ABSTRACT
Stable beryllium is monoisotopic, i.e. it consists only of the isotope 9Be. In the experiment, the beryllium powder was well mixed with the radioactive -emitter. The neutrons were emitted at various energies (i.e. at various speeds); the highest probability of capture by uranium nuclei, however, is for low speed neutrons, moving at a speed in thermal equilibrium with the surrounding material (paraffin). The thermalisation process is fastest when the neutrons are scattered at the other light nuclei present in paraffin. In each scattering process the neutrons lose speed (energy) until they have the same speed (i.e. the same energy or temperature) as the nuclei in the surrounding paraffin. Such a paraffin moderator, as it was used by Hahn, Meitner and Straßmann, is shown in Figure 3.1.
