148 research outputs found

### Application of the EXtrapolated Efficiency Method (EXEM) to infer the gamma-cascade detection efficiency in the actinide region

The study of transfer-induced gamma-decay probabilities is very useful for
understanding the surrogate-reaction method and, more generally, for
constraining statistical-model calculations. One of the main difficulties in
the measurement of gamma-decay probabilities is the determination of the
gamma-cascade detection efficiency. In [Nucl. Instrum. Meth. A 700, 59 (2013)]
we developed the Extrapolated Efficiency Method (EXEM), a new method to measure
this quantity. In this work, we have applied, for the first time, the EXEM to
infer the gamma-cascade detection efficiency in the actinide region. In
particular, we have considered the 238U(d,p)239U and 238U(3He,d)239Np
reactions. We have performed Hauser-Feshbach calculations to interpret our
results and to verify the hypothesis on which the EXEM is based. The
determination of fission and gamma-decay probabilities of 239Np below the
neutron separation energy allowed us to validate the EXEM

### Investigation of the 238U(d, p) surrogate reaction via the simultaneous measurement of γ -decay and fission probabilities

We investigated the 238U(d,p) reaction as a surrogate for the n + 238U reaction. For this purpose we measured
for the first time the γ -decay and fission probabilities of 239U∗ simultaneously and compared them to the
corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as
well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuumdiscretized
coupled-channels method and the distorted-wave Born approximation (DWBA) were used to correct
our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where fission
and γ emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas
the γ -decay probability is much higher than the neutron-induced data. We have performed calculations of the
decay probabilities with the statistical model and of the average angular momentum populated in the 238U(d,p)
reaction with the DWBA to interpret these resultsComisión Europea 26949

### Scissors resonance in the quasi-continuum of Th, Pa and U isotopes

The gamma-ray strength function in the quasi-continuum has been measured for
231-233Th, 232,233Pa and 237-239U using the Oslo method. All eight nuclei show
a pronounced increase in gamma strength at omega_SR approx 2.4 MeV, which is
interpreted as the low-energy M1 scissors resonance (SR). The total strength is
found to be B_SR = 9-11 mu_N^2 when integrated over the 1 - 4 MeV gamma-energy
region. The SR displays a double-hump structure that is theoretically not
understood. Our results are compared with data from (gamma, gamma') experiments
and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia.Comment: 11 pages, 9 figure

### Density Matrix Renormalization Group Applied to the Ground State of the XY-Spin-Peierls System

We use the density matrix renormalization group (DMRG) to map out the ground
state of a XY-spin chain coupled to dispersionless phonons of frequency $%
\omega$. We confirm the existence of a critical spin-phonon coupling $% \alpha
_c\propto \omega ^{0.7}$ for the onset of the spin gap bearing the signature of
a Kosterlitz-Thouless transition. We also observe a classical-quantum crossover
when the spin-Peierls gap $\Delta$ is of order $% \omega$. In the classical
regime, $\Delta >\omega$, the mean-field parameters are strongly renormalized
by non-adiabatic corrections. This is the first application of the DMRG to
phonons.Comment: 10 pages, 5 figures. To be published in PR

### A detector system for 'absolute' measurements of fission cross sections at n_TOF in the energy range below 200 MeV

A new measurement of the $^{235}$U(n,f) cross section was performed at the
neutron time-of-flight facility n_TOF at CERN. The experiment focused on
neutron energies from 20 MeV to several hundred MeV, and was normalized to
neutron scattering on hydrogen. This is a measurement first of its kind at this
facility, in an energy range that was until now not often explored, so the
detector development phase was crucial for its success. Two detectors are
presented, a parallel plate fission chamber (PPFC) and a recoil proton
telescope (RPT), both dedicated to perform measurements in the incident neutron
energy range from 30 MeV to 200 MeV. The experiment was designed to minimize
statistical uncertainties in the allocated run time. Several efforts were made
to ensure that the systematic effects were understood and under control. The
results show that the detectors are suited for measurements at n_TOF above 30
MeV, and indicate the path for possible future lines of development.Comment: Added acknowledgement to Euratom fundin

### X-ray Investigation of the Magneto-elastic Instability of alpha'-NaV2O5

We present an X-ray diffuse scattering study of the pretransitional
structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5.
This transition is characterized by the appearance below Tsp~35K of satellite
reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of
structural fluctuations is measured up to 90 K. These fluctuations are three
dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K
the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which
reveals the importance of transverse interactions in the stabilization of the
low temperature phase. We discuss our results within the framework of recent
theories dealing with the simultaneous occurrence of a charge ordering, a spin
gap and a lattice distortion in this intriguing compound.Comment: Accepted in PRB Rapid.comm. Corrected typos, references added,
figures improve

### Recoil Proton Telescopes and Parallel Plate Avalanche Counters for the $^{235}$U(n,f) cross section measurement relative to H(n,n)H between 10 and 450 MeV neutron energy

With the aim of measuring the $^{235}$U(n,f) cross section at the n\_TOF
facility at CERN over a wide neutron energy range, a detection system
consisting of two fission detectors and three detectors for neutron flux
determination was realized. The neutron flux detectors are Recoil Proton
Telescopes (RPT), based on scintillators and solid state detectors, conceived
to detect recoil protons from the neutron-proton elastic scattering reaction.
This system, along with a fission chamber and an array of parallel plate
avalanche counters for fission event detection, was installed for the
measurement at the n\_TOF facility in 2018, at CERN.
An overview of the performances of two RPTs - especially developed for this
measurement - and of the parallel plate avalanche counters are described in
this article. In particular, the characterization in terms of detection
efficiency by Monte Carlo simulations and response to neutron beam, the study
of the background, dead time correction and characterization of the samples,
are reported. The results of the present investigation show that the
performances of these detectors are suitable for accurate measurements of
fission reaction cross sections in the range from 10 to 450~MeV

### Fission cross section measurements for 240Pu, 242Pu

This report comprises the deliverable 1.5 of the ANDES project (EURATOM contract FP7-249671) of Task 3 "High accuracy measurements for fission" of Work Package 1 entitled "Measurements for advanced reactor systems". This deliverables provide evidence of a successful completion of the objectives of Task 3.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

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