radioactivity ap physics: m. blachly. review balance the nuclear reaction:
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Radioactivity
AP Physics: M. Blachly
Review
14 17 1 N ___ Hn
14 14 17 6 1 N C Hn
Balance the nuclear reaction:
HistoryRoentgen on Dec 22 1895, "photographed" his wife's hand
History: BecquerelIn 1896 put his wrapped photographic plates away in a darkened drawer with some crystals containing uranium. the plates were exposed during storage.
History: BecquerelThis was found to be the case with all compounds containing uranium
History: The CuriesMarie and Pierre Curie worked in Becquerel’s labWorked on isolating other elements which show activityAll three shared Nobel Prize for their work with radioactivity
History: The CuriesHad no idea radiation was harmfulMarie kept it by her bed because it glowed.Pierre carried a chuck in his pocket to show people he met.
History: The CuriesPierre is killed by horse-cartMarie isolated the most radioactively potent element, named radiumWins Nobel prize again by herself.
History: Rutherford Used a radioactive source (Pb) to investigate the types of radioactivity
Radiation TypesAlpha Particles: heavy, positive chargeBeta Particles: lighter, negative chargeGamma: no detectable mass, no charge
Radiation Types
History: Rutherford
History: RutherfordDiscovered that almost entire mass must be centered in a “nucleus” that was unimaginably small
History: Other Particles1930 Paul Dirac predicts the existence of a positronPositron found in 1932.
More Particles1932 Chadwick shot -particles at Be.
4 9 122 4 6He Be C X
RadioactivityIs a random probability eventThe decay rate is proportional to the number of radioactive nuclei
( ) NR t Nt
RadioactivityThe decay rate can be given by
0( ) tR t R e
RadioactivityThe amount of radioactive nuclei left at any time is given by the function
0( ) tN t N e
ActivitySI 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 LifeTime it takes for half of the radioactive nuclei to decay
01/ 2( )
2NN T
0
50
100
150
200
250
300
350
400
0 5 10 15 20
Time (Years)
Rad
ioac
tive
Nuc
lei (
gram
s)
Half Life
1/ 2ln 2T
1/ 20 0
12
TN N e
Half-lifeDetermine the activity of a 1 g sample of Ra-226, which has a half-life of 1600 years.
Half-life
23
21
1 mole 6.02 10 nuclei1 g226 g 1 mole
2.66 10 nuclei
N
10
1/ 2
10
1/ 2
ln 2, , 5.045 10 sec
ln 2 decays3.66 10 sec
R N TT
NRT
Half-life ExampleC-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 SolvedBone is about 10,900 years old
Activity Levels
Radioactivity Level, in Bqsource for medical therapy 100 T1 kg high-level nuclear waste 10 Tmedical diagnosis 70 M1 kg uranium 25 M1 kg low level radioactive waste 1 M1 smoke detector 30 k1 adult human 7 k1 kg fertiliser 5 k1 kg of granite 1 k
Your turnA 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.
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