Understanding Background Radiation with the help of nuclear physics

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



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.



Why?

Measurements are affected by background.

Can we shield, subtract, or discriminate?

Terrestrial: Th-U-K Cosmic rays: muons, neutrons



Chart of the nuclides; stable nuclides in black; even numbers are favored.



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.



The dance of the nucleons



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.



Does everyone have a partner?



This situation is unstable!



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.



Uranium and Thorium Decay Chains



Alpha Spec.

Right-hand side of the Chart means:• More neutrons• Longer half-life for alpha decay• Lower alpha energy

Examples include• U238• Th232



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



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



Pb214 and Bi214 indicate natural uraniumPb214 and Bi214 concentrations are equal.



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

)



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

)



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.



Optional extra slides

Cosmic rays include muons. They have very high energies: GeV, TeV … There are also neutrons at high altitudes.



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



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



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.



Cosmic Ray Conclusions

difficult to shield difficult to discriminate so we usually measure and subtract.

Understanding Background Radiation with the help of nuclear physics

Documents

s s i f i e d slide

los alamos national

beta decaybeta d

alpha spec

alpha particles

nuclides stable nuclides

alpha decayeven numbers

th232more neutrons stable