What changes in a decay

An alpha particle contains two protons and two neutrons, so alpha emission lowers atomic number Z by two and mass number A by four. In beta-minus decay, a neutron becomes a proton and emits an electron and antineutrino: Z rises by one while A stays the same. Beta-plus decay and electron capture convert a proton into a neutron, lowering Z by one. Gamma rays are electromagnetic radiation. A gamma transition from an excited daughter changes nuclear energy, not its proton or nucleon count. The IAEA technical overview distinguishes decay modes and gamma emission from an excited daughter.

Bookkeeping and a half-life example

If a symbolic nucleus has A=238 and Z=92, alpha emission leaves A=234 and Z=90. This is accounting, not proof that a particular isotope follows that path or that the transition is energetically allowed; check evaluated nuclide data.

The IAEA gives population decline as N(t)=N₀e⁻λᵗ and T½=ln2/λ. For an idealized population of 800 nuclei with a 10-day half-life, the expected count after 20 days is about 200. It does not predict when any one nucleus decays. Small samples can fluctuate around the expected count. Rutherford and Soddy’s 1902 original study found that activity in their separated ThX sample fell by half in about four days; that is a historical sample-specific result, not a universal half-life.

Scope and safety

The labels alpha, beta, and gamma do not by themselves rank practical risk. Interactions differ, while risk depends on source, energy, activity, route, and exposure duration. This article does not explain how to locate, open, or measure radioactive material. Do not handle a suspected source; follow qualified professional and regulatory procedures. The IAEA source is a uranium-exploration technical guide with a physics overview, not a handling manual.

Sources