High-accuracy determination of the neutron flux in the new experimental area n_TOF-EAR2 at CERN

Abstract

A new high flux experimental area has recently become operational at the n TOF facility at CERN. This new measuring station, n TOF-EAR2, is placed at the end of a vertical beam line at a distance of approximately 20 m from the spallation target. The characterization of the neutron beam, in terms of flux, spatial profile and resolution function, is of crucial importance for the feasibility study and data analysis of all measurements to be performed in the new area. In this paper, the measurement of the neutron flux, performed with different solid-state and gaseous detection systems, and using three neutronconverting reactions considered standard in different energy regions is reported. The results of the various measurements have been combined, yielding an evaluated neutron energy distribution in a wide energy range, from 2 meV to 100 MeV, with an accuracy ranging from 2%, at low energy, to 6% in the high-energy region. In addition, an absolute normalization of the n TOF-EAR2 neutron flux has been obtained by means of an activation measurement performed with 197 Au foils in the beam.

The fission experimental programme at the CERN n_TOF facility: status and perspectives

“…The results have been reported in Ref. [115]. Figure 10 shows the total number of neutrons per proton bunch in EAR-2 compared with the one in EAR-1, for the more commonly used smallaperture collimators (see Table 1).…”

“…Until today, Micromegas detectors have been used at n_TOF for fission measurements on 240 Pu [116], 242 Pu [138,139] and more recently on 230 Th [140], 237 Np [141] and 241 Am [142], taking advantage of the increased neutron fluence in EAR-2. Furthermore, different setups based on Micromegas detectors have been used for neutron beam characterisation and monitoring [115], for fission tagging in capture measurements of fissile isotopes (see Sect. 4.5), for (n,α) reaction studies [143] and for the characterisation of the spatial profile of the neutron beam using a recently developed variant with both mesh and anode electrodes segmented into strips [144].…”

Experimental setup and procedure for the measurement of the 7Be(n,p)7Li reaction at n_TOF

“…The 6 Li(n,α) data represent a very reliable absolute reference to normalize the 7 Be(n,p) 7 Li data, as the cross-section is an international standard from thermal up to 1 MeV neutron energy [19]. In order to prove their reliability, these data were also used to explicitly reconstruct the 6 Li(n,α) crosssection, by making use of the known neutron flux in EAR2 which had been determined by means of a set of independent measurements based on different reference reactions and employing several different detector technologies [20]. The good agreement between the so obtained cross-section and the standard one reported in the ENDF database [21] is shown in Figure 9, where the dip in the detection efficiency corresponds to the well-known resonance in the 6 Li(n,α) cross-section which has a mostly p-wave forward-backward distribution [22].…”

Development of a Neutron Imaging Station at the n_TOF Facility of CERN and Applications to Beam Intercepting Devices

“…A neutron radiography testing station has been recently developed [4,5] exploiting the neutron beam of the n_TOF Experimental Area 2, located at the shortest distance from the spallation target [6], successfully inspecting the inner structure of several targets previously irradiated at the HiRadMat facility of CERN [7] and of a spent antiproton production target used in the Antiproton Decelerator (AD) target area of CERN. The characteristics of the n_TOF neutron beam for the imaging setup will be here presented, together with the results obtained.…”