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 3 He 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 241 Am–Be neutron field at Politecnico di Milano , which allowed the estimation of the overall uncertainty of the simulated response matrix of about ± 2%.