Half-Life and Activity Practice Questions
1. If you have a radioactive sample of material what fraction will decay after 1, 2, 3, 4
and 5 half-lives.
2. If 12.5g of a 100g sample remains after 2 hours, what is its half-life?
3. Calculate the activity of a sample if 1.1 × 1015 decays occur in 294s.
4. Potassium-44 has a half life of 20 minutes and decays to form a stable isotope of
Calcium. It’s initial activity is 7.9 × 1016 Bq. What would be the activity after one
hour. What would the ratio of the potassium atoms to calcium atoms be after one
hour?
5. Is a measured count rate the same as activity? Why/Why not?
Stretch and Challenge Questions
1. In our dice model of radioactive decay, is it better to use 10, 100 or 1,000 dice, why?
2. A Geiger counter placed next to a source of radiation registers 8,127 counts in 42s,
after 2 days it’s count rate is measured to be 4.5 counts s-1? The background count in
42s is 63.
3. The atomic mass number of an element is the mass (in grams) of a certain number of
atoms in a given quantity of a substance known as a mole. It is also the number of
nucleons in a substance. How many atoms are in 1 mole of the substance?
(Hint: Uranium-235, mass of a proton= 1.67 × 10-27 kg.
Half-Life and Activity Practice Questions
1. If you have a radioactive sample of material what fraction will decay after 1, 2, 3, 4
and 5 half-lives.
2. If 12.5g of a 100g sample remains after 2 hours, what is its half-life?
3. Calculate the activity of a sample if 1.1 × 1015 decays occur in 294s.
4. Potassium-44 has a half life of 20 minutes and decays to form a stable isotope of
Calcium. It’s initial activity is 7.9 × 1016 Bq. What would be the activity after one
hour. What would the ratio of the potassium atoms to calcium atoms be after one
hour?
5. Is a measured count rate the same as activity? Why/Why not?
Stretch and Challenge Questions
1. In our dice model of radioactive decay, is it better to use 10, 100 or 1,000 dice, why?
2. A Geiger counter placed next to a source of radiation registers 8,127 counts in 42s,
after 2 days it’s count rate is measured to be 4.5 counts s-1? The background count in
42s is 63.
3. The atomic mass number of an element is the mass (in grams) of a certain number of
atoms in a given quantity of a substance known as a mole. It is also the number of
nucleons in a substance. How many atoms are in 1 mole of the substance?
(Hint: Uranium-235, mass of a proton= 1.67 × 10-27 kg.