The emission of electron in β decay is accompanied by the emission of an antineutrino ($\stackrel{‾}{\nu }$); in β+ (positron) decay a neutrino (ν) is generated. Neutrinos are neutral particles with very small (possiblly, even zero) mass compared to electrons. They have only weak interaction with other particles. They are, therefore, very difficult to detect, since they can penetrate large quantity of matter (even earth) without any interaction.