Neutron and gamma ray coincidence measurements of a high-multiplication, subcritical assembly of weapons-grade plutonium

Correlated neutrons and gamma rays were measured from a subcritical highly-multiplying assembly of plutonium at the Device Assembly Facility (DAF) located at the Nevada National Security Site (NNSS). The assembly was a 4.5 kg α-phase plutonium metal sphere composed of 94%
²³⁹
Pu and 6%
²⁴⁰
Pu by weight. Passive measurements were performed with the bare source and with multiple shielding configurations including polyethylene, steel, and tungsten. Data were collected using two EJ-309 liquid scintillators, two EJ-299 plastic scintillators, and one CeBr
₃
detector. Fast neutron-gamma timing correlations were analyzed and the feasibility of using measurements of late-arriving neutrons or late-arriving gamma rays to estimate the multiplication of the assembly was studied. Simulations were performed with MCNPX-PoliMi and GEANT4 and their results were compared to the experimental results.