Radioactive dating Flashcards
1
Q
How is 14C formed?
A
- formed in atmosphere as a result of cosmic rays knocking out neutrons from the nuclei
- there neutrons then collide with nitrogen nuclei to form carbon-14 nuclei
- (1/0) n + (14/7) N → (14 / 6) C + (1/1) P
- there neutrons then collide with nitrogen nuclei to form carbon-14 nuclei
2
Q
How does 14C enter plants?
A
- as a result of photosynthesis
- so a small percentage of the carbon content of any plant is carbon-14
3
Q
What is the half-life of 14C and why is it important?
A
- 5570
- will have a negligible decay during the liftetime of a plant
4
Q
How is a sample dated from the amount of 14C?
A
- once the plant has died, no further carbon is taken in so the propotion of carbon-14 in the dead tree is propotional to the number of atoms still to decay
5
Q
How is the date of the carbon measured
A
- activity is propotional to the number of atoms still to decay
- measuring the activity of the dead sample enables its age to be calculated
- provided the activity of the same mass of living wood is known
- this is taken to be N0 in the decay equation
6
Q
Why is carbon dating not used for samples thought to be less than 100 years old?
A
- half life is much greater than time left
- unreliable
- not used for anything over 10000 or summit as well like that probs don’t need to know any specific numbers just common sense
7
Q
What radioactive isotope do ancient rocks contain? What does it decay into
A
- (40/19) K
- this decays into (40/18) Ar via electron capture
- also decays into calcium by beta minus emission
- 8x more likely
8
Q
How does argon dating work?
A
- measuring propotion of argon-40 to potassium-40
9
Q
Why cant the K be measured directly
A
half life of 1250 million years
10
Q
For every N potassium isotopes how many argon are there?
A
- for every one 1 argon
- as there must have been N+9 potassium atoms
- as 8 decaued into calcium
- and there are N remaining
- if N = 1, N0 = 9
