Radioactivity Decay Constant Activity Exponential decay By Hannah I. Terhorst

Decay Constant The decay constant λ of the radio-nuclide is the probability that an individual nucleus will decay within a unit of time. λ = decay constant Exp. 0.2 s -1 = every nucleus has a 0.2 chance of decaying in one second 1

Activity The activity A of a source is the number of its nuclei that decay in unit time. Unit: 1 becquerel (Bq) is an activity of 1 decay per second 2

Exponential decay  the number of nuclei in a sample always decreases in the same way  t ½ = half time  different half-lives vary widely  from seconds to millions of years It is the time taken for half the radioactive nuclei to decay.  Exponential decay curve 3

 The higher the probability of decay λ  the more rapidly nuclide decay  the shorter is half-life 4

The age of rocks  Measuring how many 238 U changed to 206 Pb  Uranium-238 has a very long half-life of 4500 million years  stable nuclide lead-206  Other nuclides in the decay chain can be ignored 5

Carbon dating  Carbon-14 ( 14 C) half-life of 5730 years  All living things take 14 C more usual 12 C (photosynthesis)  14 C slowly decays  percentage slowly decreases  Used to date bones, wood, paper, cloths 6

Exp. Egyptian mummy  Step 1  Step 2 7

Source  Book page 342, 343, 345