64 research outputs found
Elastic alpha-scattering of 112Sn and 124Sn at astrophysically relevant energies
The cross sections for the elastic scattering reactions
{112,124}Sn(a,a){112,124}Sn at energies above and below the Coulomb barrier are
presented and compared to predictions for global alpha-nucleus potentials. The
high precision of the new data allows a study of the global alpha-nucleus
potentials at both the proton and neutron-rich sides of an isotopic chain. In
addition, local alpha-nucleus potentials have been extracted for both nuclei,
and used to reproduce elastic scattering data at higher energies. Predictions
from the capture cross section of the reaction 112Sn(a,g)116Te at
astrophysically relevant energies are presented and compared to experimental
data.Comment: 20 pages, 10 figures, accepted for publication in Phys. Rev.
Excited states in neutron rich boron isotopes
Publisher's version/PDFNew experimental results on in-beam gamma spectroscopy of neutron rich boron isotopes are presented for [superscript 17]B and its neutron removal fragments [superscript 14,15]B, after scattering with a H[subscript 2] target. A gamma transition for [superscript 17]B is observed at 1089 [plus or minus] 15 keV. The fragment [superscript 15]B is observed abundantly associated with a gamma transition of 1336 [plus or minus] 10 keV. This suggests for the first time a core-excited structure for [superscript 17]B thereby providing a new insight into its structure. Observations for [superscript 12,14]B are also presented.
The data set provides a useful systematic study of first excited states of neutron rich boron isotopes showing the dramatic drop in excitation energy beyond N = 8
Determining the 7Li(n,gamma) cross section via Coulomb dissociation of 8Li
The applicability of Coulomb dissociation reactions to determine the cross
section for the inverse neutron capture reaction was explored using the
reaction 8Li(gamma,n)7Li. A 69.5 MeV/nucleon 8Li beam was incident on a Pb
target, and the outgoing neutron and 7Li nucleus were measured in coincidence.
The deduced (n,gamma) excitation function is consistent with data for the
direct capture reaction 7Li(n,gamma)8Li and with low-energy effective field
theory calculations.Comment: Accepted for publication in Phys. Rev.
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.
Measurement of 25Mg(p; gamma)26Al resonance strengths via gamma spectrometry
The COMPTEL instrument performed the first mapping of the 1.809 MeV photons
in the Galaxy, triggering considerable interest in determing the sources of
interstellar 26Al. The predicted 26Al is too low compared to the observation,
for a better understanding more accurate rates for the 25Mg(p; gamma)26Al
reaction are required. The 25Mg(p;gamma)26Al reaction has been investigated at
the resonances at Er= 745; 418; 374; 304 keV at Ruhr-Universitat-Bochum using a
Tandem accelerator and a 4piNaI detector. In addition the resonance at Er = 189
keV has been measured deep underground laboratory at Laboratori Nazionali del
Gran Sasso, exploiting the strong suppression of cosmic background. This low
resonance has been studied with the 400 kV LUNA accelerator and a HPGe
detector. The preliminary results of the resonance strengths will be reported.Comment: Accepted for publication in Journal of Physics
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.
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
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