the Gamma Ray Burst 051117A as observed by Swift. The superb
quality of the early X-ray light-curve and spectra of this source, one of the
brightest seen by the X-ray Telescope at such early times, allows an
unprecedented look at the spectral and temporal evolution of the prompt and
early afterglow emission for this GRB and allows us to place stringent limits
on the detection of lines. The X-ray light-curve at early times is
characteristic of a shot-noise process, with individual shots well-modelled by
a fast-rise and exponential decay spanning a broad range in rise-times and
decay rates. A temporal spectral analysis of the early light-curve shows that
the photon index and source intensity are highly correlated with the spectrum
being significantly harder when brighter, consistent with the movement of the
peak of the Band function to lower energies following individual flares. The
high quality spectrum obtained from the first orbit of WT mode data, enables us
to place a 3 sigma upper limit on the strength of any emission line features of
EW < 15 eV, assuming a narrow emission-line of 100 eV at the peak of the
effective area (abridged).Comment: Accepted 15/3/2007 - To appear in A&
Collectivity of 0\u3csup\u3e+\u3c/sup\u3e States in \u3csup\u3e160\u3c/sup\u3eGd
Excited 0+ states in 160Gd have been examined with the (n,nâČÎł) reaction at incident neutron energies up to 2.8 MeV. Gamma-ray excitation functions and angular distribution measurements allow the confirmation of the existence of 0+ states at 1379.70 keV and 1558.30 keV, but we reject the assignments of additional previously suggested 0+ candidates. Limits on the level lifetimes of the observed 0+ states permit an evaluation of the collectivity of these states
Probing the low-lying level structure of 94Zr through ÎČÂŻ decay
223-227Low-lying states of 94Zr are populated following b- decay of 94Y, and the emitted g rays from 94Zr are detected using the 8p spectrometer composed of 20 Compton-suppressed HPGe detectors. High- statistics coincidence data have been used for the placement of very weak decay branches in the level scheme. Combining the results of level lifetimes from a previous experiment and the precisely measured branching ratio values of the weak decay branches from the present experiment, it is possible to extract the B(E2) values for all the possible decay branches from a given level. These values are helpful for proper identification of the collective and non-collective states of 94Zr. The experimental findings have been compared with predictions from shell-model calculations with a limited valence space; however, these calculations are inadequate in reproducing all of the measured spectroscopic quantities
ÎČ-decay Half-lives of Neutron-rich Nuclides in the A = 100 â 110 Mass Region
ÎČ-decay half-lives of neutron-rich nuclides in the A = 100â110 mass region have been measured using an implantation station installed inside of the Summing NaI(Tl) (SuN) detector at the National Superconducting Cyclotron Laboratory. Accurate half-lives for these nuclides are important for nuclear astrophysics, nuclear structure, and nuclear technology. The half-lives from the present work are compared with previous measurements, showing overall good agreement
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Estimation of (\u3cem\u3en, f\u3c/em\u3e) Cross Sections by Measuring Reaction Probability Ratios
Neutron-induced reaction cross sections on unstable nuclei are inherently difficult to measure due to target activity and the low intensity of neutron beams. In an alternative approach, named the âsurrogateâ technique, one measures the decay probability of the same compound nucleus produced using a stable beam on a stable target to estimate the neutron-induced reaction cross section. As an extension of the surrogate method, in this paper we introduce a new technique of measuring the fission probabilities of two different compound nuclei as a ratio, which has the advantage of removing most of the systematic uncertainties. This method was benchmarked in this report by measuring the probability of deuteron-induced fission events in coincidence with protons, and forming the ratio P[236U(d,pf)]/P [238U(d,pf)], which serves as a surrogate for the known cross section ratio of 236U(n, f)/238U(n, f). In addition, the P[238U(d, d f)]/P [236U(d, df)] ratio as a surrogate for the 237U(n, f)/235U(n, f) cross section ratio was measured for the first time in an unprecedented range of excitation energies
Estimation of (\u3cem\u3en, f\u3c/em\u3e) Cross Sections by Measuring Reaction Probability Ratios
Neutron-induced reaction cross sections on unstable nuclei are inherently difficult to measure due to target activity and the low intensity of neutron beams. In an alternative approach, named the âsurrogateâ technique, one measures the decay probability of the same compound nucleus produced using a stable beam on a stable target to estimate the neutron-induced reaction cross section. As an extension of the surrogate method, in this paper we introduce a new technique of measuring the fission probabilities of two different compound nuclei as a ratio, which has the advantage of removing most of the systematic uncertainties. This method was benchmarked in this report by measuring the probability of deuteron-induced fission events in coincidence with protons, and forming the ratio P[236U(d,pf)]/P [238U(d,pf)], which serves as a surrogate for the known cross section ratio of 236U(n, f)/238U(n, f). In addition, the P[238U(d, d f)]/P [236U(d, df)] ratio as a surrogate for the 237U(n, f)/235U(n, f) cross section ratio was measured for the first time in an unprecedented range of excitation energies
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