Radioactive Decay – Alpha, Beta, Gamma

Radioactivity is the spontaneous emission of particles or energy from an unstable nucleus. Some nuclei have too many or too few neutrons, making them unstable. To become more stable, they emit radiation in the form of alpha particles, beta particles, or gamma rays.

This process is random—we cannot predict when a particular nucleus will decay, but we can measure the average rate of decay for a large sample using half-life.

In alpha decay, the nucleus emits an alpha particle—a helium nucleus containing 2 protons and 2 neutrons (⁴₂He). This reduces the mass number by 4 and the atomic number by 2.

Alpha particles are strongly ionizing but have low penetrating power—they're stopped by paper or a few centimetres of air.

In beta decay, a neutron converts into a proton, emitting a high-speed electron (beta particle) and an antineutrino. The mass number stays the same, but the atomic number increases by 1.

Beta particles are moderately ionizing and moderately penetrating—they're stopped by a few millimetres of aluminium.

After alpha or beta decay, the nucleus is often left in an excited state. It releases excess energy as a gamma ray—a high-energy photon. Gamma decay doesn't change the mass number or atomic number.

Gamma rays are weakly ionizing but highly penetrating—they require thick lead or concrete to significantly reduce their intensity.

The half-life is the time taken for half the radioactive nuclei in a sample to decay. After one half-life, 50% of the original nuclei remain. After two half-lives, 25% remain. After three, 12.5% remain.