Experimental Evidence for the Existence of a Neutrino

Radioactive ${\mathrm{Be}}^7$ was deposited on a platinum foil by means of a new evaporation technique. An electron multiplier tube was employed to count the recoil nuclei produced in the reaction, ${\mathrm{Be}}^7+e_{\mathrm{K}}\to {\mathrm{Li}}^7+\eta +Q$. The maximum energy of the recoils was about 40 to 45 electron volts compared with the value of 58 electron volts to be expected for a neutrino of zero rest mass. An attempt was made to detect coincidences caused by the emission in opposite directions of a gamma-ray and a recoil nucleus. The observed coincidences were less than two percent of those expected for gamma-ray recoils. Apparently the recoils were caused by the emission of a neutrino and not by the emission of a gamma-ray.