3,443 research outputs found
The Importance of Parity-Dependence of the Nuclear Level Density in the Prediction of Astrophysical Reaction Rates
A simple description for obtaining the parity distribution of nuclear levels
in the pf + g9/2 shell as a function of excitation energy was recently derived.
We implement this in a global nuclear level density model. In the framework of
the statistical model, cross sections and astrophysical reaction rates are
calculated in the Fe region and compared to rates obtained with the common
assumption of an equal distribution of parities. We find considerable
differences, especially for reactions involving particles in the exit channel.Comment: 4 pages, to appear in the proceedings of CGS11 (Prague), World
Scientifi
Measurement of (α,n) reaction cross sections of erbium isotopes for testing astrophysical rate predictions
Date of Acceptance: 30/01/2015The γ-process in core-collapse and/or type Ia supernova explosions is thought to explain the origin of the majority of the so-called p nuclei (the 35 proton-rich isotopes between Se and Hg). Reaction rates for γ-process reaction network studies have to be predicted using Hauser-Feshbach statistical model calculations. Recent investigations have shown problems in the prediction of α-widths at astrophysical energies which are an essential input for the statistical model. It has an impact on the reliability of abundance predictions in the upper mass range of the p nuclei. With the measurement of the 164,166Er(α,n)167,169Yb reaction cross sections at energies close to the astrophysically relevant energy range we tested the recently suggested low energy modification of the α+nucleus optical potential in a mass region where γ-process calculations exhibit an underproduction of the p nuclei. Using the same optical potential for the α-width which was derived from combined 162Er(α,n) and 162Er(α,γ) measurement makes it plausible that a low-energy modification of the optical α+nucleus potential is needed.Peer reviewedFinal Accepted Versio
Coulomb suppression of the stellar enhancement factor
It is commonly assumed that reaction measurements for astrophysics should be
preferably performed in the direction of positive Q value to minimize the
impact of the stellar enhancement factor, i.e. the difference between the
laboratory rate and the actual stellar rate. We show that the stellar effects
can be minimized in the charged particle channel, even when the reaction Q
value is negative. As a demonstration, the cross section of the astrophysically
relevant 85Rb(p,n)85Sr reaction has been measured by activation between 2.16 <
Ec.m. < 3.96 MeV and the astrophysical reaction rate for (p,n) as well as (n,p)
is directly inferred from the data. The presented arguments are also relevant
for other alpha and proton-induced reactions in the p and rp processes.
Additionally, our results confirm a previously derived modification of a global
optical proton potential.Comment: submitted to PR
70Ge(p,gamma)71As and 76Ge(p,n)76As cross sections for the astrophysical p process: sensitivity of the optical proton potential at low energies
The cross sections of the 70Ge(p,gamma)71As and 76Ge(p,n)76As reactions have
been measured with the activation method in the Gamow window for the
astrophysical p process. The experiments were carried out at the Van de Graaff
and cyclotron accelerators of ATOMKI. The cross sections have been derived by
measuring the decay gamma-radiation of the reaction products. The results are
compared to the predictions of Hauser-Feshbach statistical model calculations
using the code NON-SMOKER. Good agreement between theoretical and experimental
S factors is found. Based on the new data, modifications of the optical
potential used for low-energy protons are discussed.Comment: Accepted for publication in Phys. Rev.
Abundance Uncertainties Obtained With the PizBuin Framework For Monte Carlo Reaction Rate Variations
Uncertainties in nucleosynthesis models originating from uncertainties in
astrophysical reaction rates were estimated in a Monte Carlo variation
procedure. Thousands of rates were simultaneously varied within individual,
temperature-dependent errors to calculate their combined effect on final
abundances. After a presentation of the method, results from application to
three different nucleosynthesis processes are shown: the -process and
the s-process in massive stars, and the main s-process in AGB stars
(preliminary results). Thermal excitation of nuclei in the stellar plasma and
the combined action of several reactions increase the final uncertainties above
the level of the experimental errors. The total uncertainty, on the other hand,
remains within a factor of two even in processes involving a large number of
unmeasured rates, with some notable exceptions for nuclides whose production is
spread over several stellar layers and for s-process branchings.Comment: 8 pages, 4 figures; Proceedings of OMEG 2017, Daejeon, Korea, June
27-30, 2017; to appear in AIP Conf. Pro
Alpha-induced reactions for the astrophysical p-process: the case of 151Eu
The cross sections of 151Eu(alpha,gamma)155Tb and 151Eu(alpha,n)154Tb
reactions have been measured with the activation method. Some aspects of the
measurement are presented here to illustrate the requirements of experimental
techniques needed to obtain nuclear data for the astrophysical p-process
nucleosynthesis. Preliminary cross section results are also presented and
compared with the predictions of statistical model calculations.Comment: Accepted for publication in Journal of Physics Conference Series,
proceeding of the Nuclear Physics in Astrophysics IV. conferenc
Parity-Dependence in the Nuclear Level Density
Astrophysical reaction rates are sensitive to the parity distribution at low
excitation energies. We combine a formula for the energy-dependent parity
distribution with a microscopic-macroscopic nuclear level density. This
approach describes well the transition from low excitation energies, where a
single parity dominates, to high excitations where the two densities are equal.Comment: 4 pages, 3 figures; contribution to Nuclei In The Cosmos VIII, to
appear in Nucl. Phys.
Alpha-induced cross sections of 106Cd for the astrophysical p-process
The 106Cd(alpha,gamma)110Sn reaction cross section has been measured in the
energy range of the Gamow window for the astrophysical p-process scenario. The
cross sections for 106Cd(alpha,n)109Sn and for 106Cd(alpha,p)109In below the
(alpha,n) threshold have also been determined. The results are compared with
predictions of the statistical model code NON-SMOKER using different input
parameters. The comparison shows that a discrepancy for 106Cd(alpha,gamma)110Sn
when using the standard optical potentials can be removed with a different
alpha+106Cd potential. Some astrophysical implications are discussed.Comment: 10 pages, 9 figures, accepted for publication in Phys. Rev
Alpha Clustering and the stellar nucleosynthesis of carbon
The astrophysical S--factor and reaction rates for the triple--alpha process
are calculated in the direct--capture model. It is shown that the stellar
carbon production is extremely sensitive to small variations in the N--N
interaction.Comment: 2 pages LaTe
The Path to Improved Reaction Rates for Astrophysics
This review focuses on nuclear reactions in astrophysics and, more
specifically, on reactions with light ions (nucleons and alpha particles)
proceeding via the strong interaction. It is intended to present the basic
definitions essential for studies in nuclear astrophysics, to point out the
differences between nuclear reactions taking place in stars and in a
terrestrial laboratory, and to illustrate some of the challenges to be faced in
theoretical and experimental studies of those reactions. The discussion
revolves around the relevant quantities for astrophysics, which are the
astrophysical reaction rates. The sensitivity of the reaction rates to the
uncertainties in the prediction of various nuclear properties is explored and
some guidelines for experimentalists are also provided.Comment: 100 pages, 33 figures, 1 table; accepted for publication in Int. J.
Mod. Phys. E (scheduled for February 2011 issue); the formatting here differs
in that it includes a table of contents and numbered paragraphs
5.4.2.1-5.4.2.10; v2: updated references; v3: typos fixed; v4: final typo
fix, content similar to published version
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