424 research outputs found
Electronic structure and magnetic properties of epitaxial FeRh(001) ultra-thin films on W(100)
Epitaxial FeRh(100) films (CsCl structure, thick), prepared
{\it in-situ} on a W(100) single crystal substrate, have been investigated via
valence band and core level photoemission. The presence of the
temperature-induced, first-order, antiferromagnetic to ferromagnetic
(AF FM) transition in these films has been verified via linear
dichroism in photoemission from the Fe 3 levels. Core level spectra indicate
a large moment on the Fe atom, practically unchanged in the FM and AF phases.
Judging from the valence band spectra, the metamagnetic transition takes place
without substantial modification of the electronic structure. In the FM phase,
the spin-resolved spectra compare satisfactorily to the calculated
spin-polarized bulk band structure.Comment: 7 pages, 5 figure
Structural and magnetic dynamics of a laser induced phase transition in FeRh
We use time-resolved x-ray diffraction and magnetic optical Kerr effect to
study the laser induced antiferromagnetic to ferromagnetic phase transition in
FeRh. The structural response is given by the nucleation of independent
ferromagnetic domains (\tau_1 ~ 30ps). This is significantly faster than the
magnetic response (\tau_2 ~ 60ps) given by the subsequent domain realignment.
X-ray diffraction shows that the two phases co-exist on short time-scales and
that the phase transition is limited by the speed of sound. A nucleation model
describing both the structural and magnetic dynamics is presented.Comment: 5 pages, 3 figures - changed to reflect version accepted for PR
Reactor monitoring and safeguards using antineutrino detectors
Nuclear reactors have served as the antineutrino source for many fundamental
physics experiments. The techniques developed by these experiments make it
possible to use these very weakly interacting particles for a practical
purpose. The large flux of antineutrinos that leaves a reactor carries
information about two quantities of interest for safeguards: the reactor power
and fissile inventory. Measurements made with antineutrino detectors could
therefore offer an alternative means for verifying the power history and
fissile inventory of a reactors, as part of International Atomic Energy Agency
(IAEA) and other reactor safeguards regimes. Several efforts to develop this
monitoring technique are underway across the globe.Comment: 6 pages, 4 figures, Proceedings of XXIII International Conference on
Neutrino Physics and Astrophysics (Neutrino 2008); v2: minor additions to
reference
Antineutrino emission and gamma background characteristics from a thermal research reactor
The detailed understanding of the antineutrino emission from research
reactors is mandatory for any high sensitivity experiments either for
fundamental or applied neutrino physics, as well as a good control of the gamma
and neutron backgrounds induced by the reactor operation. In this article, the
antineutrino emission associated to a thermal research reactor: the OSIRIS
reactor located in Saclay, France, is computed in a first part. The calculation
is performed with the summation method, which sums all the contributions of the
beta decay branches of the fission products, coupled for the first time with a
complete core model of the OSIRIS reactor core. The MCNP Utility for Reactor
Evolution code was used, allowing to take into account the contributions of all
beta decayers in-core. This calculation is representative of the isotopic
contributions to the antineutrino flux which can be found at research reactors
with a standard 19.75\% enrichment in U. In addition, the required
off-equilibrium corrections to be applied to converted antineutrino energy
spectra of uranium and plutonium isotopes are provided. In a second part, the
gamma energy spectrum emitted at the core level is provided and could be used
as an input in the simulation of any reactor antineutrino detector installed at
such research facilities. Furthermore, a simulation of the core surrounded by
the pool and the concrete shielding of the reactor has been developed in order
to propagate the emitted gamma rays and neutrons from the core. The origin of
these gamma rays and neutrons is discussed and the associated energy spectrum
of the photons transported after the concrete walls is displayed.Comment: 14 pages, 11 figures, Data in Appendix A and B (13 pages
New antineutrino energy spectra predictions from the summation of beta decay branches of the fission products
In this paper, we study the impact of the inclusion of the recently measured
beta decay properties of the Tc, Mo, and
Nb nuclei in an updated calculation of the antineutrino energy spectra
of the four fissible isotopes U, and Pu. These
actinides are the main contributors to the fission processes in Pressurized
Water Reactors. The beta feeding probabilities of the above-mentioned Tc, Mo
and Nb isotopes have been found to play a major role in the component
of the decay heat of Pu, solving a large part of the
discrepancy in the 4 to 3000\,s range. They have been measured using the Total
Absorption Technique (TAS), avoiding the Pandemonium effect. The calculations
are performed using the information available nowadays in the nuclear
databases, summing all the contributions of the beta decay branches of the
fission products. Our results provide a new prediction of the antineutrino
energy spectra of U, Pu and in particular of U for
which no measurement has been published yet. We conclude that new TAS
measurements are mandatory to improve the reliability of the predicted spectra.Comment: 10 pages, 2 figure
Nuclear break-up of 11Be
The break-up of 11Be was studied at 41AMeV using a secondary beam of 11Be
from the GANIL facility on a 48Ti target by measuring correlations between the
10Be core, the emitted neutrons and gamma rays. The nuclear break-up leading to
the emission of a neutron at large angle in the laboratory frame is identified
with the towing mode through its characteristic n-fragment correlation. The
experimental spectra are compared with a model where the time dependent
Schrodinger equation (TDSE) is solved for the neutron initially in the 11 Be. A
good agreement is found between experiment and theory for the shapes of neutron
experimental energies and angular distributions. The spectroscopic factor of
the 2s orbital is tentatively extracted to be 0.46+-0.15. The neutron emission
from the 1p and 1d orbitals is also studied
First measurements with a new -electron detector for spectral shape studies
The shape of the spectrum corresponding to the electrons emitted in
decay carries a wealth of information about nuclear structure and fundamental
physics. In spite of that, few dedicated measurements have been made of
-spectrum shapes. In this work we present a newly developed detector for
electrons based on a telescope concept. A thick plastic scintillator is
employed in coincidence with a thin silicon detector. First measurements
employing this detector have been carried out with mono-energetic electrons
from the high-energy resolution electron-beam spectrometer at Bordeaux. Here we
report on the good reproduction of the experimental spectra of mono-energetic
electrons using Monte Carlo simulations. This is a crucial step for future
experiments, where a detailed Monte Carlo characterization of the detector is
needed to determine the shape of the -electron spectra by deconvolution
of the measured spectra with the response function of the detector. A chamber
to contain two telescope assemblies has been designed for future -decay
experiments at the Ion Guide Isotope Separator On-Line facility in
Jyv\"askyl\"a, aimed at improving our understanding of reactor antineutrino
spectra
SoLid : Search for Oscillations with Lithium-6 Detector at the SCK-CEN BR2 reactor
Sterile neutrinos have been considered as a possible explanation for the recent reactor and Gallium anomalies arising from reanalysis of reactor flux and calibration data of previous neutrino experiments. A way to test this hypothesis is to look for distortions of the anti-neutrino energy caused by oscillation from active to sterile neutrino at close stand-off (similar to 6-8m) of a compact reactor core. Due to the low rate of anti-neutrino interactions the main challenge in such measurement is to control the high level of gamma rays and neutron background.
The SoLid experiment is a proposal to search for active-to-sterile anti-neutrino oscillation at very short baseline of the SCK center dot CEN BR2 research reactor.
This experiment uses a novel approach to detect anti-neutrino with a highly segmented detector based on Lithium-6. With the combination of high granularity, high neutron-gamma discrimination using 6LiF:ZnS(Ag) and precise localization of the Inverse Beta Decay products, a better experimental sensitivity can be achieved compared to other state-of-the-art technology. This compact system requires minimum passive shielding allowing for very close stand off to the reactor. The experimental set up of the SoLid experiment and the BR2 reactor will be presented. The new principle of neutrino detection and the detector design with expected performance will be described. The expected sensitivity to new oscillations of the SoLid detector as well as the first measurements made with the 8 kg prototype detector deployed at the BR2 reactor in 2013-2014 will be reported
Substrate Induced Strain Field in FeRh Epilayers Grown on Single Crystal MgO (001) Substrates
Equi-atomic FeRh is highly unusual in that it undergoes a first order meta-magnetic phase transition from an antiferromagnet to a ferromagnet above room temperature (Tr ≈ 370 K). This behavior opens new possibilities for creating multifunctional magnetic and spintronic devices which can utilise both thermal and applied field energy to change state and functionalise composites. A key requirement in realising multifunctional devices is the need to understand and control the properties of FeRh in the extreme thin film limit (tFeRh < 10 nm) where interfaces are crucial. Here we determine the properties of FeRh films in the thickness range 2.5–10 nm grown directly on MgO substrates. Our magnetometry and structural measurements show that a perpendicular strain field exists in these thin films which results in an increase in the phase transition temperature as thickness is reduced. Modelling using a spin dynamics approach supports the experimental observations demonstrating the critical role of the atomic layers close to the MgO interface
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