Radioactivity

Becquerel discovered radioactivity by accident in 1896. He lit pieces of uranium-potassium sulphate with visible light, wrapped them in black paper and put a piece of silver between the package and a photographic plate.

After several hours the developed plate was blackened: the compound had given off something that passed through the black paper and the silver.

Later experiments showed radioactivity to be a nuclear process: an unstable nucleus decays. This is radioactive decay.

Three kinds occur in nature: α-decay, in which a helium nucleus ⁴₂He is emitted; β-decay, in which electrons or positrons are emitted; and γ-decay, in which high-energy photons (hundreds of keV or more) are emitted.

A positron has the same mass as an electron and an exactly opposite charge; the two are a particle–antiparticle pair. When an electron and a positron meet they annihilate, giving gamma-ray photons.

Radioactivity signals an unstable nucleus. Stable light nuclei have neutrons and protons roughly 1 : 1; for heavy nuclei the ratio rises to about 3 : 2, as extra neutrons offset the repulsion between protons. Nuclei far from these ratios are unstable.

Only about 10% of known isotopes are stable. The rest have been made in laboratories, by bombarding stable nuclei with α, p, d, n or other particles, or identified in astronomical observations.