Beta spectrum : a sharing of energy:
In a beta decay like that of a bismuth-210 nucleus, the decay energy is shared between the nucleus, an electron and an antineutrino. As the nucleus, is much more massive than the two others (its mass is 320 000 times the mass of the electron) it takes away a negligible amount of kinetic energy. The energy is therefore shared between the electron and the antineutrino. The antineutrino escaping detection, one observe only the beta electron with a variable energy. The figure display the characteristic energy repartition of beta electrons – the beta spectrum – of the bismuth-decay
IN2P3