Extended range Bonner sphere spectrometer for high-elevation neutron measurements

A Bonner sphere spectrometer was designed for the measurement of cosmic neutrons at high elevation within the INFN-based project SAMADHA. The spectrometer consists of 8 moderating spheres (6 polyethylene and 2 polyethylene plus high atomic number inserts), each embedding a cylindrical 3He proportional counter. The response matrix was calculated with MCNP6. In view of the very low counting rates expected in the environment, specific design criteria were adopted to prevent non-neutron signals. The spectrometer was exposed in a reference 241Am–Be neutron field at Politecnico di Milano, which allowed the estimation of the overall uncertainty of the simulated response matrix of about ± 2%.