Understanding Background Radiation with the help of nuclear physics

1. Understanding Background Radiation with the help of nuclear physics Mike McNaughton

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.

3. Why? • Measurements are affected by background. • Can we shield, subtract, or discriminate? • Terrestrial: Th-U-K • Cosmic rays: muons, neutrons

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

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.

6. The dance of the nucleons

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.

8. Does everyone have a partner?

9. This situation is unstable!

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 • U234Th230Ra226Rn222Po218 • Alpha decay of even-even: few gammas, • and these few gammas have low energies.

11. Uranium and Thorium Decay Chains

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

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: Pb214Bi214Po214

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

15. Pb214 and Bi214 indicate natural uranium Pb214 and Bi214 concentrations are equal.

16. Bi214 vs U238 for natural and refined U Refined uranium does not have Bi214.

17. Bi214 vs U238 for natural and refined U Refined uranium does not have Bi214.

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.

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

20. MeV, GeV, TeV, PeV, EeV Mega: big Giga: Gigantic Tera is like tetra: (1000)4 Peta is like penta: (1000)5 Exais like hexa: (1000)6

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

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.

23. Cosmic Ray Conclusions difficult to shield difficult to discriminate so we usually measure and subtract.