Beta-positive (β+) decay

Meaning: In β+ decay a proton in the nucleus becomes a neutron, and the nucleus emits a positron (e⁺, the electron's antiparticle: same mass, charge +1) and an electron neutrino. Z falls by 1 and A is unchanged. It occurs in proton-rich nuclei, which have too few neutrons, and moves them toward stability. Electron capture competes and gives the same daughter. The positron soon annihilates with an electron, producing two 511 keV gamma photons.

Equation: parent (A, Z) → daughter (A, Z − 1) + e⁺ + ν, where A is the mass number and Z the atomic number (both dimensionless counts), e⁺ is the positron (charge +1.6 × 10⁻¹⁹ C) and ν is the neutrino (charge 0).

On the MCAT: Given a parent nuclide, find the daughter by balancing A and Z, for example ¹¹C → ¹¹B + e⁺ + ν. Compare with α decay (A − 4, Z − 2), β− decay (Z + 1) and γ emission (no change in A or Z). PET scans detect the annihilation photons.

Common confusion: Students treat the positron as a proton, or let A change. The positron is not a nucleon, so A stays the same, and the nucleus loses one unit of positive charge, so Z drops by 1. In β− decay a neutron becomes a proton and Z rises by 1.