1. 22.2 Radioactive decay Radioactive decay and its random nature Why do radioactive substances emit nuclear radiation? This process is called radioactive decay or disintegration. unstable nuclei of radioactive substances stable nuclei emit ,  and  radiation

2. 22.2 Radioactive decay  Radioactive decay is a random process. We cannot predict when a particular nucleus will decay.  The behaviour of a large number of radioactive nuclei is predictable. Simulation: Radioactive decay

3. 22.2 Radioactive decay Activity and half-life The activity of a radioactive sample is the number of unstable nuclei which decay per unit time.  Activity = rate of radioactive decay  Unit for activity: becquerel (Bq) or s  1  1 Bq (or 1 s  1 ) = 1 disintegration per second

4. 22.2 Radioactive decay  Activity of a radioactive sample is directly proportional to the number of undecayed nuclei. Number of undecayed nuclei More nuclei decayed Activity  Activity gradually decreases over time.

5. 22.2 Radioactive decay  Activity decreases by half every 20 minutes. Decay curve of a certain radioactive sample Activity / Bq 8000 6000 4000 2000 0 102030405060708090time / min half-life

6. 22.2 Radioactive decay  For all radioactive samples, the time taken for the activity to decrease by half is a constant.  This time interval is called half-life. The half-life of a radioactive sample is the time taken for the activity of the sample to decrease to half of its original value.

7. 22.2 Radioactive decay  The number of undecayed nuclei decreases by half in each half-life. The half-life of a radioactive sample is the time taken for half of the undecayed nuclei in the sample to decay. number of undecayed nuclei / 10 20 86428642 0 12341234 number of half-lives

8. 22.2 Radioactive decay That’s the end of Section 22.2 Check Point Key Ideas Previous Page Section 22.3 Exit