MCNP Sim of Jack Converter

Simulation inputs

Geometry

Side profile of converter. Distances re in cm. There are thin (0.001 inch) titanium plates on the far left and far right of the model, but they but the are too small to be seen. The three 0.15 cm thick tungsten plates are each separated by 0.2 cm of water.

Front view of convertor. Distances are in cm.

Materials legend for figures:

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Assumptions

The beam spot size was given a diameter of 0.5 cm, which was determined by the size of a burn mark on a converter that got hot during irradiation. Electrons are 42 MeV and mono energetic.

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Results

Energy spectrum of neutrons

The plot below shows the energy distribution of neutrons exiting the converter at three different intervals of neutron angle w.r.t. the incident electron beam. The neutron energy spectrum of the neutron-induced fission of U235 is also shown for comparison.

misc

The difference between the U235 neutron spectrum and the energy spectrum from the converter may be partially due to moderation of neutrons by H2O. The plot below demonstrates this point, which registers the energy of each photo-neutron immediately after emission, as opposed to after the photo-neutrons exit the converter.

Below is a plot of the rate of neutron production as the beam traverses through the converter. Zero corresponds to where the beam enters the titanium window.