8,831 research outputs found
Search for low lying dipole strength in the neutron rich nucleus Ne
Coulomb excitation of the exotic neutron-rich nucleus Ne on a
Pb target was measured at 58 A.MeV in order to search for low-lying E1
strength above the neutron emission threshold. Data were also taken on an
Al target to estimate the nuclear contribution. The radioactive beam
was produced by fragmentation of a 95 A.MeV Ar beam delivered by the
RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged
fragment hodoscope and a neutron wall. Using the invariant mass method in the
Ne+n channel, we observe a sizable amount of E1 strength between 6 and
10 MeV. The reconstructed Ne angular distribution confirms its E1
nature. A reduced dipole transition probability of B(E1)=0.490.16
is deduced. For the first time, the decay pattern of low-lying
strength in a neutron-rich nucleus is obtained. The results are discussed in
terms of a pygmy resonance centered around 9 MeV
Stresses in isostatic granular systems and emergence of force chains
Progress is reported on several questions that bedevil understanding of
granular systems: (i) are the stress equations elliptic, parabolic or
hyperbolic? (ii) how can the often-observed force chains be predicted from a
first-principles continuous theory? (iii) How to relate insight from isostatic
systems to general packings? Explicit equations are derived for the stress
components in two dimensions including the dependence on the local structure.
The equations are shown to be hyperbolic and their general solutions, as well
as the Green function, are found. It is shown that the solutions give rise to
force chains and the explicit dependence of the force chains trajectories and
magnitudes on the local geometry is predicted. Direct experimental tests of the
predictions are proposed. Finally, a framework is proposed to relate the
analysis to non-isostatic and more realistic granular assemblies.Comment: 4 pages, 2 figures, Corrected typos and clkearer text, submitted to
Phys. Rev. Let
Multifractal Dimensions for Branched Growth
A recently proposed theory for diffusion-limited aggregation (DLA), which
models this system as a random branched growth process, is reviewed. Like DLA,
this process is stochastic, and ensemble averaging is needed in order to define
multifractal dimensions. In an earlier work [T. C. Halsey and M. Leibig, Phys.
Rev. A46, 7793 (1992)], annealed average dimensions were computed for this
model. In this paper, we compute the quenched average dimensions, which are
expected to apply to typical members of the ensemble. We develop a perturbative
expansion for the average of the logarithm of the multifractal partition
function; the leading and sub-leading divergent terms in this expansion are
then resummed to all orders. The result is that in the limit where the number
of particles n -> \infty, the quenched and annealed dimensions are {\it
identical}; however, the attainment of this limit requires enormous values of
n. At smaller, more realistic values of n, the apparent quenched dimensions
differ from the annealed dimensions. We interpret these results to mean that
while multifractality as an ensemble property of random branched growth (and
hence of DLA) is quite robust, it subtly fails for typical members of the
ensemble.Comment: 82 pages, 24 included figures in 16 files, 1 included tabl
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Vaccine uptake in the Irish Travelling community: an audit of general practice records.
BACKGROUND: Compared to the general population, the Traveller community has substantial health inequalities. Vaccination coverage in Traveller children is estimated to be low and Travellers are at higher risk of vaccine-preventable diseases due to their social circumstances. METHODS: Audit of vaccination history of Traveller (n = 214) and non-Traveller (n = 776) children registered at a general practice in England. The Green Book childhood immunization schedule was used as a reference standard. RESULTS: There was significantly lower coverage for Traveller children compared to non-Traveller children for all vaccinations in the routine childhood immunization schedule. The percentage of children completing the schedule at all time points was significantly lower in the Traveller community. CONCLUSIONS: Traveller communities have significantly lower uptake of vaccinations, and therefore Travellers' children should be targeted by general practitioners for catch-up vaccination to improve outcomes for individuals and local herd immunity.This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Oxford University Press
The Role of Friction in Compaction and Segregation of Granular Materials
We investigate the role of friction in compaction and segregation of granular
materials by combining Edwards' thermodynamic hypothesis with a simple
mechanical model and mean-field based geometrical calculations. Systems of
single species with large friction coefficients are found to compact less.
Binary mixtures of grains differing in frictional properties are found to
segregate at high compactivities, in contrary to granular mixtures differing in
size, which segregate at low compactivities. A phase diagram for segregation
vs. friction coefficients of the two species is generated. Finally, the
characteristics of segregation are related directly to the volume fraction
without the explicit use of the yet unclear notion of compactivity.Comment: 9 pages, 6 figures, submitted to Phys. Rev.
Nonequilibrium brittle fracture propagation: Steady state, oscillations and intermittency
A minimal model is constructed for two-dimensional fracture propagation. The
heterogeneous process zone is presumed to suppress stress relaxation rate,
leading to non-quasistatic behavior. Using the Yoffe solution, I construct and
solve a dynamical equation for the tip stress. I discuss a generic tip velocity
response to local stress and find that noise-free propagation is either at
steady state or oscillatory, depending only on one material parameter. Noise
gives rise to intermittency and quasi-periodicity. The theory explains the
velocity oscillations and the complicated behavior seen in polymeric and
amorphous brittle materials. I suggest experimental verifications and new
connections between velocity measurements and material properties.Comment: To appear in Phys. Rev. Lett., 6 pages, self-contained TeX file, 3
postscript figures upon request from author at [email protected] or
[email protected], http://cnls-www.lanl.gov/homepages/rafi/rafindex.htm
Probing pre-formed alpha particles in the ground state of nuclei
In this Letter, we report on alpha particle emission through the nuclear
break-up in the reaction 40Ca on a 40Ca target at 50A MeV. It is observed that,
similarly to nucleons, alpha particles can be emitted to the continuum with
very specific angular distribution during the reaction. The alpha particle
properties can be understood as resulting from an alpha cluster in the daughter
nucleus that is perturbed by the short range nuclear attraction of the
collision partner and emitted. A time-dependent theory that describe the alpha
particle wave-function evolution is able to reproduce qualitatively the
observed angular distribution. This mechanism offers new possibilities to study
alpha particle properties in the nuclear medium.Comment: 4 pages, 3 figure
Structure of unbound neutron-rich He studied using single-neutron transfer
The 8He(d,p) reaction was studied in inverse kinematics at 15.4A MeV using
the MUST2 Si-CsI array in order to shed light on the level structure of 9He.
The well known 16O(d,p)17O reaction, performed here in reverse kinematics, was
used as a test to validate the experimental methods. The 9He missing mass
spectrum was deduced from the kinetic energies and emission angles of the
recoiling protons. Several structures were observed above the neutron-emission
threshold and the angular distributions were used to deduce the multipolarity
of the transitions. This work confirms that the ground state of 9He is located
very close to the neutron threshold of 8He and supports the occurrence of
parity inversion in 9He.Comment: Exp\'erience GANIL/SPIRAL1/MUST
Spontaneous state switching in realistic mean-field model of visual cortex with heteroclinic channel
Proton Driven Plasma Wakefield Acceleration
Plasma wakefield acceleration, either laser driven or electron-bunch driven,
has been demonstrated to hold great potential. However, it is not obvious how
to scale these approaches to bring particles up to the TeV regime. In this
paper, we discuss the possibility of proton-bunch driven plasma wakefield
acceleration, and show that high energy electron beams could potentially be
produced in a single accelerating stage.Comment: 13 pages, 4 figure
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