819 research outputs found
Nuclear clustering and the electron screening puzzle
Electron screening changes appreciably the magnitude of astrophysical nuclear
reactions within stars. This effect is also observed in laboratory experiments
on Earth, where atomic electrons are present in the nuclear targets.
Theoretical models were developed over the past 30 years and experimental
measurements have been carried out to study electron screening in thermonuclear
reactions. None of the theoretical models were able to explain the high values
of the experimentally determined screening potentials. We explore the
possibility that the "electron screening puzzle" is due to nuclear
clusterization and polarization effects in the fusion reactions. We will
discuss the supporting arguments for this scenario.Comment: 6 pages, 3 figures. Invited talk at the 8th Nuclear Physics in
Astrophysics International Conference, June 18-23, 2017, Catania, Ital
The electron screening puzzle and nuclear clustering
Accurate measurements of nuclear reactions of astrophysical interest within,
or close to, the Gamow peak, show evidence of an unexpected effect attributed
to the presence of atomic electrons in the target. The experiments need to
include an effective "screening" potential to explain the enhancement of the
cross sections at the lowest measurable energies. Despite various theoretical
studies conducted over the past 20 years and numerous experimental
measurements, a theory has not yet been found that can explain the cause of the
exceedingly high values of the screening potential needed to explain the data.
In this letter we show that instead of an atomic physics solution of the
"electron screening puzzle", the reason for the large screening potential
values is in fact due to clusterization effects in nuclear reactions, in
particular for reaction involving light nuclei.Comment: 6 pages, 2 figures, accepted for publication in Physics Letters
Measurement of the B 10 (p,α0) Be 7 cross section from 5 keV to 1.5 MeV in a single experiment using the Trojan horse method
For the first time the astrophysical factor of the B10(p,α0)Be7 reaction has been measured over a wide energy range, from 5 keV to 1.5 MeV, via the Trojan horse method (THM) applied to the quasifree H2(B10,αBe7)n reaction. Therefore, the S(E) factor has been recast into absolute units by scaling in the energy range 200 keV-1.2 MeV to a recent measurement using the activation method, leading to a normalization uncertainty of 4%. An R-matrix fit of the THM data was performed, to parametrize the S factor, obtain spectroscopic information on the populated resonances, and compare with other recent experiments. Finally, a new determination of the screening potential Ue has been obtained, Ue=240±50 eV, with a much smaller error than our previous measurement
Big Bang nucleosynthesis revisited via Trojan Horse Method measurements
Nuclear reaction rates are among the most important input for understanding
the primordial nucleosynthesis and therefore for a quantitative description of
the early Universe. An up-to-date compilation of direct cross sections of
2H(d,p)3H, 2H(d,n)3He, 7Li(p,alpha)4He and 3He(d,p)4He reactions is given.
These are among the most uncertain cross sections used and input for Big Bang
nucleosynthesis calculations. Their measurements through the Trojan Horse
Method (THM) are also reviewed and compared with direct data. The reaction
rates and the corresponding recommended errors in this work were used as input
for primordial nucleosynthesis calculations to evaluate their impact on the 2H,
3,4He and 7Li primordial abundances, which are then compared with observations.Comment: 22 pages, 7 figures, accepted for publication in The Astrophysical
Journa
Benign nocturnal alternating hemiplegia of childhood: a new case with unusual findings
It has been described a neuro developmental disorder labelled “Benign nocturnal alternating hemiplegia of childhood” (BNAHC)
characterized by recurrent attacks of nocturnal hemiplegia without progression to neurological or intellectual impairment. We
report a female patient who at 11 months revealed a motionless left arm, unusual crying without impairment of consciousness
and obvious precipitating factors. The attacks occur during sleep in the early morning with lack of ictal and interictal electroencephalographic
abnormalities, progressive neurological deficit, and cognitive impairment. Unlike previous reports of BNAHC our
patient come from a family with a history of both migraine, hemiplegic migraine, and sleep disorders. Our study remarks on the
typical features described in previous studies and stresses the uncommon aspects that could help to identify the disorder which is
likely to have been underestimated. Despite some clinical similarities between BNAHC and familiar hemiplegic migraine and alternating
hemiplegia of childhood, the genetic analyses of our patient did not reveal genetic mutations found in both disorders
Indirect techniques in nuclear astrophysics. Asymptotic Normalization Coefficient and Trojan Horse
Owing to the presence of the Coulomb barrier at astrophysically relevant
kinetic energies it is very difficult, or sometimes impossible, to measure
astrophysical reaction rates in the laboratory. That is why different indirect
techniques are being used along with direct measurements. Here we address two
important indirect techniques, the asymptotic normalization coefficient (ANC)
and the Trojan Horse (TH) methods. We discuss the application of the ANC
technique for calculation of the astrophysical processes in the presence of
subthreshold bound states, in particular, two different mechanisms are
discussed: direct capture to the subthreshold state and capture to the
low-lying bound states through the subthreshold state, which plays the role of
the subthreshold resonance. The ANC technique can also be used to determine the
interference sign of the resonant and nonresonant (direct) terms of the
reaction amplitude. The TH method is unique indirect technique allowing one to
measure astrophysical rearrangement reactions down to astrophysically relevant
energies. We explain why there is no Coulomb barrier in the sub-process
amplitudes extracted from the TH reaction. The expressions for the TH amplitude
for direct and resonant cases are presented.Comment: Invited talk on the Conference "Nuclear Physics in Astrophysics II",
Debrecen, Hungary, 16-20 May, 200
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