Abstract--We describe a neutron/gamma pulse shape discrimination (PSD) system that overcomes count rate limitations of previous methods for distinguishing neutrons from gammas in liquid scintillation detectors. Previous methods of PSD usually involve pulse shaping time constants that allow throughput of tens of thousands counts per second. Time correlated measurements require many millions of counts per second to accurately characterize nuclear material samples. To rapidly inspect many test articles, a high- throughput system is desired. To add neutron - gamma distinction to the analysis provides a much desired enhancement to the characterizations. However, if the PSD addition significantly slows down the inspection throughput, this PSD feature defeats any analysis advantage. Our goal for the fast PSD system is to provide sorted timing pulses to a fast, multi-channel, time-correlation processor at rates approaching several million counts per second enabling high throughput, enhanced inspection of nuclear materials. I. INTRODUCTION he sensitivity of time-dependent coincidence signatures between two or more detectors to the attributes of fissile materials has been demonstrated using the Nuclear Materials Identification System (NMIS) [1]–[6]. Attributes of fissile materials are related to features extracted from the NMIS signatures. Pulse shape discrimination (PSD) of the neutron and gamma Manuscript received October 20, 2003. This work was supported in part by the U.S. Department of Energy under contract DE-AC05-00OR22725. William L. Bryan is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6006 USA (telephone: 865-576-6844, e-mail: bryanwl@ornl.gov). Charles L. Britton is with the Oak Ridge National Laboratory, Oak Ridge, TN 37830-6006 USA (telephone: 865-574-1878, email: brittoncl@ornl.gov). John T. Mihalczo is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6010 USA (telephone: 865-574-5577, e-mail: mihalczojt@ornl.gov). John S. Neal is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6010 USA (telephone: 865-576-8275, e-mail: nealjs1@ornl.gov). Sara A. Pozzi is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6010 USA (telephone: 865-574-5699, e-mail: pozzisa@ornl.gov). Raymond W. Tucker is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6010 USA (telephone: 865-576-0947, e-mail: tuckerrwjr@ornl.gov). components of the signatures will allow extraction of new features that have been shown to be more sensitive to fissile material attributes than the features from the total signature [7], [8]. Recent work using the MCNP-PoliMi code [9] simulated detector-detector covariance functions for passive measurements on plutonium spheres and cylinders. The total signatures were divided into four components according to the detected particles: neutron-neutron, photon-photon, neutron-photon, and photon-neutron. These simulations demonstrated the ability to distinguish plutonium metal from oxide for spherical and cylindrical samples without knowing the shape or mass of the samples. In order to implement these simulations, one must use PSD techniques to separate neutron and gamma pulses. Previous methods of PSD allowed throughput of tens of thousands counts per second. Time-correlated measurements require many millions of counts per second to accurately and quickly characterize fissile material samples. We have designed a neutron/gamma PSD system that overcomes count rate limitations of previous methods for distinguishing neutrons from gammas in liquid scintillation detectors and integrates with other commercially available nuclear instrument modules (NIM). Our goal for this fast PSD system is to provide sorted timing pulses to a fast, multi-channel, time-correlation processor at rates approaching several million counts per second. II. ELECTRONIC PULSE SHAPE DISCRIMINATION METHODS Electronic PSD methods generally fall into one of three categories: (1) sensing differences in the decay times of pulses, (2) integrating pulse charge over different time intervals, and (3) digital capture and shape analysis of pulses [10], [11]. All of these methods depend on the ability to measure a difference in neutron and gamma pulse characteristics for the detector of our choice, the liquid scintillator BC501-A. The rise time, or crossover, method [12] passes individual pulses through a shaping network, producing a bipolar pulse where the “zero-crossing” is a function of pulse shape and pulse decay time. The time difference between the pulse start and the zero-crossing is converted by a time-to-amplitude-converter (TAC) into a pulse amplitude. This method suffers from its dependence on Fast Neutron - Gamma Pulse Shape Discrimination of Liquid Scintillation Signals for Time Correlated Measurements William L. Bryan, Member IEEE, Charles L. Britton, Member, IEEE, John T. Mihalczo, John S. Neal, Member, IEEE, Sara A. Pozzi, and Raymond W. Tucker T