331 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
Measurement of Conversion Coefficients in Normal and Triaxial Strongly Deformed Bands in \u3csup\u3e167\u3c/sup\u3eLu
Internal conversion coefficients have been measured for transitions in both normal deformed and triaxial strongly deformed bands in 167Lu using the Gammasphere and ICE Ball spectrometers. The results for all in-band transitions are consistent with E2 multipolarity. Upper limits are determined for the internal conversion coefficients for linking transitions between TSD Band 2 and TSD Band 1, the nw = 1 and nw = 0 wobbling bands, respectively
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
Establishing the Maximum Collectivity in Highly Deformed N=Z Nuclei
The lifetimes of the first excited 2^{+} states in the N=Z nuclei ^{80}Zr, ^{78}Y, and ^{76}Sr have been measured using the Îł-ray line shape method following population via nucleon-knockout reactions from intermediate-energy rare-isotope beams. The extracted reduced electromagnetic transition strengths yield new information on where the collectivity is maximized and provide evidence for a significant, and as yet unexplained, odd-odd vs even-even staggering in the observed values. The experimental results are analyzed in the context of state-of-the-art nuclear density-functional model calculations
Trimetric Imaging of the Martian Ionosphere Using a CubeSat Constellation
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143020/1/6.2017-5252.pd
Spectroscopy of proton-rich 79Zr : Mirror energy differences in the highly-deformed fpg shell
Energy differences between isobaric analogue states have been extracted for the A=79, 79Zr/79Y mirror pair following their population via nucleon-knockout reactions from intermediate-energy rare-isotope beams. These are the heaviest nuclei where such measurements have been made to date. The deduced mirror energy differences (MED) are compared with predictions from a new density-functional based approach, incorporating isospin-breaking effects of both Coulomb and nuclear charge-symmetry breaking and configuration mixing
Probing the role of proton cross-shell excitations in Ni 70 using nucleon knockout reactions
The neutron-rich Ni isotopes have attracted attention in recent years because of the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in Ni70 following Îł-ray tagged one-proton, one-neutron, and two-proton knockout from Cu71, Ni71, and Zn72 rare-isotope beams, respectively. Using variations observed in the relative transition intensities, signaling the changed population of specific final states in the different reactions, the role of neutron and proton configurations in excited states of Ni70 is probed schematically, with the goal of identifying those that carry, as leading configuration, proton excitations across the Z=28 shell closure. Such states are suggested in the literature to form a collective structure associated with prolate deformation. Adding to the body of knowledge for Ni70, 29 new transitions are reported, of which 15 are placed in its level scheme
Neurology
Contains reports on eight research projects.U.S. Navy (Office of Naval Research (Nonr-1841(70))U. S. Public Health Service (MH-06175-02)U. S. Air Force (AF49(638)-1313)U. S. Public Health Service (B-3055-4)U. S. Public Health Service (B-3090-4
Spectral and timing properties of a dissipative GRB photosphere
We explore the observational appearance of the photosphere of an
ultrarelativistic flow with internal dissipation of energy as predicted by the
magnetic reconnection model. Previous study of the radiative transfer in the
photospheric region has shown that gradual dissipation of energy results in a
hot photosphere. There, inverse Compton scattering of the thermal radiation
advected with the flow leads to powerful photospheric emission with spectral
properties close to those of the observed prompt GRB emission. Here, we build
on that study by calculating the spectra for a large range of the
characteristics of the flow. An accurate fitting formula is given that provides
the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy
range (in the central engine frame) as a function of the basic physical
parameters of the flow. It facilitates the direct comparison of the model
predictions with observations, including the variability properties of the
lightcurves. We verify that the model naturally accounts for the observed
clustering in peak energies of the E*f(E) spectrum. In this model, the Amati
relation indicates a tendency for the most luminous bursts to have more energy
per baryon. If this tendency also holds for individual GRB pulses, the model
predicts the observed narrowing of the width of pulses with increasing photon
energy.Comment: 10 pages, 6 figures, minor changes, A&A, in pres
Dipole Strength Distributions from HIGS Experiments
A series of photon scattering experiments has been performed on the double-beta decay partners 76Ge and 76Se, in order to investigate their dipole response up to the neutron separation threshold. Gamma-ray beams from bremsstrahlung at the S-DALINAC and from Compton-backscattering at HIGS have been used to measure absolute cross sections and parities of dipole excited states, respectively. The HIGS data allows for indirect measurement of averaged branching ratios, which leads to significant corrections in the observed excitation cross sections. Results are compared to statistical calculations, to test photon strength functions and the Axel-Brink hypothesi
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