Neutron pinball

The Simulation

Photons with a Bremsstrahlung energy distribution are incident upon 64 4 mm dia. thorium cylinders with 1 cm center-to-center spacing (see the neutron flux plot below). Photofission neutrons were simulated using the MCNPX ACE model. Upon the creation of a neutron, its initial direction of travel is logged and compared with its direction of travel when it leaves the simulation area. The target configuration pictured below is surrounded by a r=100cm cylinder (axis going into the page) of air. All cylinders have a height of 76 cm.

Prompt neutron flux from photofission in thorium cylinders

Result

14.5% of neutrons scattered in the thorium cylinders alone, and 16.5% when the air was included. The histogram below shows that when a neutron scatters, it almost certainly does so at an unacceptable angle.

Non-zero deflection angles of neutrons

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