Neutron pinball
The Simulation
Photons with a Bremsstrahlung energy distribution are incident upon 64 thorium cylinders which are .4 cm in dia. and 8 cm in height, and 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 once it leaves the simulation area. The target is surrounded by a cylindrical volume of air with a radius of 150cm and a height equal to the vertical extent of the detectors (76cm).
Results
14.5% of neutrons scattered in the thorium cylinders alone, and 16.5% scattered when air was included. The histogram below shows that if a neutron did scatter, it most likely did so at an unacceptable angle.
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