Fission Chain Restart Theory

Abstract

Fast nanosecond timescale neutron and gamma-ray counting can be performed with a (liquid) scintillator array. Fission chains in metal evolve over a timescale of tens of nanoseconds. If the metal is surrounded by moderator, neutrons leaking from the metal can thermalize and diffuse in the moderator. With finite probability, the diffusing neutrons can return to the metal and restart the fast fission chain. The timescale for this restart process is microseconds. A theory describing time evolving fission chains for metal surrounded by moderator, including this restart process, is presented. This theory is sufficiently simple for it to be implemented for real-time analysis.

Keywords:

• Mathematical modeling
• fast neutron counting
• gamma-ray counting

Acknowledgments

This work was performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. This work was developed under DOE funding. It is a pleasure to thank Ron Wurtz for his many inspirational insights and James Wong for developing application tools based on this work. We also gratefully thank Dave Heinrichs and Phil Kerr for thermal neutron data and Garry Guethlein for discussions.