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

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

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

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 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, γ ) 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 γ 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

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

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,γ) 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