Radioactive decay

Watch unpredictable nuclear events combine into a predictable exponential law.

Iodine-131 · β⁻ decayRunning

Undecayed parentDaughter nucleusRecent decay

Elapsed time0.00 days

Remaining500

Decayed0

Half-lives0.00

Detected counts0

Theory500.0

Nuclei remaining

Measured activity

Event-time histogram

Random individuals, reliable populations

No clock tells us when one nucleus will decay. But every nucleus has the same constant probability per unit time, so large samples closely follow an exponential curve.

What half-life means

After one half-life, each original nucleus has a 50% chance of remaining. A real finite sample will not necessarily contain exactly half because its decay events fluctuate statistically.

Activity and detection

Activity is the true decay rate A=λN. A detector records only a fraction of emissions and may also count background radiation, so measured counts fluctuate around the expected value.

Memoryless process: An old undecayed nucleus has the same future decay probability as an otherwise identical new one. Radiation trails and flashes are schematic; isotope half-lives are converted to a display-scaled internal unit.