Penetration of Fission Neutrons Through Water
In the high flux experimental pile that is being planned, water will be, at least in some directions, practically the only neutron shield. The calculation of the effectiveness of such a neutron shield is greatly influenced by the variable cross section of the most important constituent of water, viz., hydrogen. This cross section is low at high energies, so that high energy neutrons can penetrate water relatively easily. However, once a neutron has suffered a collision, its energy will decrease and it will suffer a second slowing-down collision after a much shorter distance of travel. It will then again lose energy and the succeeding collisions will come at decreasing distances. As a result, one gains the impression that the penetration of neutrons is determined by the high energy end of the primary neutron spectrum and that the total penetration may not be much greater than the penetration before the first collision.
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