193 research outputs found
Neutron-induced background by an alpha-beam incident on a deuterium gas target and its implications for the study of the 2H(alpha,gamma)6Li reaction at LUNA
The production of the stable isotope Li-6 in standard Big Bang
nucleosynthesis has recently attracted much interest. Recent observations in
metal-poor stars suggest that a cosmological Li-6 plateau may exist. If true,
this plateau would come in addition to the well-known Spite plateau of Li-7
abundances and would point to a predominantly primordial origin of Li-6,
contrary to the results of standard Big Bang nucleosynthesis calculations.
Therefore, the nuclear physics underlying Big Bang Li-6 production must be
revisited. The main production channel for Li-6 in the Big Bang is the
2H(alpha,gamma)6Li reaction. The present work reports on neutron-induced
effects in a high-purity germanium detector that were encountered in a new
study of this reaction. In the experiment, an {\alpha}-beam from the
underground accelerator LUNA in Gran Sasso, Italy, and a windowless deuterium
gas target are used. A low neutron flux is induced by energetic deuterons from
elastic scattering and, subsequently, the 2H(d,n)3He reaction. Due to the
ultra-low laboratory neutron background at LUNA, the effect of this weak flux
of 2-3 MeV neutrons on well-shielded high-purity germanium detectors has been
studied in detail. Data have been taken at 280 and 400 keV alpha-beam energy
and for comparison also using an americium-beryllium neutron source.Comment: Submitted to EPJA; 13 pages, 8 figure
22Ne and 23Na ejecta from intermediate-mass stars: The impact of the new LUNA rate for 22Ne(p,gamma)23Na
We investigate the impact of the new LUNA rate for the nuclear reaction NeNa on the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience hot-bottom burning. To this aim we use the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range , and metallicities , , and . We find that the new LUNA measures have much reduced the nuclear uncertainties of the Ne and Na AGB ejecta, which drop from factors of to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of Na, the uncertainties that still affect the Ne and Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge-up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance. Results for the corresponding chemical ejecta are made publicly available
First Direct Measurement of the ^{17}O(p,\gamma)^{18}F Reaction Cross-Section at Gamow Energies for Classical Novae
Classical novae are important contributors to the abundances of key isotopes,
such as the radioactive ^{18}F, whose observation by satellite missions could
provide constraints on nucleosynthesis models in novae. The
^{17}O(p,\gamma)^{18}F reaction plays a critical role in the synthesis of both
oxygen and fluorine isotopes but its reaction rate is not well determined
because of the lack of experimental data at energies relevant to novae
explosions. In this study, the reaction cross section has been measured
directly for the first time in a wide energy range Ecm = 200 - 370 keV
appropriate to hydrogen burning in classical novae. In addition, the E=183 keV
resonance strength, \omega \gamma=1.67\pm0.12 \mueV, has been measured with the
highest precision to date. The uncertainty on the ^{17}O(p,\gamma)^{18}F
reaction rate has been reduced by a factor of 4, thus leading to firmer
constraints on accurate models of novae nucleosynthesis.Comment: accepted by Phys. Rev. Let
Preparation and characterisation of isotopically enriched TaO targets for nuclear astrophysics studies
The direct measurement of reaction cross sections at astrophysical energies
often requires the use of solid targets of known thickness, isotopic
composition, and stoichiometry that are able to withstand high beam currents
for extended periods of time. Here, we report on the production and
characterisation of isotopically enriched TaO targets for the study of
proton-induced reactions at the Laboratory for Underground Nuclear Astrophysics
facility of the Laboratori Nazionali del Gran Sasso. The targets were prepared
by anodisation of tantalum backings in enriched water (up to 66% in O
and up to 96% in O). Special care was devoted to minimising the presence
of any contaminants that could induce unwanted background reactions with the
beam in the energy region of astrophysical interest. Results from target
characterisation measurements are reported, and the conclusions for proton
capture measurements with these targets are drawn.Comment: accepted to EPJ
Complete results for five years of GNO solar neutrino observations
We report the complete GNO solar neutrino results for the measuring periods
GNO III, GNO II, and GNO I. The result for GNO III (last 15 solar runs) is
[54.3 + 9.9 - 9.3 (stat.)+- 2.3 (syst.)] SNU (1 sigma) or [54.3 + 10.2 - 9.6
(incl. syst.)] SNU (1 sigma) with errors combined. The GNO experiment is now
terminated after altogether 58 solar exposure runs that were performed between
May 20, 1998 and April 9, 2003. The combined result for GNO (I+II+III) is [62.9
+ 5.5 - 5.3 (stat.) +- 2.5 (syst.)] SNU (1 sigma) or [62.9 + 6.0 - 5.9] SNU (1
sigma) with errors combined in quadrature. Overall, gallium based solar
observations at LNGS (first in GALLEX, later in GNO) lasted from May 14, 1991
through April 9, 2003. The joint result from 123 runs in GNO and GALLEX is
[69.3 +- 5.5 (incl. syst.)] SNU (1 sigma). The distribution of the individual
run results is consistent with the hypothesis of a neutrino flux that is
constant in time. Implications from the data in particle- and astrophysics are
reiterated.Comment: 22 pages incl. 9 Figures and 8 Tables. to appear in: Physics Letters
B (accepted April 13, 2005) PACS: 26.65.+t ; 14.60.P
Testing the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds
The B-type pulsators known as \beta Cephei and Slowly Pulsating B (SPB) stars
present pulsations driven by the \kappa mechanism, which operates thanks to an
opacity bump due to the iron group elements. In low-metallicity environments
such as the Magellanic Clouds, \beta Cep and SPB pulsations are not expected.
Nevertheless, recent observations show evidence for the presence of B-type
pulsator candidates in both galaxies. We seek an explanation for the excitation
of \beta Cep and SPB modes in those galaxies by examining basic input physics
in stellar modelling: i) the specific metal mixture of B-type stars in the
Magellanic Clouds; ii) the role of a potential underestimation of stellar
opacities. We first derive the present-day chemical mixtures of B-type stars in
the Magellanic Clouds. Then, we compute stellar models for that metal mixture
and perform a non-adiabatic analysis of these models. In a second approach, we
simulate parametric enhancements of stellar opacities due to different iron
group elements. We then study their effects in models of B stars and their
stability. We find that adopting a representative chemical mixture of B stars
in the Small Magellanic Cloud cannot explain the presence of B-type pulsators
there. An increase of the opacity in the region of the iron-group bump could
drive B-type pulsations, but only if this increase occurs at the temperature
corresponding to the maximum contribution of Ni to this opacity bump. We
recommend an accurate computation of Ni opacity to understand B-type pulsators
in the Small Magellanic Cloud, as well as the frequency domain observed in some
Galactic hybrid \beta Cep-SPB stars.Comment: 16 pages, 12 figures. Accepted for publication in MNRA
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.
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
Recent Advances in Modeling Stellar Interiors
Advances in stellar interior modeling are being driven by new data from
large-scale surveys and high-precision photometric and spectroscopic
observations. Here we focus on single stars in normal evolutionary phases; we
will not discuss the many advances in modeling star formation, interacting
binaries, supernovae, or neutron stars. We review briefly: 1) updates to input
physics of stellar models; 2) progress in two and three-dimensional evolution
and hydrodynamic models; 3) insights from oscillation data used to infer
stellar interior structure and validate model predictions (asteroseismology).
We close by highlighting a few outstanding problems, e.g., the driving
mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant
eruptions seen in luminous blue variables such as eta Car and P Cyg, and the
solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density
Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special
issue of Astrophysics and Space Science; 7 pages; 5 figure
A new study of the Ne(p,)Na reaction deep underground: Feasibility, setup, and first observation of the 186 keV resonance
The Ne(p,)Na reaction takes part in the neon-sodium
cycle of hydrogen burning. This cycle is active in asymptotic giant branch
stars as well as in novae and contributes to the nucleosythesis of neon and
sodium isotopes. In order to reduce the uncertainties in the predicted
nucleosynthesis yields, new experimental efforts to measure the
Ne(p,)Na cross section directly at the astrophysically
relevant energies are needed. In the present work, a feasibility study for a
Ne(p,)Na experiment at the Laboratory for Underground
Nuclear Astrophysics (LUNA) 400\,kV accelerator deep underground in the Gran
Sasso laboratory, Italy, is reported. The ion beam induced -ray
background has been studied. The feasibility study led to the first observation
of the = 186\,keV resonance in a direct experiment. An experimental
lower limit of 0.12\,\,10\,eV has been obtained for the
resonance strength. Informed by the feasibility study, a dedicated experimental
setup for the Ne(p,)Na experiment has been developed. The
new setup has been characterized by a study of the temperature and pressure
profiles. The beam heating effect that reduces the effective neon gas density
due to the heating by the incident proton beam has been studied using the
resonance scan technique, and the size of this effect has been determined for a
neon gas target.Comment: Minor errors corrected; final versio
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