Characterization of the New n_TOF Neutron Beam: Fluence, Profile and Resolution

Abstract

After a halt of four years, the n_TOF spallation neutron facility at CERN has resumed operation in November 2008 with a new spallation target characterized by an improved safety and engineering design, resulting in a more robust overall performance and effcient cooling. The first measurement during the 2009 run has aimed at the full characterization of the neutron beam. Several detectors, such as calibrated fi ssion chambers, the n_TOF Silicon Monitor, a MicroMegas detector with ¹⁰B and ²³⁵U samples, as well as liquid and solid scintillators have been used in order to characterize the properties of the neutron fluence. The spatial profile of the beam has been studied with a specially designed "X-Y" MicroMegas which provided a 2D image of the beam as a function of neutron energy. Both properties have been compared with simulations performed with the FLUKA code. The characterization of the resolution function is based on results from simulations which have been verified by the study of narrow capture resonances of ⁵⁶Fe, which were measured as part of a new campaign of (n,γ) measurements on Fe and Ni isotopes.