131 research outputs found
Event-by-event study of prompt neutrons from 239Pu(n,f)
Employing a recently developed Monte Carlo model, we study the fission of
240Pu induced by neutrons with energies from thermal to just below the
threshold for second chance fission. Current measurements of the mean number of
prompt neutrons emitted in fission, together with less accurate measurements of
the neutron energy spectra, place remarkably fine constraints on predictions of
microscopic calculations. In particular, the total excitation energy of the
nascent fragments must be specified to within 1 MeV to avoid disagreement with
measurements of the mean neutron multiplicity. The combination of the Monte
Carlo fission model with a statistical likelihood analysis also presents a
powerful tool for the evaluation of fission neutron data. Of particular
importance is the fission spectrum, which plays a key role in determining
reactor criticality. We show that our approach can be used to develop an
estimate of the fission spectrum with uncertainties several times smaller than
current experimental uncertainties for outgoing neutron energies up to 2 MeV.Comment: 17 pages, 20 figure
Proton-Rich Nuclear Statistical Equilibrium
Proton-rich material in a state of nuclear statistical equilibrium (NSE) is
one of the least studied regimes of nucleosynthesis. One reason for this is
that after hydrogen burning, stellar evolution proceeds at conditions of equal
number of neutrons and protons or at a slight degree of neutron-richness.
Proton-rich nucleosynthesis in stars tends to occur only when hydrogen-rich
material that accretes onto a white dwarf or neutron star explodes, or when
neutrino interactions in the winds from a nascent proto-neutron star or
collapsar-disk drive the matter proton-rich prior to or during the
nucleosynthesis. In this paper we solve the NSE equations for a range of
proton-rich thermodynamic conditions. We show that cold proton-rich NSE is
qualitatively different from neutron-rich NSE. Instead of being dominated by
the Fe-peak nuclei with the largest binding energy per nucleon that have a
proton to nucleon ratio close to the prescribed electron fraction, NSE for
proton-rich material near freeze-out temperature is mainly composed of Ni56 and
free protons. Previous results of nuclear reaction network calculations rely on
this non-intuitive high proton abundance, which this paper will explain. We
show how the differences and especially the large fraction of free protons
arises from the minimization of the free energy as a result of a delicate
competition between the entropy and the nuclear binding energy.Comment: 4 pages, 7 figure
The Ultraviolet flash accompanying GRBs from neutron-rich internal shocks
In the neutron-rich internal shocks model for Gamma-ray Burts (GRBs), the
Lorentz factors (LFs) of ions shells are variable, so are the LFs of
accompanying neutron shells. For slow neutron shells with a typical LF tens,
the typical beta-decay radius reads R_{\beta,s} several 10^{14} cm, which is
much larger than the typical internal shocks radius 10^{13} cm, so their impact
on the internal shocks may be unimportant. However, as GRBs last long enough
(T_{90}>20(1+z) s), one earlier but slower ejected neutron shell will be swept
successively by later ejected ion shells in the range 10^{13}-10^{15} cm, where
slow neutrons have decayed significantly. We show in this work that ion shells
interacting with the beta-decay products of slow neutron shells can power a
ultraviolet (UV) flash bright to 12th magnitude during the prompt gamma-ray
emission phase or slightly delayed, which can be detected by the upcoming
Satellite SWIFT in the near future.Comment: 6 pages (2 eps figures), accepted for publication in ApJ
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Estimate of the 9Be(n,el) Cross-section Uncertainties for the ENDL99 and ENDF/B-VII Evaluations
Uncertainties for the ENDL99 and ENDF/B-VII evaluations of the {sup 9}Be (n, el) cross section have been estimated for incident neutron energies up to 20 MeV. The uncertainties were obtained by extracting the spread of the experimental data about the evaluations, using a sophisticated procedure to ensure smoothness of the uncertainty as a function of energy. The technique used to obtain the uncertainties is described briefly in this report, and the resulting error bands are given for the two evaluations
The difficulty in using orphan afterglows to measure gamma-ray burst beaming
If gamma-ray burst (GRB) emission is strongly collimated then GRBs occur
throughout the Universe at a rate much higher than is detected. Since the
emission from the optical afterglow is thought to be more isotropic than the
gamma-ray emission, it has been hypothesized that a search for orphan
afterglows (those without the triggering GRB) would allow strong constraints to
be placed on the degree of GRB collimation. We show here that, within the
context of leading models of GRB jet evolution, measurement of the GRB beaming
angle using optical orphan searches is extremely difficult, perhaps impossible
in practice. This is because in the leading model of GRB jets, the effective
afterglow beaming angle scales with the jet angle for small angles, and so the
ratio of detected orphan afterglows to GRBs is independent of the jet opening
angle. Thus, the number of expected afterglow detections is the same for
moderate jet angles (e.g. 20 deg) as for arbitrarily small jet angles (<< 0.1
deg). For nearly isotropic GRB geometry, or for radio afterglow searches in
which the jet has become non-relativistic, the ratio of afterglows to GRBs may
give information on collimation. However, using a simple model we estimate the
expected number of orphan detections in current supernova surveys, and find
this number to be less than one, for all jet opening angles. Even for future
supernova surveys, the small detection rate and lack of dependence on
collimation angle appear to ruin the prospects of determining GRB beaming by
this method. Radio searches may provide the best hope to find the missing
orphans.Comment: 12 pages, 3 figures, submitted to Ap
Nuclear Composition of Gamma-Ray Burst Fireballs
We study three processes that shape the nuclear composition of GRB fireballs:
(1) neutronization in the central engine, (2) nucleosynthesis in the fireball
as it expands and cools, and (3) spallation of nuclei in subsequent internal
shocks. The fireballs are found to have a neutron excess and a marginally
successful nucleosynthesis. They are composed of free nucleons,
alpha-particles, and deuterium. A robust result is the survival of a
significant neutron component, which has important implications. First, as
shown in previous works, neutrons can lead to observable multi-GeV neutrino
emission. Second, as we show in an accompanying paper, neutrons impact the
explosion dynamics at radii up to 10^{17} cm and change the mechanism of the
GRB afterglow emission.Comment: 33 pages, accepted to Ap
2000 Philip C. Jessup
The State of Kuraca and the Republic of Senhava have submitted their differences concerning the vaccine trials to the International Court of Justice for resolution through a Special Agreement, in accordance with Article 40(1) of the Statute of the International Court of Justice
Implications of Neutron Decoupling in Short Gamma Ray Bursts
Roughly half of the observed gamma-ray bursts (GRBs) may arise from the
shocking of an ultra-relativistic shell of protons with the interstellar medium
(ISM). Any neutrons originally present in the GRB fireball may, depending on
the characteristics of the central engine, dynamically decouple as the fireball
accelerates. This leads to outflow consisting of separate fast proton and slow
neutron components. We derive detailed implications of neutron decoupling for
the observed lightcurves of short bursts. We show that the collision of a
neutron decayed shell with a decelerating outer shell is expected to result in
an observable second peak in the GRB lightcurve. There may be substantial
optical emission associated with such an event, so the upcoming Swift satellite
may be able to place constraints on models for short bursts. We also discuss
interesting inferences about central engine characteristics allowed by existing
BATSE data and a consideration of neutron decoupling.Comment: 12 pages, 3 figure
Nucleosynthesis in Early Supernova Winds II: The Role of Neutrinos
One of the outstanding unsolved riddles of nuclear astrophysics is the origin
of the so called ``p-process'' nuclei from A = 92 to 126. Both the lighter and
heavier p-process nuclei are adequately produced in the neon and oxygen shells
of ordinary Type II supernovae, but the origin of these intermediate isotopes,
especially 92,94Mo and 96,98Ru, has long been mysterious. Here we explore the
production of these nuclei in the neutrino-driven wind from a young neutron
star. We consider such early times that the wind still contains a proton excess
because the rates for electron neutrino and positron captures on neutrons are
faster than those for the inverse captures on protons. Following a suggestion
by Frohlich et al. 2005, we also include the possibility that, in addition to
the protons, alpha-particles, and heavy seed, a small flux of neutrons is
maintained by the reaction p(bar(nu_e),e+)n. This flux of neutrons is critical
in bridging the long waiting points along the path of the rp-process by (n,p)
and (n,gamma) reactions. Using the unmodified ejecta histories from a recent
two-dimensional supernova model by Janka et al. 2003, we find synthesis of
p-rich nuclei up to 102Pd. However, if the entropy of these ejecta is increased
by a factor of two, the synthesis extends to 120Te. Still larger increases in
entropy, that might reflect the role of magnetic fields or vibrational energy
input neglected in the hydrodynamical model, result in the production of
numerous r-, s-, and p-process nuclei up to A approximately 170, even in winds
that are proton-rich
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