Alpha Decay

Alpha decay is a type of radioactive decay where an unstable atomic nucleus emits an alpha particle—consisting of:

• 2 protons
• 2 neutrons (identical to a helium-4 nucleus)

This process reduces the parent nucleus’s atomic number by 2 and mass number by 4.

Alpha decay occurs primarily in heavy, proton-rich nuclei (typically atomic number > 82).

The alpha particle is preformed within the nucleus and escapes via quantum tunneling through the Coulomb barrier—a phenomenon forbidden by classical physics but permitted by quantum mechanics.

The decay energy is released as kinetic energy of the alpha particle and the recoiling daughter nucleus.

We call this a downgrade where the atom becomes more of an atom as stillness.

Examples

This is used in:

• Smoke detectors – Americium-241 emits alpha particles that ionize air, triggering an alarm when smoke disrupts the current.
• Nuclear batteries – Alpha decay provides long-lasting power for spacecraft (e.g., Voyager, Mars rovers) using radioisotope thermoelectric generators (RTGs).
• Cancer therapy – Targeted alpha therapy (TAT) uses isotopes like Actinium-225 to destroy tumor cells.

Key Feature: Geiger–Nuttall Rule

The decay constant lambda is a ratio of the alpha particle’s:

• space as range R
• time as half-life:

Shorter half-lives correspond to higher alpha energies. Physics calls this a hallmark of quantum tunneling.