Pulse-shape Discrimination for Identification of Neutron Sources Using the Bc-501a Liquid Scintillator

Fast and accurate identification of neutron sources is very important in nuclear nonproliferation and international safeguards applications. Commonly, the identification of neutron spectra relies on unfolding procedures, which requires the solution of an ill-posed problem. In this paper, we present a new application of an existing technique, which can be used for neutron source identification without employing energy-spectrum unfolding techniques. This technique is based on the acquisition of measured neutron pulse-height spectra by using a liquid scintillation detector BC-501A and a fast waveform digitizer TDS-5104. The technique employs an optimized digital pulse-shape discrimination method based on the standard charge integration method to discriminate neutrons from gamma rays. The accuracy of the neutron-source identification method was tested on three neutron sources: californium-252 (Cf-252), americium-beryllium (Am-Be), and americium-lithium (Am-Li). Several source-shielding configurations were investigated to assess the influence of potential shielding of the source on the accuracy of the method. This aspect was addressed by using lead and polyethylene shielding blocks. The measured pulse-height distributions were compared with the distributions simulated with the MCNP-PoliMi code, and excellent agreement was obtained for the Cf-252 and the Am-Be sources. The agreement for the Am-Li source was less favorable due to the uncertainty of measured data. The final results show that for all tested source-shielding configurations, the identification of the Cf-252, Am-Be, and Am-Li sources by direct analysis of the measured pulse-height distributions is very straightforward, and no subsequent neutron energy-spectrum unfolding is required.