Penetration of Fission Neutrons Through Water

The above expectations are quite well borne out in the following calculations. They show, therefore, that the situation is quite different from that considered in CP-137, in which a calculation with an energy independent cross section was carried out. As a consequence of the energy independence of the cross section, the total penetration was much greater than the penetration before the first collision. Hence the picture obtained in the present note is very different from the picture of CP-137. 

For similar reasons, an accurate value for the cross section is necessary only for high energies. In this region the cross section of hydrogen can be represented by 

It is remarkable that as long as the scattering is spherically symmetric, this is within 6% the maximum possible scattering cross section. According to mea- 

Let us first calculate the straight penetration to a distance x without any energy loss. This is clearly given by 

The last part of (9) can be obtained by differentiation from (3) divided by /3. 

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E. P. Wigner and G. Young, Penetration of Fission Neutrons Through Water , Mon P-283, April 9, 1947. 

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