The document discusses radioactive decay and half-lives. It provides examples of using the concept of half-lives to calculate the age of materials by measuring the ratio of radioactive elements to their decay products. Specifically, it discusses how popcorn kernels and radioisotopes both undergo random decay over time that follows predictable patterns, and gives examples of using half-life calculations to determine the age of a radioactive rock sample and a school's radioactive source.

2. Lesson 5 Half Life

5. Half life The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… At start there are 16 radioisotopes After 1 half life half have decayed (that’s 8) After 3 half lives another 2 have decayed (14 altogether) After 2 half lives another half have decayed (12 altogether) = radioisotope = new atom formed

7. A radioactive decay graph Time Count 1 half life 2 half lives 3 half lives 4 half lives

8. Dating materials using half-lives Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. Answer : The sample was originally completely uranium… … of the sample was uranium Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old. 8 8 4 8 2 8 1 8 1 half life later… 1 half life later… 1 half life later…

9. How old is our source? Imagine that the school has a highly dangerous radioactive source called Barhamium which decays into the much safer and more stable Urquartium ! We know that the half-life of Barhamium is five years . When the source was bought it was pure Barhamium and gave out 500 units of radiation per minute. Recently the source has been tested and found to be giving out only 62.5 radiations per minute. Can you work out how old the source is?

10. How old is our source? Time (Years) Activity (Rads/minute) 0 500 5 250 10 125 15 62.5

11. An exam question… (Higher Paper) Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years