109 research outputs found
The n_TOF facility at CERN: performances and first physics results
The neutron Time of Flight (n_TOF) facility at CERN is a source of a wide-range (1 eV<En<250 MeV) flux
of neutrons, generated by spallation of 20-GeV/c protons onto a solid lead target. The goal of the n_TOF is to provide
unprecedented precision in neutron kinetic energy determination, which will in turn bring the much-needed precision in
neutron-induced cross-section measurements.
The unique features of the n_TOF facility (instantaneously very intense neutron flux, low duty cycle, high resolution,
and low background) make possible the measurement of highly radioactive isotopes usually available in small quantities.
Such measurements are vital for a range of studies in fields as diverse as nuclear technology, astrophysics, and
fundamental nuclear physics.
In this paper, the characteristics of the n_TOF facility will be described, together with the main features of the highperformance
detectors and acquisition system used for cross-section measurements, with a summary of the results and
experience acquired during the first years of operation.Postprint (published version
Measurements of 90,91,92,93,94,96-Zr neutron capture cross sections at the n_TOF facility at CERN
Neutron capture cross sections for zirconium isotopes have important implications in the field of
nuclear astrophysics. In particular they play a key role in ascertaining the neutron density in the
He burning zone of the Red Giant stars. Neutron capture cross sections of 90,91,92,93,94,96Zr have
been measured over the energy range from 1 eV to 1 MeV at the spallation neutron facility
n_TOF at CERN. Based on these data, capture resonance strengths and Maxwellian-averaged
cross sections were determined with much improved accuracy.Postprint (published version
Measurement of the 197-Au(n,gamma ) cross section at n_TOF, - a step forward towards a new standard for capture cross section measurements
The accurate knowledge of the 197Au(n,g ) reaction cross section is of great importance, since this
reaction if often used as a reference in capture cross section measurements relevant to Nuclear
Astrophysics, as well as for neutron flux determination in nuclear power reactors. With the aim
of improving the accuracy of the neutron capture cross section on 197Au, extensive measurements
were performed at the n_TOF facility at CERN with two detection systems: a total absorption
calorimeter and a set of C6D6 detectors. The capture yield and the resonance parameters have
been determined in the energy range from 1 eV to 5 keV and compared with evaluated data
files. The present capture kernels are on average in good agreement with tabulated data, although
sizable differences are observed for several resonances. A few new resonances are also reported.Postprint (published version
Neutron cross section measurements at n-TOF for ADS related studies
A neutron Time-of-Flight facility (n_TOF) is available at CERN since 2001. The
innovative features of the neutron beam, in particular the high instantaneous flux, the wide
energy range, the high resolution and the low background, make this facility unique for
measurements of neutron induced reactions relevant to the field of Emerging Nuclear
Technologies, as well as to Nuclear Astrophysics and Fundamental Nuclear Physics. The
scientific motivations that have led to the construction of this new facility are here presented.
The main characteristics of the n_TOF neutron beam are described, together with the features
of the experimental apparata used for cross-section measurements. The main results of the first
measurement campaigns are presented. Preliminary results of capture cross-section
measurements of minor actinides, important to ADS project for nuclear waste transmutation,
are finally discussed.Postprint (published version
Neutron capture measurement on the s-process termination isotopes lead and bismuth
Resonance cross sections relevant for the termination of the s-process reaction path have been
determined for 204,206,207Pb and 209Bi at the CERN neutron time-of-flight spectrometer n_TOF.
Themeasurementswere carried out in the neutron energy range from1 eV up to 500 keV. By using
a system of C6D6-detectors with optimized neutron sensitivity, the main corrections of previous
measurements related to backgrounds from scattered neutrons could be practically eliminated.
Other corrections were thoroughly treated by control measurements with additional samples for
determination of the ambient background and of background from in-beam g -rays as well as by
detailed analyses via Monte Carlo simulations. The Maxwellian averaged cross sections of the
four isotopes and their implications for the s-process abundance contributions in the Pb/Bi region
are discussed.Postprint (published version
Neutron Capture cross section of 90Zr: Bottleneck in the s-process reaction flow
The neutron capture cross sections of the Zr isotopes have important implications in nuclear astrophysics and
for reactor design. The small cross section of the neutron magic nucleus 90Zr, which accounts for more than 50%
of natural zirconium represents one of the key isotopes for the stellar s-process, because it acts as a bottleneck
in the neutron capture chain between the Fe seed and the heavier isotopes. The same element, Zr, also is an
important component of the structural materials used in traditional and advanced nuclear reactors. The (n, Îł )
cross section has been measured at CERN, using the n TOF spallation neutron source. In total, 45 resonances
could be resolved in the neutron energy range below 70 keV, 10 being observed for the first time thanks to the
high resolution and low backgrounds at n TOF. On average, the Îł widths obtained in resonance analyses with
the R-matrix code SAMMY were 15% smaller than reported previously. By these results, the accuracy of the
Maxwellian averaged cross section for s-process calculations has been improved by more than a factor of 2.Postprint (published version
Experimental challenges for the Re/Os clock
The stellar neutron capture cross sections of 186Os and 187Os are fundamental for the Re/Os
cosmo-chronometer since they are de ning the s-process abundance of 187Os. Subtraction of the
s-component from the solar 187Os abundance yields the radiogenic contribution to 187Os due to
b -decays of 187Re (t1=2 = 42.3 Gy) originating from r-process nucleosynthesis. The laboratory
cross section of 187Os requires a signi cant correction for the effect of the low-lying excited state
at 9.75 keV, which is strongly populated under stellar conditions. This reliability of the theoretical
correction can be greatly improved by an experimental determination of the cross section for inelastic
scattering to the 9.75 keV state. In this work, high resolution time-of- ight measurements
of (n;g ) cross sections of 186;187;188Os from 1 eV to 1 MeV performed at the CERN n_TOF facility
are reported. The inferred stellar cross sections differ from previously recommended values.
In addition, the inelastic scattering cross section has been measured at 30 keV neutron energy via
time-of- ight at the Karlsruhe 3.7 MV Van de Graaff. The implications of these results for the
Re/Os clock are brie y discussed.Postprint (published version
Measurement of the neutron capture cross section of 236-U
In this paper we describe the 236U(n,Îł) reaction cross section measurement at
the GELINA white pulsed neutron source of the Institute for Reference Materials
and Measurements (IRMM) in Geel. The sample was placed in the neutron
beam at a flight station located at a nominal distance of 30 m from the neutron
source. Neutron capture gamma rays were detected by two C6D6-based liquid
scintillator gamma-ray detectors as a function of the neutron time-of-flight
using the pulse height weighting technique. The pulse height weighting function
has been derived from Monte Carlo simulations of the detector response
to mono-energetic gamma rays. The shape of the neutron flux was measured
with a 10B chamber, placed about 60 cm upstream in the neutron beam. The
capture yield in the resolved resonance region up to 3 keV has been derived
and will be presented here. The analysis of the capture yield in terms of Rmatrix
resonance parameters is planned for the near future.Postprint (published version
Measurements of the 90,91,92,94,96Zr neutron capture cross-section at n_TOF
The neutron capture cross sections of 90,91,92,94,96Zr, which play a key role for the determination
of the neutron density in the He burning zone of the Red Giant stars, were measured over the
energy range from 1 eV to 1 MeV at the spallation neutron facility n_TOF at CERN. Based on
these data the capture resonance strength and the Maxwellian-averaged cross section at 30 keV
were calculated.Postprint (published version
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