The three types of decay

Not all nuclei are stable: some spontaneously transform into others while emitting radiation. This is radioactivity, discovered in 1896 by Henri Becquerel in uranium salts (Nobel Prize 1903 together with Pierre and Marie Curie). Three classic types of decay are distinguished, historically named after the first three letters of the Greek alphabet.

Principle — Three classic types of decay

• $\alpha$ decay: the nucleus emits an $\alpha$ particle $= {}^4_2\text{He}$, made of 2 protons + 2 neutrons. Typical of heavy nuclei (uranium, radium). ${}^A_Z X \to {}^{A-4}_{Z-2}Y + {}^4_2\text{He}$
• $\beta^-$ decay: a neutron transforms into a proton + an electron + an antineutrino. The nucleus increases $Z$ by one while keeping $A$ constant. $n \to p + e^- + \bar\nu_e$
• $\gamma$ decay: the nucleus, left in an excited state after another decay, emits a $\gamma$ photon (typical energy $0{,}1$–$10\;\text{MeV}$) and drops to a lower-energy state. $Z$ and $A$ do not change.

The three radiations behave in profoundly different ways. In $\alpha$ decay the nucleus changes element, losing two units of charge; in $\beta^-$ decay it moves up one place in the periodic table while keeping the same mass number; in $\gamma$ decay it remains the same nucleus, which simply “de-excites” as an atom would by emitting light, but with energies a million times greater.

Historical context — Rutherford's three letters

The three Greek letters $\alpha$, $\beta$, $\gamma$ were introduced by Ernest Rutherford between 1899 and 1903 on the basis of the radiations’ penetrating power: $\alpha$ is stopped by a sheet of paper, $\beta$ by an aluminium foil a few millimetres thick, $\gamma$ requires a lead wall several centimetres thick. Their identities — helium nuclei, electrons, photons — were discovered only in the following years (Simonyi 2012).

▶ The Most Radioactive Places on Earth — Veritasium

Collegamenti

Argomenti: Fisica nucleare Concetti: Decadimento radioattivo Oggetti: Nucleo atomico

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