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Electrons and positrons are produced by the nuclear transformations of protons and neutrons known as beta decay.

In the process of beta decay, a beta ray of high energy and speed is produced from the emission of the radioactive decay of an atomic nucleus. The process in which a proton is converted to a neutron is called $尾+$decay. Again, in the process in which a neutron is converted to a proton, we get $尾-$decay.

$尾+$decay

${}_Z{}^{A}X鈫�{}_{Z-1}{}^{A}Y+{}_{+1}{}^{0}e+{}_0{}^0\stackrel{炉}{{谓}_e}$

$尾-$decay

${}_Z{}^{A}X鈫�{}_{Z+1}{}^{A}Y+{}_{+1}{}^{0}e+{}_0{}^0谓_e$

Since the proton is positively charged, the emitted particle must be a positron (as opposed to the negatively charged electron) in accordance with the law of charge conservation.

Hence, a positron is produced with an anti-neutrino in this decay.

In this case, the initial state had zero charge (the neutron is neutral), so the sum of charges in the final state must be zero. Since there is a proton in the final state, there should also be an electron (as opposed to a positron), so that q = 0.

Hence, an electron is produced with a neutrino in this decay.