Carbon-14 dating works for samples between 500 and 60,000 years old

Radioactive Decay & Half-Life — AQA A-Level Physics

Key Definition

Radiocarbon dating — A method of estimating the age of organic material by measuring the ratio of carbon-14 to carbon-12 and comparing it to the ratio in living organisms.

Carbon-14 is produced when cosmic ray neutrons collide with nitrogen-14 in the atmosphere: ¹₀n + ¹⁴₇N → ¹⁴₆C + ¹₁p.
Living organisms constantly absorb and replace carbon, so the C-14 to C-12 ratio stays roughly constant while alive.
After death, no new C-14 is absorbed. The C-14 decays with a half-lifeThe time taken for half the number of radioactive nuclei in a sample to decay, or for the activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. to halve. of ~5730 years.
By measuring the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). C-14 activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. and comparing it to the initial ratio, the age can be calculated.

Reliability limits

Samples < 500 years old: activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. is too high to detect small changes accurately. The C-14/C-12 ratio is too close to the living value.
Samples > 60,000 years old (~10 half-lives): activity is too low to distinguish from background radiation.