**TAP 514- 2: Half Life**
The half-life of strontium-90 is 27 years. The half-life of sodium-24 is 15 hours.
1. Sketch two curves on one set of axes to show how the number of atoms of each change with time.
Two samples are prepared, one containing 10^{20} atoms of strontium and the other containing 10^{20} atoms of sodium.
2. Which of the two samples has the highest activity?
3. A sample of iodine-131, with half-life 8.04 days, has an activity of 7.4 x 10^{7} becquerel.
Calculate the activity of the sample after 4 weeks?
4. ^{234}Th has a half life of 24.1 days.
(a) What fraction of a sample remains after 96.4 days?
(b) What fraction of a sample remains after 241 days?
**Answers and worked solutions**
1.
2. Sodium, since a longer half-life means the source is less active.
3. First find the number of half-lives in 4 weeks:
The activity will be:
4
(a) 96.4/24.1=4 so 4 half lives so (1/2)^{4} = =1/16 or .0625
(b) 241/24.1=10 so 10 half lives (1/2)^{10} = 1/1024 or 9.8 x 10^{-4}
**External references**
This activity is taken from Advancing Physics chapter 10, 20S which was an adaptation of Revised Nuffield Advanced Physics question 15 section F. |