Half-Life

Question 1

Define half-life in terms of the number of nuclei of a radioactive isotope in a sample.

Answer 1

The half-life of a radioactive isotope is the time taken for the number of nuclei of that isotope in a sample to halve.

Question 2

True or false? It is possible to predict exactly how long it will take for a certain radioactive nucleus to decay

Answer 2

False
Radioactive decay is a random process, so it is impossible to predict when a certain nucleus will decay or which nucleus will decay next

Question 3

Activity is measured in becquerels. What is 1 Bq?

Answer 3

1 Bq is 1 decay per second

Question 4

What is meant by the activity of a radioactive source? Why does it decline over time?

Answer 4

Activity is the rate at which a source of unstable nuclei decays
Activity declines because each time a radioactive nucleus decays, one more radioactive nucleus disappears. So over time, the number of unstable nuclei that could decay decreases

Question 5

Sample Y has a half-life of 8 hours. It has an initial activity of 200 Bq. Calculate the activity of sample Y after 1 day.

Answer 5

25Bq
1 day = 24 hours, so 1 day = 24 / 8 = 3 half-lives
Activity after 1 half-life = 200 / 2 = 100 Bq
After 2 half-lives = 100 / 2 = 50 Bq
After 3 half-lives = 50 / 2 = 25Bq