43 research outputs found
Study of the 12C+12C fusion reactions near the Gamow energy
The fusion reactions 12C(12C,a)20Ne and 12C(12C,p)23Na have been studied from
E = 2.10 to 4.75 MeV by gamma-ray spectroscopy using a C target with ultra-low
hydrogen contamination. The deduced astrophysical S(E)* factor exhibits new
resonances at E <= 3.0 MeV, in particular a strong resonance at E = 2.14 MeV,
which lies at the high-energy tail of the Gamow peak. The resonance increases
the present non-resonant reaction rate of the alpha channel by a factor of 5
near T = 8x10^8 K. Due to the resonance structure, extrapolation to the Gamow
energy E_G = 1.5 MeV is quite uncertain. An experimental approach based on an
underground accelerator placed in a salt mine in combination with a high
efficiency detection setup could provide data over the full E_G energy range.Comment: 4 Pages, 4 figures, accepted for publication in Phys. Rev. Let
Experimental Validation of the Largest Calculated Isospin-Symmetry-Breaking Effect in a Superallowed Fermi Decay
A precision measurement of the gamma yields following the beta decay of 32Cl
has determined its isobaric analogue branch to be (22.47^{+0.21}_{-0.19})%.
Since it is an almost pure Fermi decay, we can also determine the amount of
isospin-symmetry breaking in this superallowed transition. We find a very large
value, delta_C=5.3(9)%, in agreement with a shell-model calculation. This
result sets a benchmark for isospin-symmetry-breaking calculations and lends
support for similarly-calculated, yet smaller, corrections that are currently
applied to 0+ -> 0+ transitions for tests of the Standard Model
Activation measurement of the 3He(a,g)7Be reaction cross section at high energies
The astrophysically important 3He(a,g)7Be reaction was studied at high
energies where the available experimental data are in contradiction. A thin
window 3He gas cell was used and the cross section was measured with the
activation method. The obtained cross sections at energies between Ec.m. = 1.5
and 2.5 MeV are compared with the available data and theoretical calculations.
The present results support the validity of the high energy cross section
energy dependence observed by recent experiments.Comment: 16 pages, 5 figure
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.
Experimental Limit to Interstellar 244Pu Abundance
Short-lived nuclides, now extinct in the solar system, are expected to be
present in the interstellar medium (ISM). Grains of ISM origin were recently
discovered in the inner solar system and at Earth orbit and may accrete onto
Earth after ablation in the atmosphere. A favorable matrix for detection of
such extraterrestrial material is presented by deep open-sea sediments with
very low sedimentation rates (0.8-3 mm/kyr). We report here on the measurement
of Pu isotopic abundances in a 1-kg deep-sea dry sediment collected in 1992 in
the North Pacific. Our measured value of (3+-3)x10^5 244Pu atoms in the
Pu-separated fraction of the sample shows no excess over the expected
stratospheric nuclear fallout content and under reasonable assumptions we
derive a limit of 2x10^-11 g-244Pu/g-ISM for the abundance of 244Pu in ISM.Comment: 10 p, 1 fig, LateX(AASTeX) Accepted for publication in ApJL, aug 2,
200
A New Precision Measurement of the 7Be(p,gamma)8B Cross section with an Implanted 7Be Target
The 7Be(p,gamma)8B reaction plays a central role in the evaluation of solar
neutrino fluxes. We report on a new precision measurement of the cross section
of this reaction, following our previous experiment with an implanted 7Be
target, a raster scanned beam and the elimination of the backscattering loss.
The new measurement incorporates a more abundant 7Be target and a number of
improvements in design and procedure. The point at Elab=991 keV was measured
several times under varying conditions, yielding a value of S17(Ec.m. =850 keV)
= 24.0(5) eV b. Measurements were carried out at lower energies as well. Due to
the precise knowledge of the implanted 7Be density profile it was possible to
reconstitute both the off- and on resonance parts of the cross section and to
obtain from the entire set of measurements an extrapolated value of
S17(0)=21.2(7) eV b.Comment: 4 Pages, 3 Figure
Revision of the 15N(p,{\gamma})16O reaction rate and oxygen abundance in H-burning zones
The NO cycle takes place in the deepest layer of a H-burning core or shell,
when the temperature exceeds T {\simeq} 30 {\cdot} 106 K. The O depletion
observed in some globular cluster giant stars, always associated with a Na
enhancement, may be due to either a deep mixing during the RGB (red giant
branch) phase of the star or to the pollution of the primordial gas by an early
population of massive AGB (asymptotic giant branch) stars, whose chemical
composition was modified by the hot bottom burning. In both cases, the NO cycle
is responsible for the O depletion. The activation of this cycle depends on the
rate of the 15N(p,{\gamma})16O reaction. A precise evaluation of this reaction
rate at temperatures as low as experienced in H-burning zones in stellar
interiors is mandatory to understand the observed O abundances. We present a
new measurement of the 15N(p,{\gamma})16O reaction performed at LUNA covering
for the first time the center of mass energy range 70-370 keV, which
corresponds to stellar temperatures between 65 {\cdot} 106 K and 780 {\cdot}106
K. This range includes the 15N(p,{\gamma})16O Gamow-peak energy of explosive
H-burning taking place in the external layer of a nova and the one of the hot
bottom burning (HBB) nucleosynthesis occurring in massive AGB stars. With the
present data, we are also able to confirm the result of the previous R-matrix
extrapolation. In particular, in the temperature range of astrophysical
interest, the new rate is about a factor of 2 smaller than reported in the
widely adopted compilation of reaction rates (NACRE or CF88) and the
uncertainty is now reduced down to the 10% level.Comment: 6 pages, 5 figure
LUNA: a Laboratory for Underground Nuclear Astrophysics
It is in the nature of astrophysics that many of the processes and objects
one tries to understand are physically inaccessible. Thus, it is important that
those aspects that can be studied in the laboratory be rather well understood.
One such aspect are the nuclear fusion reactions, which are at the heart of
nuclear astrophysics. They influence sensitively the nucleosynthesis of the
elements in the earliest stages of the universe and in all the objects formed
thereafter, and control the associated energy generation, neutrino luminosity,
and evolution of stars. We review an experimental approach for the study of
nuclear fusion reactions based on an underground accelerator laboratory, named
LUNA.Comment: Invited Review; accepted for publication in Reports on Progress in
Physics; 26 pages; 27 figure
An actively vetoed Clover gamma-detector for nuclear astrophysics at LUNA
An escape-suppressed, composite high-purity germanium detector of the Clover
type has been installed at the Laboratory for Underground Nuclear Astrophysics
(LUNA) facility, deep underground in the Gran Sasso Laboratory, Italy. The
laboratory gamma-ray background of the Clover detector has been studied
underground at LUNA and, for comparison, also in an overground laboratory.
Spectra have been recorded both for the single segments and for the virtual
detector formed by online addition of all four segments. The effect of the
escape-suppression shield has been studied as well. Despite their generally
higher intrinsic background, escape-suppressed detectors are found to be well
suited for underground nuclear astrophysics studies. As an example for the
advantage of using a composite detector deep underground, the weak ground state
branching of the Ep = 223 keV resonance in the 24Mg(p,gamma)25Al reaction is
determined with improved precision.Comment: 8 pages, 6 figures, 3 tables; as accepted by Eur. Phys. J.