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Abstract
Neutron multiplicity analysis has been a valuable technique for safeguards measurements of plutonium oxide and mixed oxides. Historically, most of these measurements have been performed using shift register based electronics. The shift register data acquisition lacks certain flexibility because the basic coincidence parameters (e.g., pre-delay, gate width, and long delay) must be fixed prior to the start of the measurement and the values may potentially, therefore, be sub optimal. List mode or time stamped data acquisition records the arrival time of each pulse thereby preserving the history of the pulse stream and allowing analysis and reanalysis using software analogs to the shift register circuit with adjustable parameters. Until recently, the data rates encountered in the assay of modest amounts of plutonium using efficient multiplicity counters were beyond the capacity of readily available personal computers. The calibration of the large epithermal neutron multiplicity counter (LEMC) for assay of plutonium scrap materials is used as a vehicle to compare the performance of the multiplicity shift register and a commercially available list mode acquisition module.
Abstract
The study demonstrates the advantages of an innovative list-mode multispectral data acquisition system that allows simultaneous creation of several different single, summed, coincident and anticoincident spectra with a single measurement. One of the consequences of list-mode data file offline processing is a reconstructed spectrum with Compton continuum suppression and without any full-energy peak efficiency deduction owing to true coincidence summing. The spectrometer is designed to read out analogue signal from preamplifier of gamma-ray detectors and to digitalize it using DGF/Pixie-4 software and card package (XIA LLC). This is realized by converting an Ortec Compton suppression data acquisition system into an all-digital spectrometer. Instead of using its timing electronic chain to determine the coincidence event, the analog signals from primary and guard detectors were connected directly into the Pixie-4 card for pulse height and time coincident measurement by individually logging and time stamping each electronic pulse. The data acquired in list-mode included coincidence and anticoincidence events consisting of records of energy and timestamp from primary and guard detectors. Every event was stored in a text file for offline processing and spectral reconstruction. A sophisticated computer simulation was also created with the goals of obtaining a better understanding of the experimental results and calculating efficiency.
Abstract
Highly selective and sensitive γ-ray detection was performed by coincidence and anticoincidence event analysis after list-mode data acquisition using an HPGe spectrometer equipped with NaI(Tl) and plastic scintillation detectors. In order to obtain the most suitable detection of specific nuclides, coincidence or anticoincidence spectra could be freely constructed by extracting events with particular time and energy correlations. Although the detector arrangement of this system was the same as that of a typical Compton suppression spectrometer, background counts were drastically reduced and γ-rays of particular nuclides could be selectively detected by using γ-γ, γ-X, γ-X-X, and γ-β+ coincidences.