1,522 research outputs found
A method to correct differential nonlinearities in subranging analog-to-digital converters used for digital gamma-ray spectroscopy
The influence on -ray spectra of differential nonlinearities (DNL) in
subranging, pipelined analog-to-digital converts (ADCs) used for digital
-ray spectroscopy was investigated. The influence of the DNL error on
the -ray spectra, depending on the input count-rate and the dynamic
range has been investigated systematically. It turned out, that the DNL becomes
more significant in -ray spectra with larger dynamic range of the
spectroscopy system. An event-by-event offline correction algorithm was
developed and tested extensively. This correction algorithm works especially
well for high dynamic ranges
Velocity correlations in dense granular gases
We report the statistical properties of spherical steel particles rolling on
an inclined surface being driven by an oscillating wall. Strong dissipation
occurs due to collisions between the particles and rolling and can be tuned by
changing the number density. The velocities of the particles are observed to be
correlated over large distances comparable to the system size. The distribution
of velocities deviates strongly from a Gaussian. The degree of the deviation,
as measured by the kurtosis of the distribution, is observed to be as much as
four times the value corresponding to a Gaussian, signaling a significant
breakdown of the assumption of negligible velocity correlations in a granular
system.Comment: 4 pages, 4 Figure
Collision statistics of driven granular materials
We present an experimental investigation of the statistical properties of
spherical granular particles on an inclined plane that are excited by an
oscillating side-wall. The data is obtained by high-speed imaging and particle
tracking techniques. We identify all particles in the system and link their
positions to form trajectories over long times. Thus, we identify particle
collisions to measure the effective coefficient of restitution and find a broad
distribution of values for the same impact angles. We find that the energy
inelasticity can take on values greater than one, which implies that the
rotational degrees play an important role in energy transfer. We also measure
the distance and the time between collision events in order to directly
determine the distribution of path lengths and the free times. These
distributions are shown to deviate from expected theoretical forms for elastic
spheres, demonstrating the inherent clustering in this system. We describe the
data with a two-parameter fitting function and use it to calculated the mean
free path and collision time. We find that the ratio of these values is
consistent with the average velocity. The velocity distribution are observed to
be strongly non-Gaussian and do not demonstrate any apparent universal
behavior. We report the scaling of the second moment, which corresponds to the
granular temperature, and higher order moments as a function of distance from
the driving wall. Additionally, we measure long time correlation functions in
both space and in the velocities to probe diffusion in a dissipative gas.Comment: 12 pages, 4 figures, uses revtex
Non-Gaussian velocity distributions in excited granular matter in the absence of clustering
The velocity distribution of spheres rolling on a slightly tilted rectangular
two dimensional surface is obtained by high speed imaging. The particles are
excited by periodic forcing of one of the side walls. Our data suggests that
strongly non-Gaussian velocity distributions can occur in dilute granular
materials even in the absence of significant density correlations or
clustering. When the surface on which the particles roll is tilted further to
introduce stronger gravitation, the collision frequency with the driving wall
increases and the velocity component distributions approach Gaussian
distributions of different widths.Comment: 4 pages, 5 figures. Additional information at
http://physics.clarku.edu/~akudrolli/nls.htm
Propagating front in an excited granular layer
A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated
vertically on a flat plate, shows remarkable ordering transitions and
cooperative behavior just below 1g maximum acceleration. We study the stability
of a quiescent disordered or ``amorphous'' state formed when the acceleration
is switched off in the excited ``gaseous'' state. The transition from the
amorphous state back to the gaseous state upon increasing the plate's
acceleration is generally subcritical: An external perturbation applied to one
bead initiates a propagating front that produces a rapid transition. We measure
the front velocity as a function of the applied acceleration. This phenomenon
is explained by a model based on a single vibrated particle with multiple
attractors that is perturbed by collisions. A simulation shows that a
sufficiently high rate of interparticle collisions can prevent trapping in the
attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May
199
Grain Dynamics in a Two-dimensional Granular Flow
We have used particle tracking methods to study the dynamics of individual
balls comprising a granular flow in a small-angle two-dimensional funnel. We
statistically analyze many ball trajectories to examine the mechanisms of shock
propagation. In particular, we study the creation of, and interactions between,
shock waves. We also investigate the role of granular temperature and draw
parallels to traffic flow dynamics.Comment: 17 pages, 24 figures. To appear in Phys.Rev.E. High res./color
figures etc. on http://www.nbi.dk/CATS/Granular/GrainDyn.htm
Shape coexistence in the very neutron-rich odd-odd
Microsecond isomers of neutron-rich nuclei in the masses A=96 and 98 were reinvestigated at the ILL reactor (Grenoble). These nuclei were produced by thermal-neutron induced fission of Pu. The detection is based on time correlation between fission fragments selected by the Lohengrin mass spectrometer, and the rays and conversion electrons from the isomers. A new level scheme of Rb is proposed. We have found that the ground state and low-lying levels of this nucleus are rather spherical, while a rotational band develops at 461 keV energy. This bans has properties consistent with a Nilsson assignment and a deformation . It is fed by a microsecond isomer consistent with a sperical configuration. It is interesting to note that the same unique-parity states and are present in the same nucleus in a deformed and in a spherical configuration. The neighbouring odd-odd nucleus Y presents a strong analogy with Rb and is also discussed
Characterization of an Oct1 orthologue in the channel catfish, Ictalurus punctatus: A negative regulator of immunoglobulin gene transcription?
BACKGROUND: The enhancer (Eμ3') of the immunoglobulin heavy chain locus (IGH) of the channel catfish (Ictalurus punctatus) has been well characterized. The functional core region consists of two variant Oct transcription factor binding octamer motifs and one E-protein binding μE5 site. An orthologue to the Oct2 transcription factor has previously been cloned in catfish and is a functionally active transcription factor. This study was undertaken to clone and characterize the Oct1 transcription factor, which has also been shown to be important in driving immunoglobulin gene transcription in mammals. RESULTS: An orthologue of Oct1, a POU family transcription factor, was cloned from a catfish macrophage cDNA library. The inferred amino acid sequence of the catfish Oct1, when aligned with other vertebrate Oct1 sequences, revealed clear conservation of structure, with the POU specific subdomain of catfish Oct1 showing 96% identity to that of mouse Oct1. Expression of Oct1 was observed in clonal T and B cell lines and in all tissues examined. Catfish Oct1, when transfected into both mammalian (mouse) and catfish B cell lines, unexpectedly failed to drive transcription from three different octamer-containing reporter constructs. These contained a trimer of octamer motifs, a fish V(H )promoter, and the core region of the catfish Eμ3' IGH enhancer, respectively. This failure of catfish Oct1 to drive transcription was not rescued by human BOB.1, a co-activator of Oct transcription factors that stimulates transcription driven by catfish Oct2. When co-transfected with catfish Oct2, Oct1 reduced Oct2 driven transcriptional activation. Electrophoretic mobility shift assays showed that catfish Oct1 (native or expressed in vitro) bound both consensus and variant octamer motifs. Putative N- and C-terminal activation domains of Oct1, when fused to a Gal4 DNA binding domain and co-transfected with Gal4-dependent reporter constructs were transcriptionally inactive, which may be due in part to a lack of residues associated with activation domain function. CONCLUSION: An orthologue to mammalian Oct1 has been found in the catfish. It is similar to mammalian Oct1 in structure and expression. However, these results indicate that the physiological functions of catfish Oct1 differ from those of mammalian Oct1 and include negative regulation of transcription
Identification of mixed-symmetry states in an odd-mass nearly-spherical nucleus
The low-spin structure of 93Nb has been studied using the (n,n' gamma)
reaction at neutron energies ranging from 1.5 to 3.0 MeV and the 94Zr(p,2n
gamma)93Nb reaction at bombarding energies from 11.5 to 19 MeV. States at
1779.7 and 1840.6 keV, respectively, are proposed as mixed-symmetry states
associated with the coupling of a proton hole in the p_1/2 orbit to the 2+_1,ms
state in 94Mo. These assignments are derived from the observed M1 and E2
transition strengths to the symmetric one-phonon states, energy systematics,
spins and parities, and comparison with shell model calculations.Comment: 5 pages, 3 figure
Clustering transitions in vibro-fluidized magnetized granular materials
We study the effects of long range interactions on the phases observed in
cohesive granular materials. At high vibration amplitudes, a gas of magnetized
particles is observed with velocity distributions similar to non-magnetized
particles. Below a transition temperature compact clusters are observed to form
and coexist with single particles. The cluster growth rate is consistent with a
classical nucleation process. However, the temperature of the particles in the
clusters is significantly lower than the surrounding gas, indicating a
breakdown of equipartition. If the system is quenched to low temperatures, a
meta-stable network of connected chains self-assemble due to the anisotropic
nature of magnetic interactions between particles.Comment: 4 pages, 5 figure
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