907 research outputs found
Dynamic scaling of fronts in the quantum XX chain
The dynamics of the transverse magnetization in the zero-temperature XX chain
is studied with emphasis on fronts emerging from steplike initial magnetization
profiles. The fronts move with fixed velocity and display a staircase like
internal structure whose dynamic scaling is explored both analytically and
numerically. The front region is found to spread with time sub-diffusively with
the height and the width of the staircase steps scaling as t^(-1/3) and t^1/3,
respectively. The areas under the steps are independent of time, thus the
magnetization relaxes in quantized "steps" of spin-flips.Comment: 4 pages, 3 eps figures, RevTe
Multidimensional NMR identifies the conformational shift essential for catalytic competence in the 60-kDa Drosophila melanogaster dUTPase trimer
The catalytic mechanism of dUTP pyrophosphatase (dUTPase), responsible
for the prevention of uracil incorporation into DNA, involves ordering
of the flexible C terminus of the enzyme. This conformational shift is
investigated by multidimensional NMR on the Drosophila enzyme. Flexible
segments of the homotrimer give rise to sharp resonances in the
H-1-N-15 heteronuclear single-quantum coherence (HSQC) spectra, which
are clearly distinguishable from the background resonances of the well
folded protein globule. Binding of the product dUMP or the analogues
dUDP and alpha,beta-imino-dUTP to the enzyme induces a conformational
change reflected in the disappearance of eight sharp resonances. This
phenomenon is interpreted as nucleotide binding-induced ordering of
some residues upon the folded protein globule. Three-dimensional
N-15-edited H-1-N-15 HSQC total correlation spectroscopy (TOCSY) and
H-1-N-15 HSQC nuclear Overhauser effect spectroscopy measurements
allowed clear assignment of these eight specific resonance peaks. The
residues identified correspond to the conserved C-terminal sequence
motif, indicating that (i) this conformational shift is amenable to NMR
studies in solution even in the large trimeric molecule and (ii)
formation of the closed enzyme conformer in the case of the Drosophila
enzyme does not require the complete triphosphate chain of the
substrate. NMR titration of the enzyme with the nucleotide ligands as
well as kinetic data indicated significant deviation from the model of
independent active sites within the homotrimer. The results suggest
allosterism in the eukaryotic dUTPase
STUDY OF THE EARLY ROOT DEVELOPMENT OF IMPORTANT ARABLE GRASS WEEDS IN MAIZE I. LARGE CRABGRASS (DIGITARIA SANGUINALIS (L.) SCOP)
We have studied the primary root development of the one of the most important grass weeds - large crabgrass (Digitaria sanguinalis (L.) Scop). The primary, secondary and tertiary root development was studied in laboratory (thermostat), and in pots in 1994, 1995 and 1997. Each year we sowed the seeds 1 cm deep in the
pots separately. The number of days required for the change of the early root system from the sowing 17 - 24 days. The average leaf number in the root change period was 4 and varied in between 3 to 5. We also observed that the date of sowing during the summer months (June, July and August) did not influence the change of the
early root system
pairs from a nuclear transition signaling an elusive light neutral boson
Electron-positron pairs have been observed in the 10.95-MeV decay
in O. The branching ratio of the ee pairs compared to the
3.84-MeV decay of the level is deduced to be
. This magnetic monopole (M0) transition cannot proceed by
-ray decay and is, to first order, forbidden for internal pair
creation. However, the transition may also proceed by the emission of a light
neutral or boson. Indeed, we do observe a sharp peak in the
angular correlation with all the characteristics belonging to the
intermediate emission of such a boson with an invariant mass of 8.5(5)
MeV/c. It may play a role in the current quest for light dark matter in the
universe.Comment: 6 page
Observation of Anomalous Internal Pair Creation in Be: A Possible Signature of a Light, Neutral Boson
Electron-positron angular correlations were measured for the isovector
magnetic dipole 17.6 MeV state (, ) ground state
(, ) and the isoscalar magnetic dipole 18.15 MeV (,
) state ground state transitions in Be. Significant
deviation from the internal pair creation was observed at large angles in the
angular correlation for the isoscalar transition with a confidence level of . This observation might indicate that, in an intermediate step, a
neutral isoscalar particle with a mass of 16.70 (stat)
(sys) MeV and was created.Comment: 5 pages, 5 figure
Proton decay from the isoscalar giant dipole resonance in Ni
Proton decay from the 3 isoscalar giant dipole resonance (ISGDR)
in Ni has been measured using the () reaction at a
bombarding energy of 386 MeV to investigate its decay properties. We have
extracted the ISGDR strength under the coincidence condition between
inelastically scattered particles at forward angles and decay protons
emitted at backward angles. Branching ratios for proton decay to low-lying
states of Co have been determined, and the results compared to
predictions of recent continuum-RPA calculations. The final-state spectra of
protons decaying to the low-lying states in Co were analyzed for a more
detailed understanding of the structure of the ISGDR. It is found that there
are differences in the structure of the ISGDR as a function of excitation
energy.Comment: Minor changes after review. Accepted for publication in Phys. Rev. C.
19 pages; 7 figure
A multi-detector array for high energy nuclear e+e- pair spectrosocopy
A multi-detector array has been constructed for the simultaneous measurement
of energy- and angular correlation of electron-positron pairs produced in
internal pair conversion (IPC) of nuclear transitions up to 18 MeV. The
response functions of the individual detectors have been measured with
mono-energetic beams of electrons. Experimental results obtained with 1.6 MeV
protons on targets containing B and F show clear IPC over a wide
angular range. A comparison with GEANT simulations demonstrates that angular
correlations of pairs of transitions in the energy range between 6 and
18 MeV can be determined with sufficient resolution and efficiency to search
for deviations from IPC due to the creation and subsequent decay into
of a hypothetical short-lived neutral boson.Comment: 20 pages, 8 figure
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