It is a well-known fact that the non-conservation of energy during β decay may be explained by assuming the existence of light neutral particles, neutrinos, which are ejected from the nucleus at every act of β emission and which enable all the conservation laws to be fulfilled. A study of the energy distribution of recoil atoms during β decay might furnish some data for testing this hypothesis. However, in order to determine the energy distribution of recoil atoms consider-able experimental difficulties have to be overcome. The energy of recoil atoms of ordinary radioactive substances is of the order of magnitude of 1 volt, i.e. of the same order of magnitude as the adsorption energy of atoms on a surface. Thus, on escaping from the'surface on which they are adsorbed, they will lose an amount of energy of the same order of magnitude as that which they possess, and their velocity distribution will therefore be completely distorted. This difficulty may be met by employing an artificial, light radioactive substance, whose recoil atoms may have energies some ten times greater than 1 volt. For this reason active carbon C, obtained by bombarding boron with deuterons according to the reaction 10B+2H→11C+n, was employed as a radioactive substance. Moreover, the fact that the emission of positrons from 11C is not accompanied by the emission of γ-rays has the advantage of simplifying the interpretation of the results.