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

## Abstract

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-neutron angular 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% ^{239}Pu, 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.

- Authors:

- Univ. of Michigan, Ann Arbor, MI (United States)
- Idaho National Lab. (INL), Idaho Falls, ID (United States)

- Publication Date:

- Research Org.:
- Univ. of Michigan, Ann Arbor, MI (United States)

- Sponsoring Org.:
- USDOE National Nuclear Security Administration (NNSA), Office of Nonproliferation and Verification Research and Development (NA-22)

- OSTI Identifier:
- 1525316

- Grant/Contract Number:
- NA0002534

- Resource Type:
- Accepted Manuscript

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

- Additional Journal Information:
- Journal Volume: 915; Journal Issue: C; Journal ID: ISSN 0168-9002

- Publisher:
- Elsevier

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY

### Citation Formats

```
Shin, Tony H., Di Fulvio, Angela, Clarke, Shaun D., Chichester, David L., and Pozzi, Sara A. Prompt fission neutron anisotropy in low-multiplying subcritical plutonium metal assemblies. United States: N. p., 2018.
Web. doi:10.1016/j.nima.2018.09.085.
```

```
Shin, Tony H., Di Fulvio, Angela, Clarke, Shaun D., Chichester, David L., & Pozzi, Sara A. Prompt fission neutron anisotropy in low-multiplying subcritical plutonium metal assemblies. United States. doi:10.1016/j.nima.2018.09.085.
```

```
Shin, Tony H., Di Fulvio, Angela, Clarke, Shaun D., Chichester, David L., and Pozzi, Sara A. Thu .
"Prompt fission neutron anisotropy in low-multiplying subcritical plutonium metal assemblies". United States. doi:10.1016/j.nima.2018.09.085. https://www.osti.gov/servlets/purl/1525316.
```

```
@article{osti_1525316,
```

title = {Prompt fission neutron anisotropy in low-multiplying subcritical plutonium metal assemblies},

author = {Shin, Tony H. and Di Fulvio, Angela and Clarke, Shaun D. and Chichester, David L. and Pozzi, Sara A.},

abstractNote = {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-neutron angular 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.},

doi = {10.1016/j.nima.2018.09.085},

journal = {Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment},

number = C,

volume = 915,

place = {United States},

year = {2018},

month = {9}

}