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Radioactivity and radioisotopes Radiocarbon Dating Other Radiometric Dating Techniques.

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Presentation on theme: "Radioactivity and radioisotopes Radiocarbon Dating Other Radiometric Dating Techniques."— Presentation transcript:


2 Radioactivity and radioisotopes Radiocarbon Dating Other Radiometric Dating Techniques

3 Radiocarbon dating Carbon-14 ( 14 C) is a radioactive isotope of Carbon that every living thing takes in during its life. How is it different from non- radioactive Carbon-12 ( 12 C)? 14 C does not come from living things, but it is produced in the Atmosphere. Cosmic Rays produce fast Neutrons that hit atoms of Nitrogen-14 ( 14 N), turning this 14 N into 14 C. This 14 C mixes with Oxygen to produce 14 CO 2 which is taken up by plants and therefore animals.

4 Radiocarbon dating Word equations Formation of 14 C: 14 N + neutron (n) 14 C + proton (p) 14 C Decay: 14 C will eventually decay back into 14 N through this process; 14 C 14 N + beta particle ( ) THE HALF-LIFE OF 14 C IS 5730 YEARS, MEANING THAT EVERY 5730 YEAR HALF THE 14 C PARTICLES IN A DEAD CREATURE WILL HAVE TURNED BACK INTO 14 N.

5 Radiocarbon dating How does it work? After a living thing dies it stops taking 14 C in, because it stops eating and breathing. Therefore, the amount of 12 C (non-radioactive Carbon) will stay the same, while the amount of 14 C will decrease. By measuring the ratio 14 C/ 12 C 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 14 C in the sample is no longer detectable.

6 Radiocarbon dating 14 C/ 12 C RATIO 14 C 12 C Constant amount (Decreases with time) 14 C not measurable Total 14 C and 12 C in specimen (e.g. wood) Moment of death OldOlder Age not measurable After death, the amount of 12 C remains constant, but the amount of 14 C decreases.

7 The need for calibration Plants discriminate between 12 C and 14 C, i.e. they take less 14 C than expected, making the specimen seem older. Factories during the industrial revolution burnt lots of coal and other fossil fuels, releasing much 12 C in the atmosphere more 12 C 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 12 C in the atmosphere, while 14 C was still produced more 14 C in the ratio after the flood would make look specimen before the flood older than they are.

8 Other radiometric dating methods These 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-206 PARENTDAUGHTER Decays to

9 Three necessary assumptions Scientists use the PARENT-DAUGHTER method to date rocks. This method works only if all the following assumptions are true. 1.We know the initial conditions (e.g. We know how much parent and daughter substance was present at the beginning) 2.The decay rate has always been constant (i.e. The half-life of parent and daughter has never changed in time) 3.The system is closed (i.e. No parent or daughter was added to or taken away from the sample)

10 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

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