306 research outputs found
Metabolism of stilbene phytoalexins in grapevines: Oxidation of resveratrol in single-cell cultures
Single-cell cultures of the fungus resistant grapevine varieties Pollux and Sirius and the susceptible ones Riesling and Optima have been established to study the metabolism of the stilbene phytoalexins resveratrol and viniferin. Cells of different varieties showed morphological differences (form and mechanical resistance) and their susceptibility to treatment with debris of killed mycelium of Bot1ytis cinerea reflected the varietal resistance under natural conditions. Most cells of Riesling and Optima were killed by toxic components of the mycelium within 60 h, whereas cultures of Pollux and Sirius showed only slight damage. Two types of peroxidases have been characterized which can take resveratrol as a substrate converting it to a brown pigment. Viniferin was observed as an intermediate product. Peroxidase activity in suscpetible varieties was reduced as compared to the resistant ones. The oxidation products of resveratrol inhibited the activity of peroxidase. The function of stilbene phytoalexins as precursors of lignin-like substances is discussed
The importance of initial-final state correlations for the formation of fragments in heavy ion collisions
Using quantum molecular dynamics simulations, we investigate the formation of
fragments in symmetric reactions between beam energies of E=30AMeV and 600AMeV.
After a comparison with existing data we investigate some observables relevant
to tackle equilibration: dsigma/dErat, the double differential cross section
dsigma/pt.dpz.dpt,... Apart maybe from very energetic E>400AMeV and very
central reactions, none of our simulations gives evidence that the system
passes through a state of equilibrium. Later, we address the production
mechanisms and find that, whatever the energy, nucleons finally entrained in a
fragment exhibit strong initial-final state correlations, in coordinate as well
as in momentum space. At high energy those correlations resemble the ones
obtained in the participant-spectator model. At low energy the correlations are
equally strong, but more complicated; they are a consequence of the Pauli
blocking of the nucleon-nucleon collisions, the geometry, and the excitation
energy. Studying a second set of time-dependent variables (radii,
densities,...), we investigate in details how those correlations survive the
reaction especially in central reactions where the nucleons have to pass
through the whole system. It appears that some fragments are made of nucleons
which were initially correlated, whereas others are formed by nucleons
scattered during the reaction into the vicinity of a group of previously
correlated nucleons.Comment: 45 pages text + 20 postscript figures Accepted for publication in
Physical Review
Breakup Conditions of Projectile Spectators from Dynamical Observables
Momenta and masses of heavy projectile fragments (Z >= 8), produced in
collisions of 197Au with C, Al, Cu and Pb targets at E/A = 600 MeV, were
determined with the ALADIN magnetic spectrometer at SIS. An analysis of
kinematic correlations between the two and three heaviest projectile fragments
in their rest frame was performed. The sensitivity of these correlations to the
conditions at breakup was verified within the schematic SOS-model. The data
were compared to calculations with statistical multifragmentation models and to
classical three-body calculations. Classical trajectory calculations reproduce
the dynamical observables. The deduced breakup parameters, however, differ
considerably from those assumed in the statistical multifragmentation models
which describe the charge correlations. If, on the other hand, the analysis of
kinematic and charge correlations is performed for events with two and three
heavy fragments produced by statistical multifragmentation codes, a good
agreement with the data is found with the exception that the fluctuation widths
of the intrinsic fragment energies are significantly underestimated. A new
version of the multifragmentation code MCFRAG was therefore used to investigate
the potential role of angular momentum at the breakup stage. If a mean angular
momentum of 0.75/nucleon is added to the system, the energy fluctuations
can be reproduced, but at the same time the charge partitions are modified and
deviate from the data.
PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.Ld, 25.75.-qComment: 38 pages, RevTeX with 21 included figures; Also available from
http://www-kp3.gsi.de/www/kp3/aladin_publications.htm
Breakup Density in Spectator Fragmentation
Proton-proton correlations and correlations of protons, deuterons and tritons
with alpha particles from spectator decays following 197Au + 197Au collisions
at 1000 MeV per nucleon have been measured with two highly efficient detector
hodoscopes. The constructed correlation functions, interpreted within the
approximation of a simultaneous volume decay, indicate a moderate expansion and
low breakup densities, similar to assumptions made in statistical
multifragmentation models.
PACS numbers: 25.70.Pq, 21.65.+f, 25.70.Mn, 25.75.GzComment: 11 pages, LaTeX with 3 included figures; Also available from
http://www-kp3.gsi.de/www/kp3/aladin_publications.htm
Time Scales in Spectator Fragmentation
Proton-proton correlations and correlations of p-alpha, d-alpha, and t-alpha
from spectator decays following Au + Au collisions at 1000 AMeV have been
measured with an highly efficient detector hodoscope. The constructed
correlation functions indicate a moderate expansion and low breakup densities
similar to assumptions made in statistical multifragmentation models. In
agreement with a volume breakup rather short time scales were deduced employing
directional cuts in proton-proton correlations.
PACS numbers: 25.70.Pq, 21.65.+f, 25.70.MnComment: 8 pages, with 5 included figures; To appear in the proceedings of the
CRIS 2000 conference; Also available from
http://www-kp3.gsi.de/www/kp3/aladin_publications.htm
Thermal and Chemical Freeze-out in Spectator Fragmentation
Isotope temperatures from double ratios of hydrogen, helium, lithium,
beryllium, and carbon isotopic yields, and excited-state temperatures from
yield ratios of particle-unstable resonances in 4He, 5Li, and 8Be, were
determined for spectator fragmentation, following collisions of 197Au with
targets ranging from C to Au at incident energies of 600 and 1000 MeV per
nucleon. A deviation of the isotopic from the excited-state temperatures is
observed which coincides with the transition from residue formation to
multi-fragment production, suggesting a chemical freeze-out prior to thermal
freeze-out in bulk disintegrations.Comment: 14 pages, 10 figures, submitted to Phys. Rev. C, small changes as
suggested by the editors and referee
Pion radii in nonlocal chiral quark model
The electromagnetic radius of the charged pion and the transition radius of
the neutral pion are calculated in the framework of the nonlocal chiral quark
model. It is shown in this model that the contributions of vector mesons to the
pion radii are noticeably suppressed in comparison with a similar contribution
in the local Nambu--Jona-Lasinio model. The form-factor for the process
gamma*pi+pi- is calculated for the -1 GeV^2<q^2<1.6 GeV^2. Our results are in
satisfactory agreement with experimental data.Comment: 7 pages, 7 figure
Multiplicity correlations of intermediate-mass fragments with pions and fast protons in 12C + 197Au
Low-energy pi+ (E < 35 MeV) from 12C+197Au collisions at incident energies
from 300 to 1800 MeV per nucleon were detected with the Si-Si(Li)-CsI(Tl)
calibration telescopes of the INDRA multidetector. The inclusive angular
distributions are approximately isotropic, consistent with multiple
rescattering in the target spectator. The multiplicity correlations of the
low-energy pions and of energetic protons (E > 150 MeV) with intermediate-mass
fragments were determined from the measured coincidence data. The deduced
correlation functions 1 + R \approx 1.3 for inclusive event samples reflect the
strong correlations evident from the common impact-parameter dependence of the
considered multiplicities. For narrow impact-parameter bins (based on
charged-particle multiplicity), the correlation functions are close to unity
and do not indicate strong additional correlations. Only for pions at high
particle multiplicities (central collisions) a weak anticorrelation is
observed, probably due to a limited competition between these emissions.
Overall, the results are consistent with the equilibrium assumption made in
statistical multifragmentation scenarios. Predictions obtained with
intranuclear cascade models coupled to the Statistical Multifragmentation Model
are in good agreement with the experimental data.Comment: 9 pages, 11 figures, subm. to EPJ
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