Characterization of Special Nuclear Material Using a Time-Correlated Pulse-Height Analysis

The ability to perform non-destructive characterization of specialnuclear materials (SNM) is extremely important to ensuring thatnuclear material is not diverted. Cross-correlation measurementsdetect multiple neutrons in nanosecond time windows, whichcan be useful in distinguishing SNM from other neutron sources.Additionally, individual correlated particle pairings can be identifiedwhen the cross-correlation measurement is performed withliquid scintillation detectors with good pulse shape discriminationcapability. Past efforts have shown that measurements utilizingthe time difference between (n, n) events can be used toidentify fission sources from other neutron sources. However,other correlated particle pairings can offer additional informationabout the source. The analysis presented here takes advantageof the timing and energy deposition of (γ, n) particle pairingsto further characterize the source by creating a time-correlatedpulse-height (TCPH) distribution. TCPH uses the height of thedetected neutron pulse along with the time difference betweenthe correlated neutron and gamma to create a surface of the pulseheight at a given time delay. For sources that do not have multiplication,the maximum possible time delay at a given pulseheight can be predicted by simple kinetics. With multiplicationpresent in the system it becomes possible to have high energyevents arriving at times later than the predicted maximum. Thisprovides a means of identifying a multiplying source from a nonmultiplyingsource. This work presents the initial development ofthis technique and preliminary measurement results from a 252Cfsource compared to results simulated with MCNPX-PoliMi.