Development of a capture-gated fast neutron detector with pulse shape discrimination using digital pulse processing
This study explores the use of digital pulse processing techniques for n/y pulse shape
discrimination (PSD) in liquid scintillators, and the application of these techniques to a
capture-gated fast neutron monitor developed using an enriched '°B-loaded liquid scintillator
(BC523A). The motivation for this study has been to develop a computationally-fast digital
PSD algorithm, which can be used to detect a weak neutron flux in the presence of a strong
gamma ray background and to assess its suitability for use as a portable neutron monitor for
fast neutron dosimetry.
BC523A can operate as a full-energy neutron spectrometer when used in the 'capturegated'
mode, where a characteristic capture time is observed between the proton recoil and
neutron capture pulses, thus producing a very clean signature for those fast neutrons which
are completely moderated within the detector volume. The use of digital waveform capture of
this double-pulse sequence is a powerful technique that allows acquiring both the timestamped
pulse amplitudes and the capture lifetime in a single data set. The capture-gated
performance of a 105 cm' BC523A detector was investigated using fast neutrons from an
Am-Be source. The measured mean neutron capture time in BC523A was 470±80 ns, which
is a factor of 5 shorter than that reported for liquid scintillators loaded with natural boron.
Due to its limited neutron detection efficiency, an extension of this technique to a large
volume (685 cm) BC523A was developed, and provided an efficiency increase by a factor of
7. The efficiency enhancement was modelled using MCNP-4C.
Good n/y separation was obtained using digital PSD applied to BC523A. The PSD figure of-
merit (FOM) was investigated for various organic scintillators, and compared between
digital and analogue pulse processing techniques. The application of digital PSD to the
capture-gate detection mode was investigated, as an additional method for suppression of