Half-life

Stability (or rate of decay) of a radioisotope is measured in half-life.

The decay of an unstable nucleus is a random event and is independent of chemical or physical conditions.

The half-life of a radioisotope is the time taken for the sample's activity to fall by half, and can be easily calculated.

Question one

Question

The mass of a radioisotope falls from 1.6g to 0.1g in 2 hours. What is the half-life of this radioisotope?

Original mass 1.6g. 1 half-life 0.8g. 2 half-lives 0.4g. 3 half-lives 0.2g. 4 half-lives 0.1g. Takes two hours, 120min. 4 half-lives equal 120min then 1 half-life equals 120 divided by four, 30min.

Question two

Question

If a 1g sample of a radioisotope with a half-life of 3 days has an activity of 32 counts sec-1, how long would it take for the activity of a 2g sample to fall to 8 counts sec-1?

1g activity 32. 2g sample activity 2x32, 64. 1 half-life activity 32. 2 half-lives 16. 3 half-lives 8. If half-life is 3 days, takes 3 half-lives for activity to drop to 8 counts then equals nine days

Radioactivity is measured in the number of nuclear decays an object has per second