When a nucleus emits a positron, its atomic number is lowered by one but its mass number stays the same. Positron emission is also known as positive beta decay and involves the decay of a proton into a neutron.
During positron emission, the proton moves from the parent nucleus to the daughter nucleus in its conversion to a neutron. The nucleus then emits a neutrino and a positron. A positron is a positive particle with the same mass as an ordinary electron, but it has the opposite charge.
Positron emission was discovered by Irène and Frédéric Joliot-Curie in 1934 as one of the three processes of beta decay.