560 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
Autofocusing imaging: Imaging with primaries, internal multiples and free-surface multiples
New Avoparcin-like Molecules from the Avoparcin Producer Amycolatopsis coloradensis ATCC 53629
Amycolatopsis coloradensis ATCC 53629 is the producer of the glycopeptide antibiotic avoparcin. While setting up the production of the avoparcin complex, in view of its use as analytical standard, we uncovered the production of a to-date not described ristosamynil-avoparcin. Ristosamynil-avoparcin is produced together with α-and β-avoparcin (overall indicated as the avoparcin complex). Selection of one high producer morphological variant within the A. coloradensis population, together with the use of a new fermentation medium, allowed to increase productivity of the avoparcin complex up to 9 g/L in flask fermentations. The selected high producer displayed a non-spore forming phenotype. All the selected phenotypes, as well as the original unselected population, displayed invariably the ability to produce a complex rich in ristosamynil-avoparcin. This suggested that the original strain deposited was not conforming to the description or that long term storage of the lyovials has selected mutants from the original population
Sub MeV Particles Detection and Identification in the MUNU detector ((1)ISN, IN2P3/CNRS-UJF, Grenoble, France, (2)Institut de Physique, Neuch\^atel, Switzerland, (3) INFN, Padova Italy, (4) Physik-Institut, Z\"{u}rich, Switzerland)
We report on the performance of a 1 m TPC filled with CF at 3
bar, immersed in liquid scintillator and viewed by photomultipliers. Particle
detection, event identification and localization achieved by measuring both the
current signal and the scintillation light are presented. Particular features
of particle detection are also discussed. Finally, the Mn
photopeak, reconstructed from the Compton scattering and recoil angle is shown.Comment: Latex, 19 pages, 20 figure
Palladium-catalyzed domino carbopalladation/5-exo-allylic amination of \u3b1-amino allenamides: an efficient entry to enantiopure imidazolidinones
Allenamides of alpha-amino acids were converted into enantiopure 2-vinylimidazolidin-4-ones by a carbopalladation/exo-cyclization process. The products were obtained in 2.5:1-5.5:1 dr, with 94-99% ee. The palladium-catalyzed carbonylative cyclization of the same substrates afforded enone structures. Starting from properly substituted allenamides, an intramolecular carbopalladation followed by intramolecular amination gave rise to tricyclic fused-ring imidazolidinones
Ruthenium-Catalyzed Decarboxylative Rearrangement of 4-Alkenyl-isoxazol-5-ones to Pyrrole Derivatives
Easily accessible isoxazol-5(4H)-ones are useful precursors of heterocycles. In this context, we report the ruthenium-catalyzed transformation of 4-alkenyl-substituted isoxazol-5-ones to afford 1H-pyrrole derivatives. The operative conditions were proven to be effective also on cyclohexane-fused isoxazolones giving 4,5,6,7-tetrahydroindoles. The reactions, which allow for access to tri-and tetra-substituted pyrroles in moderate to high yields, occur through decarboxylative ring-opening/ring-closure involving C-H functionalization of the alkenyl moiety
A Non-coded β2,2-Amino Acid with Isoxazoline Core Able to Stabilize Peptides Folding through an Unprecedented Hydrogen Bond
Dedicated to Prof. Cesare Gennari on the occasion of his 70th birthday New peptidomimetics containing a beta(2,)(2)-isoxazoline amino acid, i.e. 5-(aminomethyl)-3-phenyl-4,5-dihydroisoxazole-5-carboxylic acid (Isox-beta(2,)(2)AA), were prepared and studied by NMR and theoretical calculation. Although similar amino acid derivatives have already been prepared via 1,3-dipolar cycloaddition reaction, neither experimental details nor characterization were found and they were never used for peptide synthesis. Both enantiomers were inserted in peptide sequences to verify their ability to induce a secondary structure. We found that an unexpected conformation is given by R-Isox-beta(2,)(2)AA, inducing the folding of short peptides thanks to an unprecedented H-bond involving C=N of the isoxazoline side chain of our beta(2,)(2)-AA
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
A new FSA approach for in situ -ray spectroscopy
An increasing demand of environmental radioactivity monitoring comes both
from the scientific community and from the society. This requires accurate,
reliable and fast response preferably from portable radiation detectors. Thanks
to recent improvements in the technology, -spectroscopy with sodium
iodide scintillators has been proved to be an excellent tool for in-situ
measurements for the identification and quantitative determination of
-ray emitting radioisotopes, reducing time and costs. Both for
geological and civil purposes not only K, U, and Th have
to be measured, but there is also a growing interest to determine the
abundances of anthropic elements, like Cs and I, which are used
to monitor the effect of nuclear accidents or other human activities.
The Full Spectrum Analysis (FSA) approach has been chosen to analyze the
-spectra. The Non Negative Least Square (NNLS) and the energy
calibration adjustment have been implemented in this method for the first time
in order to correct the intrinsic problem related with the
minimization which could lead to artifacts and non physical results in the
analysis.
A new calibration procedure has been developed for the FSA method by using in
situ -spectra instead of calibration pad spectra. Finally, the new
method has been validated by acquiring -spectra with a 10.16 cm x 10.16
cm sodium iodide detector in 80 different sites in the Ombrone basin, in
Tuscany. The results from the FSA method have been compared with the laboratory
measurements by using HPGe detectors on soil samples collected in the different
sites, showing a satisfactory agreement between them. In particular, the
Cs isotopes has been implemented in the analysis since it has been
found not negligible during the in-situ measurements.Comment: accepted by Science of Total Environment: 8 pages, 10 figures, 3
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