Neutron pinball

The Simulation

Photons with a Bremsstrahlung energy distribution are incident upon 64 thorium cylindrical targets. All thorium targets have 1 cm center-to-center spacing, a dia. of 0.4 cm, and a height of 8 cm. The target geometry is shown in the neutron flux plot below. The targets are surrounded by a cylindrical room filled with air, which is given a radius equal to the distance between the target and the detectors (150 cm), and a height equal to the vertical extent of the detectors (76 cm). Upon the creation of a neutron from photofission, its initial direction of travel is logged and compared with the direction of travel as seen by a detector placed 150 cm away from the target.

A uniform plane source of Bremsstrahlung photons, traveling from left to right, induce photofission in the targets. The rate of neutron production is represented by the color.

Results

12% of neutrons scattered in the thorium cylinders alone, and 14% scattered when air was included. The histogram below shows that if a neutron did scatter, it most likely did so at an unacceptable angle.

Non-zero deflection angles of neutrons

Effect of room air

About 2% of neutrons were scattered in the air. Below is a histogram of neutron deflection angles, as seen by the detector, when neutron-thorium interaction was turned off.

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