446 research outputs found
Screening enhancement factors for laboratory CNO and rp astrophysical reactions
Cross sections of laboratory CNO and rp astrophysical reactions are enhanced
due to the presence of the multi-electron cloud that surrounds the target
nuclei. As a result the relevant astrophysical factors are overestimated unless
corrected appropriately. This study gives both an estimate of the error
committed if screening effects are not taken into account and a rough profile
of the laboratory energy thresholds at which the screening effect appears. The
results indicate that, for most practical purposes, screening corrections to
past relevant experiments can be disregarded. Regarding future experiments,
however, screening corrections to the CNO reactions will certainly be of
importance as they are closely related to the solar neutrino fluxes and the rp
process. Moreover, according to the present results, screening effects will
have to be taken into account particularly by the current and future LUNA
experiments, where screened astrophysical factors will be enhanced to a
significant degree.Comment: 6 RevTex pages + 2 ps figures. (Revised version). Accepted for
publication in Journal of Physics
First Measurement of the He3+He3-->He4+2p Cross Section down to the Lower Edge of the Solar Gamow Peak
We give the LUNA results on the cross section measurement of a key reaction
of the proton-proton chain strongly affecting the calculated neutrino
luminosity from the Sun: He3+He3-->He4+2p. Due to the cosmic ray suppression
provided by the Gran Sasso underground laboratory it has been possible to
measure the cross section down to the lower edge of the solar Gamow peak, i.e.
as low as 16.5 keV centre of mass energy. The data clearly show the cross
section increase due to the electron screening effect but they do not exhibit
any evidence for a narrow resonance suggested to explain the observed solar
neutrino flux.Comment: 5 pages, RevTeX, and 2 figures in PostScript Submitted for
publicatio
Impact of a revised Mg(p,)Al reaction rate on the operation of the Mg-Al cycle
Proton captures on Mg isotopes play an important role in the Mg-Al cycle
active in stellar H-burning regions. In particular, low-energy nuclear
resonances in the Mg(p,)Al reaction affect the production
of radioactive Al as well as the resulting Mg/Al abundance ratio.
Reliable estimations of these quantities require precise measurements of the
strengths of low-energy resonances. Based on a new experimental study performed
at LUNA, we provide revised rates of the Mg(p,)Al
and the Mg(p,)Al reactions with corresponding
uncertainties. In the temperature range 50 to 150 MK, the new recommended rate
of the Al production is up to 5 times higher than previously
assumed. In addition, at T MK, the revised total reaction rate is a
factor of 2 higher. Note that this is the range of temperature at which the
Mg-Al cycle operates in an H-burning zone. The effects of this revision are
discussed. Due to the significantly larger Mg(p,)Al
rate, the estimated production of Al in H-burning regions is less
efficient than previously obtained. As a result, the new rates should imply a
smaller contribution from Wolf-Rayet stars to the galactic Al budget.
Similarly, we show that the AGB extra-mixing scenario does not appear able to
explain the most extreme values of Al/Al, i.e. , found
in some O-rich presolar grains. Finally, the substantial increase of the total
reaction rate makes the hypothesis of a self-pollution by massive AGBs a more
robust explanation for the Mg-Al anticorrelation observed in Globular-Cluster
stars
Activation measurement of the 3He(alpha,gamma)7Be cross section at low energy
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a
major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted
by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis
calculations. The present work reports on a new precision experiment using the
activation technique at energies directly relevant to big-bang nucleosynthesis.
Previously such low energies had been reached experimentally only by the
prompt-gamma technique and with inferior precision. Using a windowless gas
target, high beam intensity and low background gamma-counting facilities, the
3He(alpha,gamma)7Be cross section has been determined at 127, 148 and 169 keV
center-of-mass energy with a total uncertainty of 4%. The sources of systematic
uncertainty are discussed in detail. The present data can be used in big-bang
nucleosynthesis calculations and to constrain the extrapolation of the
3He(alpha,gamma)7Be astrophysical S-factor to solar energies
The 3He(alpha,gamma)7Be S-factor at solar energies: the prompt gamma experiment at LUNA
The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang
nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement
of the 3He(alpha,gamma)7Be cross section has been performed at the INFN Gran
Sasso underground laboratory by both the activation and the prompt gamma
detection methods. The present work reports full details of the prompt gamma
detection experiment, focusing on the determination of the systematic
uncertainty. The final data, including activation measurements at LUNA, are
compared with the results of the last generation experiments and two different
theoretical models are used to obtain the S-factor at solar energies.Comment: Accepted for publication in Nucl. Phys.
New Energy Calibration of the CMAM 5MV Tandem Accelerator
Abstract Ion accelerators are fundamental in the ongoing research on materials for future energy sources, being the primary tool for understanding the behaviour of different classes of materials (functional, structural, diagnostic) under e.g. the intense radiation expected in fission reactors or the critical thermal operational conditions in IV generation fission reactors. The relevance of ion accelerators research extends straightforwardly to the modification and analysis of materials to be used in future developments of diverse non-nuclear sources like photovoltaic, fuel batteries, etc. From the analytical point of view, the energy of the accelerated ion needs, in many cases, to be known with a precision higher than e.g. the width of reaction resonances that are used for controlling either the yield of a reaction or the penetration depth of the ion, imposing a calibration of the accelerator terminal voltage. This paper reports on the new energy calibration performed for the 5 MV CMAM tandem accelerator
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.
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
- …