Investigation of neutron stray fields close to the two meson targets of the PSI proton accelerator facility

The High-Intensity Proton Accelerator Facility at the Paul Scherrer Institute (PSI) accelerates protons to an energy of 590 MeV with currents up to 2.4 mA, i.e. 1.4 MW beam power. The beam feeds four main experiments using individual targets. The areas adjacent to these targets are heavily shielded by several meters of iron and concrete. The neutron stray field at different positions outside the shielding close to two of these targets has been investigated using the PSI extended range Bonner sphere spectrometer (ERBSS). It consists of 10 moderator spheres made of polyethylene and 4 spheres modified with metal shells, enhancing the sensitivity for neutrons with energies >20 MeV. The data was normalized to the proton current measured by a resonance chamber upstream of the first target. Two commercially available survey instruments for neutron radiation constantly monitored the stability of the field during the measurements. The spectral neutron distribution was determined by applying Bayesian methods, which were optimized for measurements of neutron stray fields behind shielding at high-energy accelerators. The measurements within restricted access areas resulted in ambient dose equivalent rates of 25-50 μSv (h mA)-1 with significant contributions of high-energy neutrons. The comparison to doses indicated by a commercially available survey instrument suitable for measurements in fields with a high-energy neutron component showed reasonable agreement with the dose values obtained from the ERBSS measurement. However, it is desirable to apply in-field calibration factors derived from spectrum measurements to reduce the uncertainty of dose values obtained with survey instruments.