2,624 research outputs found
The 41Ca(n,α)38Ar cross section up to 100 keV neutron energy
The 41Ca(n,alpha)38Ar reaction cross section has been studied with resonance neutrons at the GELINA neutron facility of the Institute for Reference Materials and Measurements in Geel (Belgium) from a few eV up to 100 keV. A Frisch-gridded ionization chamber with methane as detector gas was installed at a 30 meter long flight path. About 20 resonances have been identified. From the cross section data obtained, the Maxwellian averaged cross section (MACS) as a function of stellar temperature has been calculated by numerical integration
245Cm fission cross section measurement in the thermal energy region
A new cross section measurement for the ^245Cm(n,f) reaction in the thermal energy region has been performed at the GELINA neutron facility of the Institute for Reference Materials and Measurements (IRMM) in Geel, Belgium. The energy of the neutrons is determined applying the time of flight method using a flight path length of about 9 m. In the present work, the incident neutron energy covers 10 meV up to a few eV. A 98.48% enriched ^245Cm sample was mounted back-to-back with a ^10B sample in the centre of a vacuum chamber together with two surface barrier detectors positioned outside the neutron beam. One detector measured the ^10B(n,a)^7Li reaction products for the neutron flux determination, while the second one registered the ^245Cm(n,f) fragments. In this way, the neutron flux can be determined simultaneously with the fission fragments. A control measurement has been performed replacing the ^245Cm sample with a ^235U sample in order to check that the well-known ^235U(n,f) cross section can be reproduced. Our measurement yielded a ^245Cm(nth,f) cross section of 2131±43±173 b and a Westcott factor gf=0.939±0.019
Ashkin-Teller universality in a quantum double model of Ising anyons
We study a quantum double model whose degrees of freedom are Ising anyons.
The terms of the Hamiltonian of this system give rise to a competition between
single and double topologies. By studying the energy spectra of the Hamiltonian
at different values of the coupling constants, we find extended gapless regions
which include a large number of critical points described by conformal field
theories with central charge c=1. These theories are part of the Z_2 orbifold
of the bosonic theory compactified on a circle. We observe that the Hilbert
space of our anyonic model can be associated with extended Dynkin diagrams of
affine Lie algebras which yields exact solutions at some critical points. In
certain special regimes, our model corresponds to the Hamiltonian limit of the
Ashkin-Teller model, and hence integrability over a wide range of coupling
parameters is established.Comment: 11 pages, minor revision
A Framework for Directional and Higher-Order Reconstruction in Photoacoustic Tomography
Photoacoustic tomography is a hybrid imaging technique that combines high
optical tissue contrast with high ultrasound resolution. Direct reconstruction
methods such as filtered backprojection, time reversal and least squares suffer
from curved line artefacts and blurring, especially in case of limited angles
or strong noise. In recent years, there has been great interest in regularised
iterative methods. These methods employ prior knowledge on the image to provide
higher quality reconstructions. However, easy comparisons between regularisers
and their properties are limited, since many tomography implementations heavily
rely on the specific regulariser chosen. To overcome this bottleneck, we
present a modular reconstruction framework for photoacoustic tomography. It
enables easy comparisons between regularisers with different properties, e.g.
nonlinear, higher-order or directional. We solve the underlying minimisation
problem with an efficient first-order primal-dual algorithm. Convergence rates
are optimised by choosing an operator dependent preconditioning strategy. Our
reconstruction methods are tested on challenging 2D synthetic and experimental
data sets. They outperform direct reconstruction approaches for strong noise
levels and limited angle measurements, offering immediate benefits in terms of
acquisition time and quality. This work provides a basic platform for the
investigation of future advanced regularisation methods in photoacoustic
tomography.Comment: submitted to "Physics in Medicine and Biology". Changes from v1 to
v2: regularisation with directional wavelet has been added; new experimental
tests have been include
Saturation effect and determination of nuclear matter denisity distribution from optical potential
A refined double folding procedure with density dependence of the effective nucleon- -nucleon interaction included is used to calculate the real part of the alpha particle — Ca potentials. We show that the experimentally determined difference between rms radii of the (real) potentials implies a larger size of the nuclear matter distribution of the Ca nucleus as compared to the Ca nucleus
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