PULSE SHAPE DISCRIMINATION FOR THE NUCLEAR MATERIALS IDENTIFICATION SYSTEM (NMIS)

Preliminary results from efforts to implement on-line, fast pulse shape discrimination (PSD) methods in the Nuclear Materials Identification System (NMIS) are discussed. Previous NMIS implementations used fast plastic scintillation detectors that did not allow discrimination of neutrons and gamma rays. Time coincidence signatures acquired by NMIS thus included contributions from both neutrons and gamma rays. It has been shown that attributes of the fissile material under investigation are related to features extracted from the signatures measured by NMIS. The ability to separate the total signature into its neutron and gamma components will lead to the possibility of extracting new features. On the basis of previous studies, these new features are more sensitive to the attributes of the fissile material than the features extracted from the total signature. PSD implementation used the method of charge integration over two different time periods. Liquid organic scintillators (BC501-A) were used to detect neutrons and gamma rays from a Cf-252 source. Pulse type was differentiated using the time-of-flight technique, and pulses were digitized for subsequent analysis using a fast digital oscilloscope. Analysis of digitized pulses provided optimum signal integration periods for pulse discrimination. Electronic circuits were designed, built, and tested to implement our PSD technique. Results to date indicate promise as an effective PSD methodology for future use in the NMIS.