Transcript UNM-LA-Sep-2012

Understanding Background Radiation
with the help of nuclear physics
Mike McNaughton
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Slide 1
Abstract
 Basic physics informs our understanding of
background radiation. The resulting
insights lead us to methods to distinguish
the materials of interest from background.
An understanding of the natural uranium
decay chain provides information on the
types and origins of natural and
anthropogenic materials.
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Slide 2
Why?
 Measurements are affected by background.
 Can we shield, subtract, or discriminate?
 Terrestrial: Th-U-K
 Cosmic rays: muons, neutrons
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Slide 3
Chart of the nuclides; stable nuclides
in black; even numbers are favored.
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Slide 4
Nuclides: odd and even
 Pairs of neutrons, pairs of protons, or pairs
of pairs are more stable.
 Even numbers are favored.
 Example: alpha particle is even-even-even.
 K-40: is very odd!
 Beta decay: odd-odd decays to even-even.
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Slide 5
The dance of the nucleons
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Slide 6
Dance of the nucleons
 Visualize a nucleus as a dance.
 The nucleons continuously reconfigure in
every possible way.
 Example: Be-8 quickly reconfigures as two
alpha particles.
 However, K-40 takes billions of years to
reconfigure as Ca-40 or Ar-40.
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Slide 7
Does everyone have a partner?
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Slide 8
This situation is unstable!
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Slide 9
Rules for alpha decay
 Even numbers are stable, e.g., U, Th
 Even-even is stable, e.g., U238, Th232
 More neutrons stable for alpha decay
(not for beta decay).
 even-even even-even
 U234Th230Ra226Rn222Po218
 Alpha decay of even-even: few gammas,
 and these few gammas have low energies.
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Slide 10
Uranium and Thorium Decay Chains
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Slide 11
Alpha Spec.
 Right-hand side of the Chart means:
• More neutrons
• Longer half-life for alpha decay
• Lower alpha energy
 Examples include
• U238
• Th232
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Slide 12
Gammas accompany beta decay
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Beta decay converts a neutron to a proton
so even-even goes to odd-odd
and odd-odd goes to even-even
two beta decays in succession.
 Pb is very stable and never emits an alpha.
 Example: Pb214Bi214Po214
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Slide 13
Gamma Spec.
 Few gammas from even-even alpha decay
 Most gammas if the parent or the product
is odd-odd
 Highest energy if the parent is odd-odd
 Examples
• Tl-208
• Bi-214
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Slide 14
Pb214 and Bi214 indicate natural uranium
Pb214 and Bi214 concentrations are equal.
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Slide 15
5
Bi214 (pCi/g)
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
U238 (pCi/g)
Bi214 vs U238 for natural and refined U
Refined uranium does not have Bi214.
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Slide 16
Bi214 and U238
7
6
Bi214 (pCi/g)
5
4
3
2
1
0
0
50
100
150
200
250
300
350
400
U238 (pCi/g)
Bi214 vs U238 for natural and refined U
Refined uranium does not have Bi214.
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Slide 17
Conclusions
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Nuclear physics helps us understand background.
Even-even nuclides contrast with odd-odd nuclides.
Useful gammas are associated with odd-odd nuclides.
The absence of Bi214 indicates refined uranium.
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Slide 18
Optional extra slides
 Cosmic rays include muons.
 They have very high energies: GeV, TeV …
 There are also neutrons at high altitudes.
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Slide 19
MeV, GeV, TeV, PeV, EeV
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Mega: big
Giga: Gigantic
Tera is like tetra: (1000)4
Peta is like penta: (1000)5
Exa is like hexa: (1000)6
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Slide 20
Muons
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Muons are like penetrating electrons.
Shielding is difficult.
10 km of air, 10 m of soil, 1 m of steel.
Rate of energy loss depends on speed.
Their speed is close to that of light.
In a beta detector, they look like betas.
Off-scale in a thick detector
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Slide 21
Cosmic Neutrons
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Almost the speed of light, so they are penetrating
Uncharged, so they are penetrating
Strong interaction with nucleons
More nucleons  more interactions  more shielding
Hydrogenous materials are not good shields.
Shielding is difficult.
Neutrons create recoil protons with a wide range of
energies so it is difficult to discriminate.
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Slide 22
Cosmic Ray Conclusions
 difficult to shield
 difficult to discriminate
 so we usually measure and subtract.
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Slide 23