Resonance Absorption of Neutrons by Spheres

Abstract

By concentrating uranium into lumps, one can decrease its resonance absorption as compared with the resonance absorption of the same amount of uranium spread out uniformly in a moderator. It is shown here that this decrease is caused principally, not by the diminished over‐all density of the neutrons at the lump, but by their changed energy distribution. This shows deep minima at the resonance lines, i.e., where the uranium absorbs most strongly. The considerations here presented show that the decrease in absorption is largest for the low energy resonances of U²³⁸ which are responsible for the bulk of the absorption in the case of uniform distribution. As a result, the resonance absorption of uranium lumps extends over a very large energy region–up to about 5000 ev. It can be decomposed, approximately, into two parts: the mass absorption, proportional to the number of U²³⁸ atoms present; and a surface absorption, proportional to the surface of the lump. The total decrease of the resonance absorption produced by lumping (the ``advantage factor'' of lumping) can be estimated to reach, under favorable conditions, a value as high as 20.