154 research outputs found
The cosmological 7Li problem from a nuclear physics perspective
The primordial abundance of 7Li as predicted by Big Bang Nucleosynthesis
(BBN) is more than a factor 2 larger than what has been observed in metal-poor
halo stars. Herein, we analyze the possibility that this discrepancy originates
from incorrect assumptions about the nuclear reaction cross sections relevant
for BBN. To do this, we introduce an efficient method to calculate the changes
in the 7Li abundance produced by arbitrary (temperature dependent)
modifications of the nuclear reaction rates. Then, considering that 7Li is
mainly produced from 7Be via the electron capture process 7Be + e -> 7Li +
nu_e, we assess the impact of the various channels of 7Be destruction.
Differently from previous analysis, we consider the role of unknown resonances
by using a complete formalism which takes into account the effect of Coulomb
and centrifugal barrier penetration and that does not rely on the use of the
narrow-resonance approximation. As a result of this, the possibility of a
nuclear physics solution to the 7Li problem is significantly suppressed. Given
the present experimental and theoretical constraints, it is unlikely that the
7Be + n destruction rate is underestimated by the 2.5 factor required to solve
the problem. We exclude, moreover, that resonant destruction in the channels
7Be + t and 7Be + 3He can explain the 7Li puzzle. New unknown resonances in 7Be
+ d and 7Be + alpha could potentially produce significant effects. Recent
experimental results have ruled out such a possibility for 7Be+d. On the other
hand, for the 7Be + alpha channel very favorable conditions are required. The
possible existence of a partially suitable resonant level in 11C is studied in
the framework of a coupled-channel model and the possibility of a direct
measurement is considered.Comment: Final version, accepted for publication in JCA
The Potential of Mixtures of Pure Fluids in ORC-based Power Units fed by Exhaust Gases in Internal Combustion Engines
Abstract ORC represents an effective challenge in the waste heat recovery from ICEs. In spite of technological aspects, its thermodynamic design still deserves attention. Mixtures of pure fluids show interesting properties able to improve exergetic efficiency of the Rankine cycle, thanks to the positive slope of the phase changing. They can reduce also ODP and GWP, helping the replacement trends of working fluids. The paper optimizes cycle exergetic efficiency considering mixtures of pure fluids. The use of hydrocarbons in mixtures is particularly suitable and when used in limited fractions with other organic fluids they loses the limits related to the flammability.R245fa is a fluid that obtains a large net power increase when used in mixtures with hydrocarbons, compared to pure fluid an optimized R245fa/benzene mixture, for instance, attains an 11% net power increase
The response of primordial abundances to a general modification of and/or of the early universe expansion rate
We discuss the effects of a possible time variation of the Newton constant
on light elements production in Big Bang Nucleosyntesis (BBN). We
provide analytical estimates for the dependence of primordial abundances on the
value of the Newton constant during BBN. The accuracy of these estimates is
then tested by numerical methods. % Moreover, we determine numerically the
response of each element to an arbitrary time-dependent modification of the
early universe expansion rate. Finally, we determine the bounds on possible
variations of which can be obtained from the comparison of
theoretical predictions and observational data.Comment: 11 pages, 2 figure
Magnetic field hourly averages from the Rome-GSFC experiment aboard Helios 1 and Helio 2
Plots of all the hourly averages computed from the solar magnetic field measurements obtained during the mission are given separately for Helios 1 and Helios 2. The magnitude and the direction of the averaged field are plotted versus the number of solar rotations as seen from Helios, counted from launch
Non-inductive components of electromagnetic signals associated with L’Aquila earthquake sequences estimated by means of inter-station impulse response functions
On 6 April 2009 at 01:32:39 UT a strong earthquake
occurred west of L’Aquila at the very shallow depth
of 9 km. The main shock local magnitude was Ml = 5.8
(Mw=6.3). Several powerful aftershocks occurred the following
days. The epicentre of the main shock occurred 6 km
away from the Geomagnetic Observatory of L’Aquila, on a
fault 15 km long having a NW-SE strike, about 140 , and
a SW dip of about 42 . For this reason, L’Aquila seismic
events offered very favourable conditions to detect possible
electromagnetic emissions related to the earthquake. The
data used in this work come from the permanent geomagnetic
Observatories of L’Aquila and Duronia. Here the results concerning
the analysis of the residual magnetic field estimated
by means of the inter-station impulse response functions in
the frequency band from 0.3 Hz to 3 Hz are shown
Are there nu_mu or nu_tau in the flux of solar neutrinos on earth?
Using the model independent method of Villante, Fiorentini, Lisi, Fogli,
Palazzo, and the rates measured in the SNO and Super-Kamiokande solar neutrino
experiment, we calculate the amount of active nu_mu or nu_tau present in the
flux of solar neutrinos on Earth. We show that the probability of
nu_e->nu_{mu,tau} transitions is larger than zero at 99.89% CL. We find that
the averaged flux of nu_{mu,tau} on Earth is larger than 0.17 times the 8B nu_e
flux predicted by the BP2000 Standard Solar Model at 99% CL. We discuss also
the consequences of possible nu_e->anti-nu_{mu,tau} or nu_e->anti-nu_e
transitions of solar neutrinos. We derive a model-independent lower limit of
0.52 at 99% CL for the ratio of the 8B nu_e flux produced in the Sun and its
value in the BP2000 Standard Solar Model.Comment: 5 pages. Added discussion on possible nu_e->anti-nu_{mu,tau} or
nu_e->anti-nu_e transition
Model based design and optimization of a fuel cell electric vehicle
© 2014 The Authors. Alternative vehicle powertrains (hybrid, hydrogen, electric) are a right answer to the emissions problem in very congested urban areas. The most effective alternative choice is surely represented by fuel cell vehicles. The design and optimization of this kind of powertrain can take a great benefit from mathematical models which include auxiliary management and control strategies of the energy fluxes: the use of a virtual platform limits the expensive and time-consuming experimental activity.
The Authors present a lumped parameter mathematical model of a hybrid vehicle fed by a fuel cell pack. The propulsion system (fuel cell and battery) is accurately designed according to energy balancing. The mechanical power is given by an electric machine, whose behavior as alternator allows the energy recovery during braking. This aspect is treated with particular care (inside the overall vehicle and powertrain modeling). Vehicle's stability and safety has been also verified before recovering energy, re-modulating the braking action.Hi-CEPS EC project “Highly integrated combustion electric propulsion system”; Italian national project “MECCANO – Industria 201
Solar neutrino event spectra: Tuning SNO to equalize Super-Kamiokande
The Super-Kamiokande (SK) and the Sudbury Neutrino Observatory (SNO)
experiments are monitoring the flux of B solar neutrinos through the electron
energy spectrum from the reactions nu_{e,mu,tau} + e --> nu_{e,mu,tau} + e and
nu_e + d --> p + p + e, respectively. We show that the SK detector response to
B neutrinos in each bin of the electron energy spectrum (above 8 MeV) can be
approximated, with good accuracy, by the SNO detector response in an
appropriate electron energy range (above 5.1 MeV). For instance, the SK
response in the bin [10,10.5] MeV is reproduced (``equalized'') within 2
percent by the SNO response in the range [7.1,11.75] MeV. As a consequence, in
the presence of active neutrino oscillations, the SK and SNO event rates in the
corresponding energy ranges turn out to be linearly related, for any functional
form of the oscillation probability. Such equalization is not spoiled by the
possible contribution of hep neutrinos (within current phenomenological
limits). In perspective, when the SK and the SNO spectra will both be measured
with high accuracy, the SK-SNO equalization can be used to determine the
absolute B neutrino flux, and to cross-check the (non)observation of spectral
deviations in SK and SNO. At present, as an exercise, we use the equalization
to ``predict'' the SNO energy spectrum, on the basis of the current SK data.
Finally, we briefly discuss some modifications or limitations of our results in
the case of sterile neutrino oscillations and of relatively large Earth matter
effects.Comment: 18 pages + 6 figure
Fusion rate enhancement due to energy spread of colliding nuclei
Experimental results for sub-barrier nuclear fusion reactions show cross
section enhancements with respect to bare nuclei which are generally larger
than those expected according to electron screening calculations. We point out
that energy spread of target or projectile nuclei is a mechanism which
generally provides fusion enhancement. We present a general formula for
calculating the enhancement factor and we provide quantitative estimate for
effects due to thermal motion, vibrations inside atomic, molecular or crystal
system, and due to finite beam energy width. All these effects are marginal at
the energies which are presently measurable, however they have to be considered
in future experiments at still lower energies. This study allows to exclude
several effects as possible explanation of the observed anomalous fusion
enhancements, which remain a mistery.Comment: 17 pages with 3 ps figure included. Revtex styl
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