What happens during positron annihilation?

Positron annihilation occurs when a positron (the antimatter counterpart of an electron) encounters an electron. The two particles destroy each other, and their mass is converted into energy in the form of two gamma photons, each with an energy of 511 keV.

These two photons are emitted in nearly opposite directions (approximately 180° apart) to conserve momentum. This process is fundamental to positron emission tomography (PET), as the detection of these paired photons allows image formation.

Positron annihilation converts the mass of a positron and electron into two oppositely directed 511 keV gamma photons detected in PET imaging.

Positron annihilation does not occur inside the nucleus itself. Instead, it takes place after the positron has been emitted during beta plus decay and has travelled a short distance through surrounding tissue.

Understanding the physics

During beta plus (β⁺) decay, a proton inside the nucleus is converted into a neutron and emits a positron. The positron carries kinetic energy and travels a short distance through tissue, losing energy through interactions with surrounding electrons.

Once sufficiently slowed, the positron encounters an electron. Because the positron and electron have equal mass but opposite charge, they annihilate each other. According to Einstein’s equation (E = mc²), their combined rest mass is converted into energy.

The rest mass of an electron (or positron) corresponds to 511 keV. Therefore, annihilation produces two gamma photons, each with 511 keV of energy, giving a total of 1.022 MeV. The photons are emitted in opposite directions to conserve both energy and momentum.

In reality, the photons are not emitted at exactly 180° apart due to slight residual motion of the positron–electron system at the moment of annihilation. This small angular deviation contributes to a limitation in PET spatial resolution known as non-collinearity.

The distance the positron travels before annihilation (known as the positron range) also affects spatial resolution. Higher-energy positrons travel further before annihilation, reducing image sharpness.

Where this matters clinically

Positron annihilation is the physical basis of PET imaging. PET scanners detect the two simultaneously emitted 511 keV photons in coincidence, allowing reconstruction of the line along which annihilation occurred. The physics of annihilation directly influences PET image resolution and radiation dose.