Time-Correlated-Pulse-Height Technique Measurements of Fissile Samples at the Device Assembly Facility

摘要

Detecting and quantifying special nuclear material via gamma spectroscopy techniques becomes exceedingly difficult in the presence of shielding and background radiation. Gamma particles are easily attenuated in high density materials, whereas the long half-lives of special nuclear material may preclude adequate signal to noise ratios. Neutrons exhibit higher penetrability over gamma rays, yet their presence does not guarantee the existence of special nuclear material. Many radioactive sources emit temporally correlated gammas and neutrons, but only for fissile and fissionable sources can these correlations extend over several reactions. Thus especially the detection of multiplying fission sources garners much attention. Whereas in non-multiplying fission sources, such as ~(252)Cf, correlated gamma-neutron-pairs arise from just one spontaneous fission event, multiplying fission sources would allow for a gamma to be temporally correlated with any neutron from a subsequent fission in the same fission chain. Organic scintillation detectors exhibit the required properties for such measurements. These include their nanosecondscale timing properties and their aptitude for pulse shape discrimination. A measurement system consisting of four 3 in by 3 in EJ-309 liquid scintillation detectors was used to measure timecorrelated gamma-neutron-pairs from the BeRP ball, a 4.5 kg plutonium sphere at the Device Assembly Facility at the Nevada National Security Site. This sphere was measured with polyethylene or tungsten reflectors of various thicknesses to attain a variety of multiplications. For a gamma-neutron-pair detected in separate detectors, the time difference between the triggering gamma and the subsequently detected neutron as well as its measured pulse height were recorded. A surface plot of this experimental data was superimposed on curves representing the neutron energy limit for a fixed time of flight assuming no multiplication. Both the experimental data and MCNPX-PoliMi simulations showed that the number of events beyond these curves were indicative of the source’s multiplication, whereas a non-multiplying source would show zero events beyond these curves.