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Monday, 27 June 2011

Homework on Half Lives

Half Life of Radon Gas

a  The random nature of the decay process
b Around 55s
   Foundation students would choose 4000 to 2000 as one half life. They would draw a horizontal line accross to the middle of the curve and draw a vertical line. They would read the time off the x axis and call that the half life
Diligent students would draw a smmoth curve to represent the average decay. They would use use more than one half life from 4000 to 500 (3 half lives) to get a more accurate reading. They would also realise that the count of 4000 may correspond to a time of 0 seconds.
c The long half life would mean that there would be no perceptable change in the radiation count during the short time available for a school lab experiment.

Ice Age Man

isotope different nuclei of the same element, having the same number of protons but different number of neutrons
unstable incorrect balance of protons and neutrons (arrangement of charge within the nucleus) leading to the ejection prticle
Beta Particle high energy electron emitted form an unstable nucleus.
Half-life time taken for half the nuclei of a given radioactive substance to decay

6 protons
(14 - 6) = 8 neutrons

c  Text editor not up to typing this. Check post on Radiocarbon dating.

d 11 460 years ago. (2 half lives)
   16-->8 (1 half life)     8 --> 4 (another half life)
e It is too old.
6000 years is roughly the 1/2 life of C-14. The parts of a tree alive now would give out 14 beta particles per minute. The wood in the trunk is dead. A part that died 6000 years ago should be producing 7 beta particle per minute not 6. It would need a few hundred years to be down to 6. You could check how many by drawing a graph of count (y axis) and time (x axis)
f There is more CO2 in the atmosphere now.
I think a better answer would be that there is more C-14 taken up during photosynthesis now than there was 6000 years ago.
Why? C-14 is heavier than C-12 different diffusion rates?
Less C-14 produced by cosmic rays due to a lower concentration of CO2 in the upper atmosphere?

Do some research over the holidays