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PH 122 (46)

Half life, activity, radioactive dating, special relativity, binding energy

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PH 122
Brian Jones

9 April Short half life means high activity 0.693N Activity = decay rate = R = t1 2 Time must be in seconds Unit of activity, Becquerel, is in seconds As the atoms decay, the activity decreases Half Life and Activity 235 238 There are two main isotopes of uranium, U and U. Two billion years ago, 23U comprised 3% of the uranium in the earth’s crust. Now, it’s prevalence has dropped to 0.7% of the uranium in the earth’s crust. Which of the isotopes has a longer half life? 238 U If you have a sample of 23U and a sample of 238U, each with the same mass (and therefore approximately the same number of atoms) which sample will have a higher activity? 235U 226 What is the activity, in Bq and Ci, of 1.0 g of Ra? Marie Curie was the discoverer of radium; can you see where the unit of activity named after her came from? 0.693N Activity = decay rate = R = t1 2 Use the “quick and dirty” form of mass – m ( Ra) = 226 u = 3.75 x 10 -25kg 0.001kgsample 21 m = 3.75x10 −25kg peratom = 2.67 x 10 atoms t 1 ( Ra) = 1600 yr = 5.05 x 10 s 10 2 R = 3.7 x 10 Bq = 1.0 Ci t1 My watch has a tritium ( 2 = 12 yr) dial. When I purchased it in 2004, it had an activity of about 100 MBq. What is the activity now? R = 100 MBq [ ½ ] 10 yr/12 yr R = 56 MBq Radioactive Dating 14 12 A scrap of parchment from the Dead Sea Scrolls was found to have a C/ C ratio that is 79.5% of the modern value. Determine the age of this parchment. t N 1 1 N 0 = [ ] 2 2 t
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