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Radioactivity

Radioactivity

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Radioactivity

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  1. Radioactivity AP Physics: M. Blachly

  2. Review Balance the nuclear reaction:

  3. History • Roentgen on Dec 22 1895, "photographed" his wife's hand

  4. History: Becquerel • In 1896 put his wrapped photographic plates away in a darkened drawer with some crystals containing uranium. • the plates were exposed during storage. 

  5. History: Becquerel • This was found to be the case with all compounds containing uranium

  6. History: The Curies • Marie and Pierre Curie worked in Becquerel’s lab • Worked on isolating other elements which show activity • All three shared Nobel Prize for their work with radioactivity

  7. History: The Curies • Had no idea radiation was harmful • Marie kept it by her bed because it glowed. • Pierre carried a chuck in his pocket to show people he met.

  8. History: The Curies • Pierre is killed by horse-cart • Marie isolated the most radioactively potent element, named radium • Wins Nobel prize again by herself.

  9. History: Rutherford • Used a radioactive source (Pb) to investigate the types of radioactivity

  10. Radiation Types • Alpha Particles: heavy, positive charge • Beta Particles: lighter, negative charge • Gamma: no detectable mass, no charge

  11. Radiation Types

  12. History: Rutherford

  13. History: Rutherford • Discovered that almost entire mass must be centered in a “nucleus” that was unimaginably small

  14. History: Other Particles • 1930 Paul Dirac predicts the existence of a positron • Positron found in 1932.

  15. More Particles • 1932 Chadwick shot -particles at Be.

  16. Radioactivity • Is a random probability event • The decay rate is proportional to the number of radioactive nuclei

  17. Radioactivity • The decay rate can be given by

  18. Radioactivity • The amount of radioactive nuclei left at any time is given by the function

  19. Activity • SI unit of activity: Becquerel • 1 Bq = 1 decay/sec • Traditional Unit: The Curie • Activity of 1 gram of Radium • 1 Ci = 3.7 x 1010 Bq

  20. Half Life • Time it takes for half of the radioactive nuclei to decay

  21. Half Life

  22. Half-life • Determine the activity of a 1 g sample of Ra-226, which has a half-life of 1600 years.

  23. Half-life

  24. Half-life Example • C-14 has a half life of 5730 years. A 50 gram bone sample shows 200 decays/min. Living bone gives an activity of 15 decays/minute/gram. How old is the bone?

  25. Half-life Example Solved • Bone is about 10,900 years old

  26. Activity Levels

  27. Your turn • A scrap of paper taken from the Dead Sea Scrolls was found (using a mass spec) to have a 14C/12C ratio of 0.795 times that found in plants living today. Estimate the age of the scroll.