Radioactivity and radioisotopes

• Radiocarbon Dating • Other Radiometric Dating Techniques

Radiocarbon datingCarbon-14 (14C) is a radioactive isotope of Carbon that every living thing takes in during its life.

• How is it different from non-radioactive Carbon-12 (12C)? 14C does not come from living things, but it is produced in the Atmosphere. Cosmic Rays produce fast Neutrons that hit atoms of Nitrogen-14 (14N), turning this 14N into 14C. This 14C mixes with Oxygen to produce 14CO2 which is taken up by plants and therefore animals.

Radiocarbon datingWord equations

• Formation of 14C:

14N + neutron (n) 14C + proton (p)

• 14C Decay:

14C will eventually decay back into 14N through this process;

14C 14N + beta particle ()

THE HALF-LIFE OF 14C IS 5730 YEARS, MEANING THAT EVERY 5730 YEAR HALF THE 14C PARTICLES IN A DEAD CREATURE WILL HAVE TURNED BACK INTO 14N.

Radiocarbon datingHow does it work?

After a living thing dies it stops taking 14C in, because it stops eating and breathing. Therefore, the amount of 12C (non-radioactive Carbon) will stay the same, while the amount of 14C will decrease. By measuring the ratio 14C/ 12C in the atmosphere and comparing it to the ratio in the dead sample scientists measure the age of that living thing from its death.

N.B.The max. age radiocarbon can give is 50000 years,

because after that limit the amount of 14C in the sample is no longer detectable.

Radiocarbon dating14C/ 12C RATIO

After death, the amount of 12C remains constant, but the amount of 14C decreases.

The need for calibration• Plants discriminate between 12C and 14C, i.e. they

take less 14C than expected, making the specimen seem older.

• Factories during the industrial revolution burnt lots of coal and other fossil fuels, releasing much 12C in the atmosphere more 12C means specimens would look older than they are.

• Some Scientists believe that the Genesis flood buried lots of carbon which became coal, oil etc., lowering the 12C in the atmosphere, while 14C was still produced more 14C in the ratio after the flood would make look specimen before the flood older than they are.

Other radiometric dating methodsThese methods use the concentrations of PARENT

and DAUGHTER products in radioactive decay chains.

PARENT is the starting radioactive substance

DAUGHTER is the substance the PARENT decays into.

Example

URANIUM-238 LEAD-206PARENT DAUGHTER

Decays to

Three necessary assumptionsScientists use the PARENT-DAUGHTER method to

date rocks. This method works only if all the following assumptions are true.

Problems with these methods• These assumptions are not provable because

they cannot be subject to direct scientific observation

• Methods should work reliably on things of known age

• Different dating techniques should consistently agree