Radioactivity AP Physics: M. Blachly
Review Balance the nuclear reaction:
History • Roentgen on Dec 22 1895, "photographed" his wife's hand
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.
History: Becquerel • This was found to be the case with all compounds containing uranium
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
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.
History: The Curies • Pierre is killed by horse-cart • Marie isolated the most radioactively potent element, named radium • Wins Nobel prize again by herself.
History: Rutherford • Used a radioactive source (Pb) to investigate the types of radioactivity
Radiation Types • Alpha Particles: heavy, positive charge • Beta Particles: lighter, negative charge • Gamma: no detectable mass, no charge
History: Rutherford • Discovered that almost entire mass must be centered in a “nucleus” that was unimaginably small
History: Other Particles • 1930 Paul Dirac predicts the existence of a positron • Positron found in 1932.
More Particles • 1932 Chadwick shot -particles at Be.
Radioactivity • Is a random probability event • The decay rate is proportional to the number of radioactive nuclei
Radioactivity • The decay rate can be given by
Radioactivity • The amount of radioactive nuclei left at any time is given by the function
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
Half Life • Time it takes for half of the radioactive nuclei to decay
Half-life • Determine the activity of a 1 g sample of Ra-226, which has a half-life of 1600 years.
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?
Half-life Example Solved • Bone is about 10,900 years old
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.