Understanding Background Radiation with the help of nuclear physics
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Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 1
Understanding Background Radiation with the help of nuclear physics
Mike McNaughton
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 2
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.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 3
Why?
Measurements are affected by background.
Can we shield, subtract, or discriminate?
Terrestrial: Th-U-K Cosmic rays: muons, neutrons
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 4
Chart of the nuclides; stable nuclides in black; even numbers are favored.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 5
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.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 6
The dance of the nucleons
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LA-UR-12-24818 U N C L A S S I F I E D Slide 7
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.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 8
Does everyone have a partner?
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LA-UR-12-24818 U N C L A S S I F I E D Slide 9
This situation is unstable!
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 10
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 U234Th230Ra226Rn222Po218 Alpha decay of even-even: few gammas, and these few gammas have low energies.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 11
Uranium and Thorium Decay Chains
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LA-UR-12-24818 U N C L A S S I F I E D Slide 12
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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LA-UR-12-24818 U N C L A S S I F I E D Slide 13
Gammas accompany beta decay 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: Pb214Bi214Po214
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 14
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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LA-UR-12-24818 U N C L A S S I F I E D Slide 15
Pb214 and Bi214 indicate natural uraniumPb214 and Bi214 concentrations are equal.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 16
Bi214 vs U238 for natural and refined URefined uranium does not have Bi214.
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
U238 (pCi/g)
Bi21
4 (p
Ci/g
)
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 17
Bi214 vs U238 for natural and refined URefined uranium does not have Bi214.
0 50 100 150 200 250 300 350 4000
1
2
3
4
5
6
7
Bi214 and U238
U238 (pCi/g)
Bi21
4 (p
Ci/g
)
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 18
Conclusions
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.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 19
Optional extra slides
Cosmic rays include muons. They have very high energies: GeV, TeV … There are also neutrons at high altitudes.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 20
Mega: big Giga: Gigantic Tera is like tetra: (1000)4
Peta is like penta: (1000)5
Exa is like hexa: (1000)6
MeV, GeV, TeV, PeV, EeV
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LA-UR-12-24818 U N C L A S S I F I E D Slide 21
Muons
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
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 22
Cosmic Neutrons
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.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
LA-UR-12-24818 U N C L A S S I F I E D Slide 23
Cosmic Ray Conclusions
difficult to shield difficult to discriminate so we usually measure and subtract.
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