Construction and characterization of a time-of-flight chopper prototype at the RA-6 experimental reactor of the Bariloche Atomic Centre (Argentina)

Choppers have been key instruments in time-of-flight (TOF) experiments in nuclear research reactors since the mid-20th century. Their main function is to chop the neutron beams at well-defined time intervals, which allows the energy of the neutrons to be determined by measuring the time of flight to the detector positions. This work presents the construction, installation and characterization of NAHUEL, a prototype chopper-based time-of-flight (TOF) instrument, that has recently been installed at the RA-6 experimental reactor of the Bariloche Atomic Centre, Argentina. Firstly, the principle of operation and design details of the chopper are described. This is followed by a description of beam shaping aimed at reducing gamma and fast neutron background, including the collimation and shielding configuration for the incident beam. The design of the detection system and data acquisition electronics for time-of-flight (TOF) spectra determination is also presented. Additionally, the experimental setup for neutron transmission measurements is outlined. The characterization and optimization of key system parameters such as background, energy resolution, and dead time of the acquisition system are addressed. To analyze the measured spectra, a fitting procedure was applied using a model combining a Maxwellian distribution, characterized by a temperature, plus an epithermal component. Transmission measurements in reference materials were performed to determine their total cross sections, which were then compared with previous experimental data. The system was proven to provide reliable measurements of transmission between 10 and 200 meV. Finally, Bragg edge transmission studies of a known polycrystalline material were conducted to assess the system’s energy resolution under different operating conditions.