# Resonance Levels for Absorption of Neutrons

## Abstract

WHEN a very thin layer of boron is irradiated with slow neutrons, the number of -particles emitted can be represented by an expression proportional to nii+P, where the terms nii correspond to the nearly homogeneous groups of neutrons absorbed selectively in certain elements; ni is the number of neutrons having kinetic energy Ei, and i is their coefficient of absorption in boron. The most important term represents the group of thermal neutrons, called by Amaldi and Fermi1 the group C. If we absorb this group in a thin sheet of cadmium, the number of -particles emitted by boron diminishes by an amount proportional to aCnCC, where c is the fraction of the group C absorbed by cadmium. Similarly, let us suppose that we absorb another group, J, in a suitable element; the corresponding decrease of the number of -particles will be proportional to aJnJJ. If, therefore, the relative numbers of groups C and J and the fractions a are known, we can determine the ratio C/J, which is equal to . In this way the energy corresponding to the resonance level of the element J can be calculated.

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## References

1. 1

Amaldi and Fermi, Ric. Scient., 2, 544 (1935).

2. 2

H. v. Halban, jun., and P. Preiswerk, NATURE, 137, 905 (1936).

3. 3

C. H. Collie, NATURE, 137, 614 (1936).