Identification of shielded neutron sources with the liquid scintillator BC-501A using a digital pulse shape discrimination method

Fast and unambiguous identification of shielded neutron sources is of paramount importance in nuclear nonproliferation and international safeguards applications. Usually, the identification of neutron spectra relies on unfolding procedures that are unstable and time consuming. In this paper, we present a new application of an existing technique, which can be used for neutron source identification by direct analysis of the pulse height spectrum, i.e., without resorting to energy spectrum unfolding. This technique is based on the acquisition of neutron and γ-ray pulses by using a liquid scintillation detector and a fast waveform digitizer. An optimized digital pulse shape discrimination method based on standard charge integration is used to discriminate neutrons from γ-rays. The neutron pulses are then analyzed to generate a pulse height distribution. The accuracy of the neutron source identification technique was tested on three neutron sources: Cf-252, Am-Be, and Am-Li. Several source-shielding configurations were tested to assess the influence of potential shielding of the source and the sensitivity of the technique. For this investigation, lead and polyethylene shielding blocks were used. The measured pulse height distributions were compared with the distributions simulated with the MCNP-PoliMi code, and very good agreement was obtained. The results show that for both shielded and unshielded configurations the identification of Cf-252, Am-Be, and Am-Li shielded sources is easily achievable by direct analysis of the measured pulse height distributions, without the need of subsequent neutron energy spectrum unfolding.