12 research outputs found
LOW ENERGY MEASUREMENT OF THE 96ZR(α, N) 99MO REACTION CROSS SECTION AND ITS IMPACT ON WEAK R-PROCESS NUCLEOSYNTHESIS
Cross section measurement of the astrophysically important 17O(p,gamma)18F reaction in a wide energy range
The 17O(p,g)18F reaction plays an important role in hydrogen burning
processes in different stages of stellar evolution. The rate of this reaction
must therefore be known with high accuracy in order to provide the necessary
input for astrophysical models.
The cross section of 17O(p,g)18F is characterized by a complicated resonance
structure at low energies. Experimental data, however, is scarce in a wide
energy range which increases the uncertainty of the low energy extrapolations.
The purpose of the present work is therefore to provide consistent and precise
cross section values in a wide energy range.
The cross section is measured using the activation method which provides
directly the total cross section. With this technique some typical systematic
uncertainties encountered in in-beam gamma-spectroscopy experiments can be
avoided.
The cross section was measured between 500 keV and 1.8 MeV proton energies
with a total uncertainty of typically 10%. The results are compared with
earlier measurements and it is found that the gross features of the 17O(p,g)18F
excitation function is relatively well reproduced by the present data.
Deviation of roughly a factor of 1.5 is found in the case of the total cross
section when compared with the only one high energy dataset. At the lowest
measured energy our result is in agreement with two recent datasets within one
standard deviation and deviates by roughly two standard deviations from a third
one. An R-matrix analysis of the present and previous data strengthen the
reliability of the extrapolated zero energy astrophysical S-factor.
Using an independent experimental technique, the literature cross section
data of 17O(p,g)18F is confirmed in the energy region of the resonances while
lower direct capture cross section is recommended at higher energies. The
present dataset provides a constraint for the theoretical cross sections.Comment: Accepted for publication in Phys. Rev. C. Abstract shortened in order
to comply with arxiv rule
Measurement of the 91 Zr(p, Îł ) 92 m Nb cross section motivated by type Ia supernova nucleosynthesis
Abstract: The synthesis of heavy, proton rich isotopes is a poorly understood astrophysical process. Thermonuclear (type Ia) supernova explosions are among the suggested sites and the abundance of some isotopes present in the early Solar System may be used to test the models. 92Nb is such an isotope and one of the reactions playing a role in its synthesis is 91Zr(p,Îł)92Nb. As no experimental cross sections were available for this reaction so far, nucleosynthesis models had to solely rely on theoretical calculations. In the present work the cross section of 91Zr(p,Îł)92m Nb has been measured at astrophysical energies by activation. The results excellently confirm the predictions of cross sections and reaction rates for 91Zr(p,Îł)92Nb, as used in astrophysical simulations.Peer reviewe
Resonance strengths in the 14N(p,Îł)15O astrophysical key reaction measured with activation
The 14N(p,gamma)15O reaction plays a vital role in various astrophysical
scenarios. Its reaction rate must be accurately known in the present era of
high precision astrophysics. The cross section of the reaction is often
measured relative to a low energy resonance, the strength of which must
therefore be determined precisely. The activation method, based on the
measurement of 15O decay, has not been used in modern measurements of the
14N(p,gamma)15O reaction. The aim of the present work is to provide strength
data for two resonances in the 14N(p,gamma)15O reaction using the activation
method. The obtained values are largely independent from previous data measured
by in-beam gamma-spectroscopy and are free from some of their systematic
uncertainties. Solid state TiN targets were irradiated with a proton beam
provided by the Tandetron accelerator of Atomki using a cyclic activation. The
decay of the produced 15O isotopes was measured by detecting the 511 keV
positron annihilation gamma-rays. The strength of the Ep = 278 keV resonance
was measured to be 13.4 +- 0.8 meV while for the Ep = 1058 keV resonance the
strength is 442 +- 27 meV. The obtained Ep = 278 keV resonance strength is in
fair agreement with the values recommended by two recent works. On the other
hand, the Ep = 1058 keV resonance strength is about 20% higher than the
previous value. The discrepancy may be caused in part by a previously neglected
finite target thickness correction. As only the low energy resonance is used as
a normalization point for cross section measurements, the calculated
astrophysical reaction rate of the 14N(p,gamma)15O reaction and therefore the
astrophysical consequences are not changed by the present results.Comment: Accepted for publication in Phys. Rev.
Cross section of -induced reactions on Au at sub-Coulomb energies
Statistical model calculations have to be used for the determination of
reaction rates in large-scale reaction networks for heavy-element
nucleosynthesis. A basic ingredient of such a calculation is the a-nucleus
optical model potential. Several different parameter sets are available in
literature, but their predictions of a-induced reaction rates vary widely,
sometimes even exceeding one order of magnitude.
This paper presents the result of a-induced reaction cross-section
measurements on gold which could be carried out for the first time very close
to the astrophysically relevant energy region. The new experimental data are
used to test statistical model predictions and to constrain the a-nucleus
optical model potential.
For the measurements the activation technique was used. The cross section of
the (a,n) and (a,2n) reactions was determined from g-ray counting, while that
of the radiative capture was determined via X-ray counting.
The cross section of the reactions was measured below E~MeV. In the
case of the Au(a,2n)Tl reaction down to 17.5~MeV with 0.5-MeV
steps, reaching closer to the reaction threshold than ever before. The cross
section of Au(a,n)Tl and Au(a,g)Tl was measured
down to E and 14.0~MeV, respectively, with 0.5-MeV steps above the
(a,2n) reaction threshold and with 1.0-MeV steps below that.
The new dataset is in agreement with the available values from the
literature, but is more precise and extends towards lower energies. Two orders
of magnitude lower cross sections were successfully measured than in previous
experiments which used g-ray counting only, thus providing experimental data at
lower energies than ever before. The new precision dataset allows us to find
the best-fit a-nucleus optical model potential and to predict cross sections in
the Gamow window with smaller uncertainties.Comment: Accepted for publication in Phys. Rev.
Low Energy measurement of the reaction cross section and its impact on weak r-process nucleosynthesis
Lighter heavy elements beyond iron and up to around silver can form in
neutrino-driven ejecta in core-collapse supernovae and neutron star mergers.
Slightly neutron-rich conditions favour a weak r-process that follows a path
close to stability. Therefore, the beta decays are slow compared to the
expansion time scales, and (,n) reactions become critical to move
matter towards heavier nuclei. The rates of these reactions are calculated with
the statistical model and their main uncertainty, at energies relevant for the
weak r-process, is the +nucleus optical potential. There are several
sets of parameters to calculate the +nucleus optical potential leading
to large deviations for the reaction rates, exceeding even one order of
magnitude. Recently the Zr(,n)Mo reaction has been
identified as a key reaction that impacts the production of elements from Ru to
Cd. Here, we present the first cross section measurement of this reaction at
energies (6.22 MeV E 12.47 MeV) relevant for the
weak r-process. The new data provide a stringent test of various model
predictions which is necessary to improve the precision of the weak r-process
network calculations. The strongly reduced reaction rate uncertainty leads to
very well-constrained nucleosynthesis yields for isotopes under
different neutrino-driven wind conditions