Grade 11 Physics Practice Exam 2026 – Complete Study Guide

Positron emission comes from a type of beta decay called beta-plus decay. In this process, a proton inside the nucleus is transformed into a neutron. To balance charge and conserve lepton number, a positron (the positively charged antimatter counterpart of the electron) and a neutrino are emitted. As a result, the atomic number decreases by one while the mass number stays the same. This happens in nuclei that have too many protons relative to neutrons, nudging the nucleus toward stability.

Other forms of decay don’t produce positrons. In beta-minus decay, a neutron becomes a proton and emits an electron (and an antineutrino). Alpha decay ejects a helium-4 nucleus, changing both the number of protons and neutrons and removing a chunk of mass. Gamma decay releases energy as photons without changing the nucleus’s proton or neutron count.

So the decay that emits a positron is beta-plus decay.