Tues. Dec 14, 2009 Phy208 Lecture 29 1 Final Exam is Mon Dec 21, 5:05 pm - 7:05 pm 2103 Chamberlin 3 equation sheets allowed About 30% on new material Rest on topics of exam1, exam2, exam3. Week15HW covers material for final, but does not count toward HW grade. From Last Time… Nuclear structure and isotopes Binding energy of nuclei
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Tues. Dec 14, 2009Phy208 Lecture 29 1 Final Exam is Mon Dec 21, 5:05 pm - 7:05 pm 2103 Chamberlin 3 equation sheets allowed About 30% on new material Rest.
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Tues. Dec 14, 2009 Phy208 Lecture 29 1
Final Exam is Mon Dec 21, 5:05 pm - 7:05 pm 2103 Chamberlin
3 equation sheets allowedAbout 30% on new materialRest on topics of exam1, exam2, exam3.
Week15HW covers material for final,but does not count toward HW grade.
From Last Time…Nuclear structure and isotopesBinding energy of nuclei
Tues. Dec 14, 2009 Phy208 Lecture 29 2
Radioactive nuclei
~ equal # neutrons and protons
Tues. Dec 14, 2009 Phy208 Lecture 29 3
Radioactive decay• Unstable nuclei decay by emitting particle
• Can be photon (light particle), or matter particle.
• Emitted particle carries away energy
– Can strip electrons from atoms (ionizing radiation)
– break apart chemical bonds in living cells (radiation damage)
• Radiation damage depends on– Energy deposited / tissue mass (1 Gy (gray) = 1J/kg)– Damaging effect of particle (RBE, relative biological effectiveness)
• Dose equivalent = (Energy deposited / tissue mass) x RBE– Units of Sv (sieverts) [older unit = rem, 1 rem=0.01 Sv]– Common units mSv (10-3Sv), mrem (10-3rem)– Common ‘safe’ limit = 500 mrem/yr (5 mSv/yr)
Tues. Dec 14, 2009 Phy208 Lecture 29 5
Radioactive tracersWorked on radioactivity
as student with Ernest Rutherford.
Lodged in nearby boarding home.
Suspected his landlady was serving meals later in week ‘recycled’ from the Sunday meat pie. His landlady denied this!
deHevesy described his first foray into nuclear medicine:
George de HevesyGeorge de Hevesy
“The coming Sunday in an unguarded moment Iadded some radioactive deposit [lead-212] to thefreshly prepared pie and on the following Wednesday,with the aid of an electroscope, I demonstrated to thelandlady the presence of the active deposit in the soufflé.”
Tues. Dec 14, 2009 Phy208 Lecture 29 6
A random process
• Radioactive decay is a random process– It has some probability of occurring.
• For one nucleus,– – r = decay rate
• For N nuclei, – # decays N = N x Prob(decay) =rNt– # decays / s = N/t =rN
€
Probdecay in Δt( ) = rΔt
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⇒ N = Noe−r t
Tues. Dec 14, 2009 Phy208 Lecture 29 7
Radioactive half-life• Example of random decay.• Start with 8,000 identical radioactive nuclei• After one half-life, half the nuclei have decayed.
t=0 t=1 yr
t=2 yr
t=3 yr
Every half-life, half the atoms decay
Undecayed nuclei
Tues. Dec 14, 2009 Phy208 Lecture 29 8
Radioactive decay question
A piece of radioactive material is initially observed to have 10,000 radioactive nuclei.
3 hours later, you measure 1,250 radiaoctive nuclei.
The half-life is
A. 1/2 hourB. 1 hourC. 3 hoursD. 8 hours
In each half-life, the number of radioactive nuclei, and hence the number of decays / second, drops by a factor of two.
After 1 half life, 5000 are left undecayed. After 2 half lives, 1/2 of these are left: 2,500After 3 half lives there are 1,250 left.
Tues. Dec 14, 2009 Phy208 Lecture 29 9
Radioactive decay question
A piece of radioactive material is initially observed to have 1,000 decays/sec.
It’s half life is 2 days.Four days later, you measure
Decay rate r (Units of s-1) Prob( nucleus decays in time t ) = r t
Activity R (Units of becquerel (1 Bq=1 s-1) orcurie (1 Ci=3.7x1010 s-1)
Mean # decays / s = rN, N=# nuclei in sample
Half-life t1/2 (Units of s) time for half of nuclei to decay = t1/2
Quantifying radioactivity
€
=ln2
r=
0.693
r
€
N = Noe−r t
Tues. Dec 14, 2009 Phy208 Lecture 29 11
Different types of radioactivity
• Three different types of decay observed:Alpha decayBeta decayGamma decay
(First three letters of Greek alphabet).
Ernest Rutherford (1899): "These experiments show that the uranium radiation is complex and that there are present at least two distinct types of radiation - one that is very readily absorbed, which will be termed for convenience the alpha-radiation, and the other of more penetrative character which will be termed the beta-radiation."
Tues. Dec 14, 2009 Phy208 Lecture 29 12
Heavy nucleus spontaneously emits alpha particle
Example of a decay
• nucleus loses 2 neutrons and 2 protons.
• It becomes a different element (Z is changed)
• Example:
€
92238U→ 2
4He+ 90234Th
92 protons146 neutrons
90 protons144 neutrons
2 protons2 neutrons
Alpha particle
Tues. Dec 14, 2009 Phy208 Lecture 29 13
Decay sequence of 238U
Number of neutrons
Num
ber
of
pro
ton
s
a decay
Tues. Dec 14, 2009 Phy208 Lecture 29 14
RadonZone 1 Highest Potential (greater than 4 pCi/L)
Zone 2 Moderate Potential (from 2 to 4 pCi/L)
http://www.radonwisconsin.com/
• Radon is in the 238U decay series
• Radon is an a emitter that presents an environmental hazard
• Inhalation of radon and its daughters can ionize lung cells increasing risk of lung cancer