1. Types of Radioactivity Three types of Radioactivity: Alphas: Positively charged; Helium nuclei; can be stopped by paper Betas: Negatively charged; electrons; stopped by light metals Gammas: Uncharged; electromagnetic radiation; stopped by thick sheets of lead

2. Examples (lots of extras on board) 238 U alpha decay: 238 U -> 234 Th + α 60 Co beta decay: 60 Co -> 60 Ni + β 65 Ni gamma decay: 65 Ni* -> 65 Ni + γ

3. Half Lives All radioactive nuclei have their own particular half-life. This is the amount of time necessary for half of the nuclei in a given sample to decay to their “daughter” nucleus. There is no way (at present) to predict the half life of a given nucleus – they are measured experimentally

4. Half Life Calculation 116 In has a half life of one hour (give or take). a)Original sample gives 480 cts/sec. How many cts/sec after two hours? After 4 hours? b)Different sample originally gave 920 cts/min. We measure 115 cts/min. How old is the sample?

5. Real life examples 14 C decays to 14 N with a half life of 5730 years, giving 15 decays/min/gram of Carbon. 200 gram sample of Carbon, get 2100 counts in one min. See board for estimate. 90 Sr has a T 1/2 of 30 years. We start with 5 kg. How long until sample is safe? (Calc is actually too complicated for us....)