The n_TOF Facility at CERN: Performances and First Physics Results

Abstract

The neutron Time of Flight (n_TOF) facility at CERN is a source of a wide-range (1 eV<E n <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.

Neutron-induced fission cross section ofnatPb andBi209from threshold to 1 GeV: An

“…Presently, n TOF is the only neutron facility that can cover the whole energy range from thermal energy to 1 GeV. More detailed descriptions can be found in the literature [6,7].…”

Au197(n,γ) cross section in the unresolved resonance region

“…These γ rays originate mostly from neutron captures in the cooling water or in the spallation target during and after the moderation process and reach the experimental area at the same time as neutrons in the energy range between 1 and 100 keV. The photon energy spectrum, which exhibits a prominent peak at 2.2 MeV from capture events in hydrogen, extends up to several MeV [12] and is energetic enough to be registered with considerable efficiency above the 200 keV threshold used in data analysis. It turned out that the spectrum taken with the Pb sample shown in Fig.…”

“…The evacuated beam pipes were stainless steel tubes (wall thickness of 0.5 mm) with 75-µm-thick Kapton windows. To avoid angular distribution effects for resonances with > 0 and to reduce the background due to in-beam γ rays from neutron captures in the water moderator [12], which are scattered by the sample preferentially in forward direction, the detectors were mounted 9.5 cm upstream of the sample, corresponding to an effective angle of 125 • relative to direction of the neutron beam.…”

“…It was determined by a series of GEANT3 [34] simulations based on the in-beam γ -ray spectrum from Ref. [12] and on a detailed replica of the experimental setup. The simulated events were analyzed in the same way as the real data, with a threshold of 200 keV on the deposited energy in the C 6 D 6 detectors.…”

The33S(n,α)30Si cross section measurement at n_TOF-EAR2 (CERN): From 0.01 eV to the resonance region

“…The neutron beam characteristics, the state-of-the-art of detectors and data acquisition systems make n TOF a unique facility for measuring neutron induced reaction cross section of very radioctive isotopes as for identifying and studying resonances in neutron cross sections. During the first 13 years of operation, the only experimental room was located underground at 185 m from the spallation target along the horizontal direction, n TOF-EAR1 [8], characterized by a high instantaneous flux (10 6 neutrons/bunch), a wide neutron energy range (from 25 meV to over 1 GeV) and good energy resolution for most of the energy range, E/E=10 −4 [9]. In 2014, a new experimental hall, n TOF-EAR2 [10,11], was built at 20 m from the target above the ground in the vertical direction.…”