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RP PROCESS NUCLEOSYNTHESIS

The rp-process (rapid proton capture process) consists of consecutive proton
captures onto seed nuclei to produce heavier elements. It is a nucleosynthesis.


RP PROCESS NUCLEOSYNTHESIS

Contents:
The r-, s-, and p-Processes in Nucleosynthesis - B.S. Meyer Looking for other
ways to read this? Stellar nucleosynthesis


Hungarian Academy of Sciences. Fingerprint nuclides. Van de Graaff accelerators.
Physical Review C - Nuclear Physics , 64 6 , Physical Review C - Nuclear
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The FAIR project is being realized in international collaboration. International
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Overview on FAIR. Research at FAIR. Busso, R. The r process, which is
responsible for the production of about half of the heavy-element abundances
including Th and U, is characterized by enormous neutron densities of to cm -3
and time scales of a few seconds. These conditions clearly point to an explosive
scenario, e. Fission can have an important influence on the termination of the
r-process and on the abundances of long-lived actinides, which are relevant for
determining the age of the Universe. In order to quantitatively understand the
role of fission in the r process, two important pieces of information are
needed: the fission-barrier heights and mass- and charge-distributions of the
fission fragments.

Since experimental information is only available for nuclei in a limited region
of the nuclide chart, the heavy r-process nuclei have to be described by
theoretical predictions. Recently, important progress has been made in
developing full microscopic approaches to nuclear fission. Nevertheless, due to
the complexity of the problem, this type of calculations is still difficult to
apply to heavy nuclei and the accuracy of these models is rather limited.

At GSI a macroscopic-microscopic approach was used to investigate the fission
contribution to the r process. A model for calculating mass- and
charge-distributions of fission fragments that can correctly predict the
transition from double-humped to single-humped distributions with decreasing
mass of the fissioning system and increasing excitation energy in the light
actinides was developed and has been used [1] to calculate fission-fragment
distributions in neutrino-induced fission of r-process nuclei.

There are 32 proton rich nuclei between Se and Hg, which can neither be produced
by the s process nor by the r process. It detected more than thermonuclear X-ray
bursts with a high throughput Galloway et al. The down selection from more than
to 37 was made in two steps: 1 requiring that the decay, as judged by eye, is
smooth; and 2 requiring that the covered dynamic range in flux is wide more than
about This naturally selects hydrogen-poor bursts and UCXBs.

The most important result of these authors is that the decay in photon count
rate and energy flux can best be modeled by a simple power law in contrast to
the commonly employed exponential decay function , and that the power-law decay
index for the energy flux is on average 1. The most common index of 1. Each
burst is consistent with a single power law, which is at odds with the theory
presented in this study. This study is a follow-up of this light curve study of
37 bursts, in which we do not make a selection based on a smooth decay.

Therefore, it includes many hydrogen-rich bursts. This 1 provides us with a data
set to study the rp process, which because of the waiting points prolongs into
the cooling phase; and 2 allows us to study the cooling over many more bursts
than 37 after we separate its contribution in the burst decay from that of the
rp process. The data set presented by the PCA is the basis of our study. It is
the best data set available on X-ray bursts because it is large and provides a
wide dynamic range in photon count rates. In Sects. In Sect.


THE R-, S-, AND P-PROCESSES IN NUCLEOSYNTHESIS - B.S. MEYER

Each PCU had two proportional counter chambers on top of each other: a top
propane layer, and a bottom xenon layer. The xenon layer is the main instrument.
The propane layer was used as an anticoincidence counter, although it did
occasionally provide scientific value since it extended the bandpass to somewhat
lower energies e. Observations were made with various combinations of PCUs. In
general, the average number of active PCUs decreased from five early in the
mission to one at the end.

The center PCU number 2, counting from 0 was almost always operational.

For event rates typical for type I bursts 10 4 s -1 , the live time fraction of
the PCA was affected by a few percent. The PCA could be simultaneously read out
by six event analyzers EAs that could be programmed in any of seven basic
read-out modes. One EA always employed the standard-1 mode, yielding photon
count rates at 0. Another often-used mode is the good-xenon mode, which provided
0.

It was only useful for faint bursts because it more easily overflowed the
telemetry than the science event mode. We employed the science event mode
because we required an energy resolution that was capable of determining the
bolometric flux, and in incidental cases we used the good-xenon mode. Some
sources only exhibited one burst in the PCA e.

Table 1 lists the burst counts per source column a. For a broad perspective of
the burst sample and an easy comparison with burst parametrizations elsewhere,
we show in Fig. We included all bad fits since we are only interested in a
general picture of timescales and did not draw any further conclusions from
these numbers. The average is The employed light curves have 1 s time resolution
and concern all RXTE-PCA bursts, except for bursts with insufficient data
coverage or almost unconstrained decay times.

Bursts are often incompletely covered or have signal-to-noise ratios that are
too low on the peak fluxes to meaningfully study the decay i. The number of
bursts left per source is specified in Table 1 column b. Much of the decrease in
the sample is due to the many faint bursts on top of a bright persistent flux
from IGR J e. We note that our analysis is biased toward bursts from 4U , 4U ,
GS , and Aql X-1 because they have the most bursts and the highest
signal-to-noise ratio.

Spectra were extracted in fine enough time bins from 1 s early on in the burst
to typically 16 s at the end of the burst up to mostly s after the burst start
time. This s time limit is longer than employed in Galloway et al. Furthermore,
we extracted for each burst a pre-burst spectrum from data taken in the same EA
read-out mode and between and 16 s before the burst start, except for a few tens
of cases when the data start later than s before the burst start, but before the
16 s mark.

Corrections were applied for the instrument dead time following the prescription
from the instrument team 1. The extracted spectra encompass all emission within
the field of view and are expected to contain the following components: burst
radiation, cosmic X-ray background, particle-induced background, emission from
other sources in the field of view, and the non-burst flux from the burst source
itself due to the accretion process. The pre-burst spectrum is considered as one
combined measurement of all the components except for the burst radiation. We
modeled it through a disk blackbody Mitsuda et al.

Each burst spectrum was modeled by a combination of this pre-burst model and a
Planck function for the burst radiation with effective temperature and
normalization as free parameters.


LOOKING FOR OTHER WAYS TO READ THIS?

It has recently been found that the non-burst accretion flux changes roughly in
tandem with the burst flux. Worpel et al. When the burst decays, f a generally
decreases to approximately unity. Support for scattering and cooling of the
corona comes also from measurements at photon energies above 30 keV e. The
latter constitutes the traditional method of modeling burst spectra and assumes
that the accretion emission is unaffected by the bursts. We find that the
population-wide perspective of the results is the same, but results on
individual sources may differ somewhat.

We applied the f a method in a somewhat different manner than Worpel et al.
First, we applied a longer exposure time for the pre-burst spectrum of usually
84 s versus 16 s by Worpel et al. Second, we fit the same spectral model to all
pre-burst spectra, while Worpel et al. We stress that we employed this model
purely empirically and ignored any physical interpretation of it. Third, we did
not distinguish between non-burst emission from the burst source and other
contributors within the field of view when determining f a , in contrast to
Worpel et al. We fit the burst spectra with a combination of the pre-burst model
disk blackbody and power law , fixing its parameters to the pre-burst values and
multiplying it with a free constant equal to f a , and a Planck function for the
burst radiation with two free parameters normalization and temperature.

Stellar Nucleosynthesis Explained in 4 Minutes


A problem with the f a model is see also Worpel et al. Consequently, there is a
coupling between the normalizations of the two components that introduces
additional uncertainty in the luminosity of the blackbody component up to tens
of percents according to Worpel et al. This decreases the diagnostic power of
the light-curve analysis and increases the uncertainties of the parameters. For
two-thirds of these spectra, the source was significantly detected in the sense
that the flux was at least three times higher than its uncertainty.

This percentage is 20 times larger than expected for purely statistical
fluctuations. From the fitted blackbody parameters, we calculated the bolometric
flux. We note that the bolometric correction, applicable from the 3—20 keV band,
is between 1.

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Our approach to the spectral modeling is, except for the free f a , traditional
and effective. Nevertheless, there are some caveats. Foremost, the effective
temperature and normalization derived from the blackbody fit are mere proxies
for the true effective temperature and emission size. Neutron star surfaces are
not blackbodies, but atmospheres that are nearly completely ionized and where
particularly the hot electrons Comptonize the radiation from below. Furthermore,
the atmosphere may contain an increased level of metals that influence the
blackbody spectrum through free-free and bound-free electron-ion interactions.

Radiation transfer calculations of model atmospheres London et al. If the
bolometric flux were unaffected, would be 1. We consider this a systematic
uncertainty in our cooling power-law decay indices that is due to the blackbody
model. While leaving f a free improves spectral fits considerably, it is
expected that not only the normalization but also the shape of the spectrum of
the accretion-induced radiation should change during a burst. If the accretion
disk corona is irradiated by the burst photons, its temperature may adjust to
the typical temperature of these photons and its spectral shape will change.

If the Poynting-Robertson effect is strong, the changing accretion rate is
expected to result in variable spectral shapes. This implies that it is
difficult to separate the accretion- and burst-induced spectra from each other,
given the limited bandpass and the similar shapes.


STELLAR NUCLEOSYNTHESIS

We assumed that the burst emission is isotropic. If any anisotropy exists and
changes during the burst, this may affect the light curves. However, the few
percent anisotropies suggested from burst oscillation measurements e. We
neglected any absorption features in the burst spectra that might change in
amplitude because the ionization degree of the photosphere changes as a result
of cooling.

Such features have been detected in a few particularly powerful bursts e. We
assumed that the spin of the neutron star has no noticeable effect on the
luminosity, either through rotational broadening of spectra by special and
general relativistic effects, from an oblateness of the neutron star surface or
because burning is confined to changing areas on the neutron star.




HINTS FOR WRITING A PERSUASIVE ESSAY

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Hints On Writing Persuasive Essays About Books Learn How to Write a Persuasive
Essay 7 Quick Tips for Writing a Great Persuasive Essay


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to produce extremely proton-rich nuclei via hot-CNO burning and the ap- and
rp-processes. Neutron-induced nucleosynthesis may occur in the neutrino-wind
driven shock front of supernova explosions initiating the r-process and
producing .