High-accuracy determination of the neutron flux at n TOF

Abstract

The neutron flux of the n TOF facility at CERN was measured, after installation of the new spallation target, with four different systems based on three neutron-converting reactions, which represent accepted cross sections standards in different energy regions. A careful comparison and combination of the different measurements allowed us to reach an unprecedented accuracy on the energy dependence of the neutron flux in the very wide range (thermal to 1 GeV) that characterizes the n TOF neutron beam. This is a pre-requisite for the high accuracy of cross section measurements at n TOF. An unexpected anomaly in the neutron-induced fission cross section of 235U is observed in the energy region between 10 and 30 keV, hinting at a possible overestimation of this important cross section, well above currently assigned uncertainties.