124 research outputs found
Microscopic calculation of proton capture reactions in mass 60-80 region and its astrophysical implications
Microscopic optical potentials obtained by folding the DDM3Y interaction with
the densities from Relativistic Mean Field approach have been utilized to
evaluate S-factors of low-energy reactions in mass 60-80 region
and to compare with experiments. The Lagrangian density FSU Gold has been
employed. Astrophysical rates for important proton capture reactions have been
calculated to study the behaviour of rapid proton nucleosynthesis for waiting
point nuclei with mass less than A=80
Comparison of the LUNA 3He(alpha,gamma)7Be activation results with earlier measurements and model calculations
Recently, the LUNA collaboration has carried out a high precision measurement
on the 3He(alpha,gamma)7Be reaction cross section with both activation and
on-line gamma-detection methods at unprecedented low energies. In this paper
the results obtained with the activation method are summarized. The results are
compared with previous activation experiments and the zero energy extrapolated
astrophysical S factor is determined using different theoretical models.Comment: Accepted for publication in Journal of Physics
Ultra-sensitive in-beam gamma-ray spectroscopy for nuclear astrophysics at LUNA
Ultra-sensitive in-beam gamma-ray spectroscopy studies for nuclear
astrophysics are performed at the LUNA (Laboratory for Underground Nuclear
Astrophysics) 400 kV accelerator, deep underground in Italy's Gran Sasso
laboratory. By virtue of a specially constructed passive shield, the laboratory
gamma-ray background for E_\gamma < 3 MeV at LUNA has been reduced to levels
comparable to those experienced in dedicated offline underground gamma-counting
setups. The gamma-ray background induced by an incident alpha-beam has been
studied. The data are used to evaluate the feasibility of sensitive in-beam
experiments at LUNA and, by extension, at similar proposed facilities.Comment: accepted, Eur. Phys. J.
Constraining the S factor of 15N(p,g)16O at Astrophysical Energies
The 15N(p,g)16O reaction represents a break out reaction linking the first
and second cycle of the CNO cycles redistributing the carbon and nitrogen
abundances into the oxygen range. The reaction is dominated by two broad
resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to
the ground state of 16O. Interference effects between these contributions in
both the low energy region (Ep < 338 keV) and in between the two resonances
(338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of
astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O
reaction has been remeasured covering the energy range from Ep=1800 keV down to
130 keV. The results have been analyzed in the framework of a multi-level
R-matrix theory and a S(0) value of 39.6 keV b has been found.Comment: 15 pages, 9 figure
Direct measurement of the 15N(p,gamma)16O total cross section at novae energies
The 15N(p,gamma)16O reaction controls the passage of nucleosynthetic material
from the first to the second carbon-nitrogen-oxygen (CNO) cycle. A direct
measurement of the total 15N(p,gamma)16O cross section at energies
corresponding to hydrogen burning in novae is presented here. Data have been
taken at 90-230 keV center-of-mass energy using a windowless gas target filled
with nitrogen of natural isotopic composition and a bismuth germanate summing
detector. The cross section is found to be a factor two lower than previously
believed.Comment: LUNA collaboration; accepted by J. Phys.
Astrophysical S-factor of the 3He(alpha,gamma)7Be reaction measured at low energy via prompt and delayed gamma detection
Solar neutrino fluxes depend both on astrophysical and on nuclear physics
inputs, namely on the cross sections of the reactions responsible for neutrino
production inside the Solar core. While the flux of solar 8B neutrinos has been
recently measured at Superkamiokande with a 3.5% uncertainty and a precise
measurement of 7Be neutrino flux is foreseen in the next future, the predicted
fluxes are still affected by larger errors. The largest nuclear physics
uncertainty to determine the fluxes of 8B and 7Be neutrinos comes from the
3He(alpha,gamma)7Be reaction. The uncertainty on its S-factor is due to an
average discrepancy in results obtained using two different experimental
approaches: the detection of the delayed gamma rays from 7Be decay and the
measurement of the prompt gamma emission. Here we report on a new high
precision experiment performed with both techniques at the same time. Thanks to
the low background conditions of the Gran Sasso LUNA accelerator facility, the
cross section has been measured at Ecm = 170, 106 and 93 keV, the latter being
the lowest interaction energy ever reached.
The S-factors from the two methods do not show any discrepancy within the
experimental errors. An extrapolated S(0)= 0.560+/-0.017 keV barn is obtained.
Moreover, branching ratios between the two prompt gamma-transitions have been
measured with 5-8% accuracy.Comment: to be published in Physical Review
The 14N(p,gamma)15O reaction studied with a composite germanium detector
The rate of the carbon-nitrogen-oxygen (CNO) cycle of hydrogen burning is
controlled by the 14N(p,gamma)15O reaction. The reaction proceeds by capture to
the ground states and several excited states in O-15. In order to obtain a
reliable extrapolation of the excitation curve to astrophysical energy, fits in
the R-matrix framework are needed. In an energy range that sensitively tests
such fits, new cross section data are reported here for the four major
transitions in the 14N(p,gamma)15O reaction. The experiment has been performed
at the Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV
accelerator placed deep underground in the Gran Sasso facility in Italy. Using
a composite germanium detector, summing corrections have been considerably
reduced with respect to previous studies. The cross sections for capture to the
ground state and to the 5181, 6172, and 6792 keV excited states in O-15 have
been determined at 359, 380, and 399 keV beam energy. In addition, the
branching ratios for the decay of the 278 keV resonance have been remeasured.Comment: Submitted to Phys. Rev.
Characterization of the LUNA neutron detector array for the measurement of the 13C(α,n)16O reaction
We introduce the LUNA neutron detector array developed for the investigation of the 13C(\u3b1, n)16O reaction towards its astrophysical s-process Gamow peak in the low-background environment of the Laboratori Nazionali del Gran Sasso (LNGS). Eighteen 3He counters are arranged in two different configurations (in a vertical and a horizontal orientation) to optimize neutron detection efficiency, target handling and target cooling over the investigated energy range E\u3b1,lab=300 12400 keV (En=2.2 122.6MeV in emitted neutron energy). As a result of the deep underground location, the passive shielding of the setup and active background suppression using pulse shape discrimination, we reached a total background rate of 1.23\ub10.12 counts/hour. This resulted in an improvement of two orders of magnitude over the state of the art allowing a direct measurement of the 13C(\u3b1, n)16O cross-section down to E\u3b1,lab=300 keV. The absolute neutron detection efficiency of the setup was determined using the 51V(p,n)51Cr reaction and an AmBe radioactive source, and completed with a Geant4 simulation. We determined a (34 \ub1 3)% and (38 \ub1 3)% detection efficiency for the vertical and horizontal configurations, respectively, for En=2.4MeV neutrons
Determination of luminosity for in-ring reactions:A new approach for the low-energy domain
Luminosity is a measure of the colliding frequency between beam and target
and it is a crucial parameter for the measurement of absolute values, such as
reaction cross sections. In this paper, we make use of experimental data from
the ESR storage ring to demonstrate that the luminosity can be precisely
determined by modelling the measured Rutherford scattering distribution. The
obtained results are in good agreement with an independent measurement based on
the x-ray normalization method. Our new method provides an alternative way to
precisely measure the luminosity in low-energy stored-beam configurations. This
can be of great value in particular in dedicated low-energy storage rings where
established methods are difficult or impossible to apply.Comment: 8 pages, 5 figure
Proton capture cross section of Sr isotopes
The (p,gamma) cross section of Sr-84,Sr-86,Sr-87 isotopes has been measured using activation technique in the proton energy range between 1.5 and 3 MeV. These reactions are relevant to the p-process of stellar nucleosynthesis and in addition, the reaction cross sections in the mass region up to 100 are also important concerning the rp-process associated with explosive hydrogen and helium burning. It is speculated that this rp-process could be responsible for the higher abundances of p-nuclei in this mass region. Natural SrF2 targets were irradiated at the 5 MV Van de Graaff accelerator of the ATOMKI. The induced gamma -activity was measured with a calibrated HPGe detector. The resulting cross sections, important for the quantitative discussion of the production of p-nuclei, are compared with the predictions of statistical model calculations using the NON-SMOKER code
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