Radioactive isotope in use

Question 1

Carbon dating

Answer 1

• Carbon dioxide is absorbed by living plants due to photosynthesis.
• Living plants and trees contain a small amount of carbon-14 as well as carbon-12.
• The proportion of carbon-14 in the atmosphere for the last 50 000 years or so is known.
• Half-life of carbon-14 is 5570 years.
• After a tree has died it cannot take in any more carbon-14.
• Knowing activity of a certain mass of living wood, we can calculate how old sample is by measuring the current activity of the same mass.

Question 2

Argon dating

Answer 2

• Ancient rocks contain argon-40
  
  • as a result of decay of potassium-40 into argon-40 by electron capture
  • or of potassium-40 by β- decay into calcium-40.
• Half-life of potassium-40 is 1250 million years.
• The age of the rock is calculated by measuring the proportion of argon-40 to potassium-40.

Question 3

For every N potassium-40 atoms present.

Answer 3

• There is one argon atom.
• Formerly N + 9 potassium atoms
• 1 that decays into argon-40 + 8 that decays into calcium-40 + N remaining
• N = N₀e^-λt

Question 4

What is a radioactive tracer?

Answer 4

A radioactive tracer is used to follow the path of a substance through a system.

Question 5

A tracer should have a:

Answer 5

• Short half-life so that it does not remain in body a long time and cause damage, but not too short so that activity becomes too weak during measurements.
• Gamma source as radiation needs to pass through body to be detected.

Question 6

Commonly used medical tracer?

Answer 6

technetium-99 half-life 6 hours

Question 7

Detecting underground pipe leaks

Answer 7

• Radioactive tracer injected into the flow.
• A detector on the surface above the pipeline is used to detected leakage.
• Injected fluid contains a beta or gamma emitter as alpha would be absorbed by the pipes.

Question 8

Modelling oil reservoirs mathematically to improve oil recovery

Answer 8

• Triated water -half life 12 years- is injected into an oil reservoir at high pressure, forcing some oil out.
• Detectors at the production wells monitor breakthrough of the radioactive isotope.

Question 9

Uptake of fertilisers in plants

Answer 9

• Plants watered with a solution containing phosporus-32 - a beta emitter with half life 14- a fertiliser.
• By measuring the radioactivity of the leaves.
• The amount of the fertiliser reaching them can be determined.

Question 10

Monitor uptake of iodine by plants

Answer 10

• Patient administered sodium iodine contain iodine-131 with a half life of 8 days.
• The activity of the thyroid and the activity of an identical sample prepared at the same time is measured 24 hours later.