769 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
Search for correlations between solar flares and decay rate of radioactive nuclei
The deacay rate of three different radioactive sources 40K, 137Cs and natTh
has been measured with NaI and Ge detectors. Data have been analyzed to search
for possible variations in coincidence with the two strongest solar flares of
the years 2011 and 2012. No significant deviations from standard expectation
have been observed, with a few 10-4 sensitivity. As a consequence, we could not
find any effect like that recently reported by Jenkins and Fischbach: a few per
mil decrease in the decay rate of 54Mn during solar flares in December 2006.Comment: 5 pages, 3 figure
Precise measurement of the 222Rn half-life: a probe to monitor the stability of radioactivity
We give the results of a study on the 222Rn decay we performed in the Gran
Sasso Laboratory (LNGS) by detecting the gamma rays from the radon progeny. The
motivation was to monitor the stability of radioactivity measuring several
times per year the half-life of a short lifetime (days) source instead of
measuring over a long period the activity of a long lifetime (tens or hundreds
of years) source. In particular, we give a possible reason of the large
periodical fluctuations in the count rate of the gamma rays due to radon inside
a closed canister which has been described in literature and which has been
attributed to a possible influence of a component in the solar irradiation
affecting the nuclear decay rates. We then provide the result of four half-life
measurements we performed underground at LNGS in the period from May 2014 to
January 2015 with radon diffused into olive oil. Briefly, we did not measure
any change of the 222Rn half-life with a 8*10^-5 precision. Finally, we provide
the most precise value for the 222Rn half-life: 3.82146(16){stat}(4){syst}
days.Comment: Accepted for publication in Physics Letters B, 6 pages, 6 figure
Search for time modulations in the decay rate of 40K and 232Th
Time modulations at per mil level have been reported to take place in the
decay constant of about 15 nuclei with period of one year (most cases) but also
of about one month or one day. In this paper we give the results of the
activity measurement of a 40K source and a 232Th one. The two experiments have
been done at the Gran Sasso Laboratory during a period of about 500 days, above
ground (40K) and underground (232Th) with a target sensitivity of a few parts
over 10^5. We also give the results of the activity measurement at the time of
the X-class solar flares which took place in May 2013. Briefly, our
measurements do not show any evidence of unexpected time dependence in the
decay rate of 40K and 232Th.Comment: version accepted for publication (Astroparticle Physics
Allylsulfones through Palladium-Catalyzed Allylic C−H Sulfonylation of Terminal Alkenes
Two previously unknown protocols for Pd-catalyzed allylic C-H sulfonylation of terminal alkenes have been developed. While the former consists of a direct Pd(II)-catalyzed oxidative C-H allylic sulfonylation in the presence of sulfinate anions, the latter involves a sequential one-pot Pd(II)-catalyzed C-H allylic acetoxylation followed by a Pd(0)-catalyzed sulfonylation. The scope of both protocols was studied on 25 examples
Intramolecular Pd(II)-Catalyzed Cyclization of Propargylamides: Straightforward Synthesis of 5-Oxazole-carbaldehydes
(Chemical Equation Presented) Direct synthesis of 2-substituted 5-oxazolecarbaldehydes was performed by intramolecular reaction of propargylamides through treatment with a catalytic amount of Pd(II) salts in the presence of a stoichiometric amount of reoxidant agent. The heterocyclization process was well-tolerated by a wide range of aryl, heteroaryl, and alkyl propargylamides. This protocol constitutes a valuable synthetic pathway to 5-oxazolecarbaldehydes, alternative to the formylation on oxazole rings, often unsatisfactory in term of regioselectivity and yields
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
Search for time modulations in the decay constant of 40K and 226Ra at the underground Gran Sasso Laboratory
Time modulations at per mil level have been reported to take place in the
decay constant of several nuclei with period of one year (most cases) but also
of about one month or one day. On the other hand, experiments with similar or
better sensitivity have been unable to detect any modulation. In this letter we
give the results of the activity study of two different sources: 40K and 226Ra.
The two gamma spectrometry experiments have been performed underground at the
Gran Sasso Laboratory, this way suppressing the time dependent cosmic ray
background. Briefly, our measurements reached the sensitivity of 3.4 and 3.5
parts over 10^6 for 40K and 226Ra, respectively (1 sigma) and they do not show
any statistically significant evidence of time dependence in the decay
constant. We also give the results of the activity measurement at the time of
the two strong X-class solar flares which took place in September 2017. Our
data do not show any unexpected time dependence in the decay rate of 40K in
correspondence with the two flares. To the best of our knowledge, these are the
most precise and accurate results on the stability of the decay constant as
function of time.Comment: Accepted for publication in Physics Letters B, 6 pages, 8 figures.
arXiv admin note: text overlap with arXiv:1311.704
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