Development of a Fast-Neutron Detector With Silicon Photomultiplier Readout

A robust neutron detector was developed to monitor the neutron flux in fast-neutron/gamma radiography (FNGR) cargo scanners. The main features of the new neutron detector were an ability to handle high count rate for statistical precision and a high damage threshold for longevity when used close to intense fast neutron sources (up to 10¹⁰ n/s). The detector comprises a fast scintillator coupled to a shielded 100 pixel Hamamatsu 1 mm times 1 mm silicon photomultiplier (SiPM) via a 220 cm length of wavelength shifting (WLS) plastic fibre. The WLS fibre was glued onto two long sides of a 10 times 10 times 50 mm scintillator wrapped in Tyvek reflector. Scintillation pulses of up to ~40 photons reached the SiPM. With the detector mounted 60 mm from a Thermo A-325 DT neutron generator (up to 6 times 10⁷ n/s) the count rate was over 4 times 10⁴ counts per second. The SiPM gain was effectively stabilised against temperature variations by automatically trimming the bias voltage with SiPM temperature. Comparison data is provided for a photodiode-scintillator neutron detector placed 134 cm from the A-325 neutron generator and for a PMT-WLS-scintillator neutron detector.