MCNP Sim of Jack Converter

Simulation inputs

MCNP input deck: File:MCNP Jack Converter.txt.

The cross-section plots below show that the vast majority photo-neutrons are produced by (G,n) on Tungsten-184.

Geometry

Profile view of converter geometry. Distances are in cm. There are two thin (0.001 inch) titanium plates, one on the far left and another on the far right of the convertor, but they are too small to be seen in this diagram. 0.2 cm of water separates each of the three 0.15 cm thick tungsten plates.

Front view of convertor geometry. Distances are in cm.

Materials legend for figures:

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Assumptions

The beam spot size was given a diameter of 0.5 based off of the size of a burn mark on a converter that got hot during irradiation. Electrons are mono energetic.

Results

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

The mean and median energy of fission neutrons from U235 is 1.94 MeV and 1.66 MeV, respectively. The mean and median energy of produced neurons are also shown on each plot.

Neutron energy spectra from 42 MeV electrons

0-7 MeV neutrons, with and without water

With water in the converter:

Without water in the converter:

0-0.1 MeV neutrons, with and without water

With water:

Without water:

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.

Photons

The plot below shows the energy distribution of photons exiting the converter at three different intervals of their angle w.r.t. the incident electron beam