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GNEPGNEPAlan J Hays
Genoa Central High School
Arkansas
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Before you can intellectually discuss nuclear energy, one
must understand nuclear physics.
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Challenges to discussing Nuclear Physics
• Homer Simpson• The “stigma” of “physics”
too difficult, therefore is to be avoided at all costs
• Society issuestoo difficult, therefore is to be avoided at all costs
• Religious issues young earthers and other controversies, therefore is to be avoided at all costs
• Mathematics is language of physics, therefore
is too difficult, therefore is to be avoided at all costs
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Misconceptions• All radioactive elements are extremely
dangerous to living things.• There is no way to protect oneself from
radioactivity.• There are very few naturally occurring
radioactive elements.• Radioactive elements only occur in
nuclear power plants and nuclear weapons.
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It’s all Earth Science to me!
Most all of the earth’s rock material contains uranium. Ratio of U-238 to U-235 0.7%
U-238 →Pb T1/2 4.5 bil yrs.
U-235 T1/2 700 million yrs.
@t = 0 yrs [U-238] = [U-235]
Ratio of T1/2U-238 to T1/2U-235 7
Age of earth 8 U-235 half-lifes 5.6 bil yrs
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Lab Sequence
• What’s hot and what’s not
• Background radiation
• Radiation and Distance
• Shielding and absorption
• Half-Life
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What’s Hot and What’s Not
• A discovery lab
• Students are given a brief on GM tube use
• Given a bag of miscellaneous materials.
• Observations
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Background Radiation
• Count the “ticks” for a specified time• From a class get 20 – 30 recordings• Sample data: counts per 30 seconds.
C
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Background radiation: 40 cps
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Radiation and Distance• Familiar “inverse square law”• Sample data: Background Radiation = 58cpm
Dist=cm Counts/30s B-R corrected
5 3283 6508
6 2086 4114
7 1376 2694
8 1047 2036
9 793 1528
10 676 1294
11 559 1060
12 533 1008
13 469 880
14 445 832
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Radiation and Distance
• Graphing
y = 144651x-2.0132
0
1000
2000
3000
4000
5000
6000
7000
0 5 10 15
The exponent of x -2; data function very closely follows the inverse square law
2
1
xky
y = 144651x-2.0132
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Shielding
• Beta source• Data:
Type Counts %
Unshielded 4388
Paper 3860 88%
Thin Plastic 528 12%
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Absorption
• Beta source and aluminum shielding• Sample data
Al thickness cpm
0.020 4673
0.040 2757
0.060 1109
0.080 553
0.100 171
0.120 39
y = 16545e-47.095x
0
1000
2000
3000
4000
5000
6000
7000
0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140
y = 16545e-47.1x
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Absorption
• Gamma source and lead shielding• Sample Data
Pb thickness cpm
0 3762
0.032 3504
0.064 3478
0.125 3148
0.250 2486
y = -4966.7x + 3743.5
2000
2200
2400
2600
2800
3000
3200
3400
3600
3800
4000
0 0.05 0.1 0.15 0.2 0.25 0.3
y = -4967x + 3744
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Half-Life• Ba-137m Isogenerator
Cs-137 Parent nuclide; half-life(t1/2) 30yrs
This decays to a metastable form(excited state) of barium(Ba-137m).
Ba-137m decays to Ba-137(ground state) by gamma emission. t1/2 2.5min.
• Freshness of samples is critical to good data
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Half-Life
• Sample data:time Counts(N) Ln(N)
1 15 2.708
2 11 2.398
3 8.4 2.128
4 6.1 1.808
5 5 1.609
6 3.5 1.253
7 2.7 0.993
8 2 0.693
y = -6.3935Ln(x) + 15.188
0
2
4
6
8
10
12
14
16
0 2 4 6 8 10
Time vs Counts
y = -6.39ln(x) + 15.2
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Half-life
• Mathematical modeling: teNN 0
teNN 0
tNN
0
ln
tNN 0lnln
bmxy
NtN
0lnln
Linear!
N = # of nuclei present at time t.
N0 = # of nuclei present at t = 0.
= decay constant (it’s constant!)
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Half-life
• Linear graph Slope =
= - 0.2852
693.02ln
21 Ty = -0.2852x + 2.9821
0.000
0.500
1.000
1.500
2.000
2.500
3.000
0 2 4 6 8 102852.0693.0
2852.02ln
21 T
minT 43.221
Ln(N) vs time
y = -0.2852x + 2.9821
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Physics Mantras• Don’t memorize; understand• To know physics is to do physics• A single good observation is worth a century of bad
philosophy• Process is more important than results• You only arrive at the right answer after making all
possible mistakes.• A theory is accepted only when the last of its
opponents dies off.• Pollyannas who believe anything is possible should be
subjected to a course in thermodynamics.