Half-life questions
1. Californium-253 has a half-life of 18 days. Calculate how long it would take for the
radioactive count in a sample to decrease from 4Bq to 0.25Bq (2 marks)
2. Carbon – 14 has a half-life of 5700 years. A count begins at 200 and many years later the
count is done again and it is now 150. How many years later was that second count done? (2
marks)
3. The radioactive count of Rn-222 takes a 30.4 days to fall from 80000Bq to 312.5Bq. What is
the half-life of Rn-222? (2 marks)
4. A man has just found what he believes to be a really old coin with his metal detector. Upon
testing in the laboratory, the coin is found to have a C-14 reading of 1.6Bq. If the coin is
believed to be 11400 years old and the half-life of C-14 is 5700 years, estimate the reading
on the coin when it was new (3 marks)
5. The isotope Pu-238 has a half-life of 87.7 years and can be used to power generators. A
generator contains 925g of Pu-238 and 1g of Pu-238 has a power density of 0.54 W/g.
Estimate the energy released per second by the generator after 263 years (3 marks)