TY - JOUR

T1 - Prompt fission neutron anisotropy in low-multiplying subcritical plutonium metal assemblies

AU - Shin, Tony H.

AU - Di Fulvio, Angela

AU - Clarke, Shaun D.

AU - Chichester, David L.

AU - Pozzi, Sara A.

N1 - Funding Information:
This research was funded in-part by the Consortium for Verification Technology, United States , under Department of Energy National Nuclear Security Administration award DE-NA0002534 .

PY - 2019/1/21

Y1 - 2019/1/21

N2 - Fission neutron anisotropy, due to the kinematics of the fission process, has been studied for non-multiplying sources and highly multiplying subcritical plutonium metal assemblies (i.e. relatively long fission chains). The studies on highly multiplying assemblies show that the observed neutron–neutron angular distribution appear isotropic, while the studies on non-multiplying sources show that the neutron–neutronangular distribution appear anisotropic. No measured data exists, however, that investigates the dependence of neutron anisotropy on multiplication for low-multiplying assemblies. We have experimentally characterized the dependence of fission neutron anisotropy on multiplication for low-multiplying plutonium metal assemblies. Here, an array of 16 organic scintillators was used to measure plutonium metal assemblies (95% 239Pu, by mass) exhibiting a leakage multiplication of 1.0722(3) to 1.6006(4). Full neutron–neutron angular distributions were measured, and the fission neutron anisotropy was quantified with the ratio of neutron–neutron coincidences observed at 180°and 90°. The results show that the neutron–neutron angular distribution becomes more isotropic as the multiplication increases. Additionally, energy–angle correlations were also characterized showing that the angular distributions are more anisotropic when observing neutrons of higher energy.

AB - Fission neutron anisotropy, due to the kinematics of the fission process, has been studied for non-multiplying sources and highly multiplying subcritical plutonium metal assemblies (i.e. relatively long fission chains). The studies on highly multiplying assemblies show that the observed neutron–neutron angular distribution appear isotropic, while the studies on non-multiplying sources show that the neutron–neutronangular distribution appear anisotropic. No measured data exists, however, that investigates the dependence of neutron anisotropy on multiplication for low-multiplying assemblies. We have experimentally characterized the dependence of fission neutron anisotropy on multiplication for low-multiplying plutonium metal assemblies. Here, an array of 16 organic scintillators was used to measure plutonium metal assemblies (95% 239Pu, by mass) exhibiting a leakage multiplication of 1.0722(3) to 1.6006(4). Full neutron–neutron angular distributions were measured, and the fission neutron anisotropy was quantified with the ratio of neutron–neutron coincidences observed at 180°and 90°. The results show that the neutron–neutron angular distribution becomes more isotropic as the multiplication increases. Additionally, energy–angle correlations were also characterized showing that the angular distributions are more anisotropic when observing neutrons of higher energy.

KW - Multiplication

KW - Neutron anisotropy

KW - Neutron–neutron angular correlations

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U2 - 10.1016/j.nima.2018.09.085

DO - 10.1016/j.nima.2018.09.085

M3 - Article

AN - SCOPUS:85054177040

VL - 915

SP - 110

EP - 115

JO - Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

JF - Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

SN - 0168-9002

ER -