Monte Carlo Simulations of the Energy Resolution Function of n_TOF at CERN

The n_TOF facility is a time-of-flight (TOF) spectrometer dedicated to studying neutron-induced reactions, mainly neutron capture and fission cross sections. The spectrometer consists of a pulsed proton beam (7 × 10¹² protons/pulse, 6-ns width, 20 GeV/c) impinging on an 80 × 80 × 60 cm³ lead target. The neutrons produced by spallation reactions reach the detector station at 185 m through an evacuated tube. There, neutron-induced reactions are studied by using the TOF technique. The facility is unique for its high instantaneous neutron flux (of the order 10⁶ neutrons/cm² per proton pulse at 185 m), an excellent energy resolution, low background conditions, and a very low duty cycle. This combination allows one to measure neutron capture and fission cross sections in the energy range from 1 eV to 250 MeV with high precision.

For the analysis of the data in the resolved resonance region up 1 MeV, a precise and accurate knowledge of the distribution of the energy resolution is mandatory. The only way to obtain the resolution function in a detailed way is to use Monte Carlo simulations together with the experimental verification with well-known resonance reactions at selected energies. Such calculations and an analytical fit of the results have been performed for the target setup of the first phase of data taking.

Monte Carlo simulations performed for the assessment and comparison of the resolution function for different target configurations are reported. The different resolution functions are compared and discussed.