Biology, Environment, Geology, Subatomic particles

Carbon 14 for dating (Part 1)

This post’s subject is carbon 14. A radioactive isotope which allows determining the age of an artifact or living being from thousand of years ago.

Carbon 14 formation

Carbon 14 is a carbon’s radioactive isotope with 6 protons, 6 electrons, and 8 neutrons. 

carbon isotopes
Isotopes are 2 atoms with the same number of protons. Source: La pizarra de Yuri.

Cosmic rays that arrives in the upper atmosphere produce neutrons, the later collide with nitrogen 14, producing carbon 14.

carbon 14 formation
Source: Quora.

The carbon 14 (C14) combines with oxygen gas to form carbon dioxide (CO_{2}), which is absorbed by plants. The latter are consumed by animals, which consequently absorb the isotope when they feed themselves.

Absorption of carbon 14
Source: Philobytes.

When an animal or plant dies, doesn’t absorb C14 anymore. This isotope decays in nitrogen 14.

C14 decay
Green arrow = balance in living beings, red arrow = unbalance in living beings. Número = number. Source: Creationwiki.

When older the organic material, less the quantity of reminiscent carbon 14 and lower the emission of beta particles (electrons).

carbon 14 decay
This isotope’s half-life is 5730 years. It means that the decay rate falls to half every 5730 years. Source: Learning Geology.
carbon 14 decay curve
Carbon 14 decay curve. Source: Grade 11 Physics.

Radioactive decay calculation

The decay curve follows the equation below.

R=R_{0}e^{-\lambda t}

  • R_{0} is the decay rate when t=0, by that, in the moment of organism’s death, in decay/seconds or becquerel (Bq).
  • R is the current decay rate.
  • \lambda is the disintegration constant, whose value is:

\lambda=\frac{ln2}{T_{1/2}}=\frac{ln2}{5730}=1,209\cdot 10^{-4}

  • t is the time passed since the organism died.
  • T_{1/2} is the isotope’s half-life. 

The formula to calculate t is just a manipulation of previous equation.

t=\frac{-ln(\frac{R}{R_{0}})}{-ln2}\cdot T_{1/2}

The equations shown serve to all radioactive isotopes and elements.

Errors in measurement

Dating by carbon 14 is only reliable for samples until 50,000 years ago. The quantity produced in the atmosphere isn’t constant, which might cause errors in measurements.

history of C14
Variation of C14 quantity was higher thousand of years in BC era. Source: (Aitken, 2001).

There was an increase in concentration of this isotope due to nuclear tests in the last century. Generating a peak in the 1960s.

Source: EarthSky.

The living beings have many other radioactive isotopes, for example, the potassium 40, whose half-life is 1.248 billion years, In the next part, I will explain the methods used to measure C14 decay.

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