Potassium in the deep Earth: Radioactivity under pressure

1. Potassium in the deep Earth: Radioactivity under pressure Kanani K. M. Lee DOANOW, March 23-25, 2007 kanani@physics.nmsu.edu http://www.physics.nmsu.edu/~kanani

2. Earth’s Deep Interior Lamb & Sington (1994)

3. Heat  Dynamics • SOURCES • Primordial: • accretion • differentiation • Radioactivity: • K, U, Th www.gridclub.com/fact_gadget/ 1001/earth/earth/99.html

4. What we “know”: • Chondritic K/U ratio ~8 x 104 (Wasserburg et al., 1964) • Terrestrial K/U ratio ~1 x 104 • What we don’t know: • Why is there a discrepancy in the K/U ratios? • K lost to space during accretion?!? • K incorporated in the deep Earth during accretion?!?

5. Fe-K alloying at high P/T Diamond-Anvil Cell experiments Ab-initio QM calculations Lee & Jeanloz, GRL, 2003; Lee et al., GRL, 2004

6. Very HOT early Earth. • Long-lived magma ocean? • Core-Mantle boundary reactions? Heat  Dynamics • Up to 20% of the Earth’s power generated from 40K decay in the core!! • Geodynamo • Mantle convection www.gridclub.com/fact_gadget/ 1001/earth/earth/99.html

7. Earth’s current P/T conditions 300 K ~2000 K ~3000 K ? ~6000 K ? or greater?

8.  radioactive decay schemes - decay + decay e- capture

9. A bit of Electron Capture History • 1947: Electron capture decay of 7Be predicted to be affected by extra-nuclear environments: Segré, Daudel • Late 1940’s-1950’s: lots of theory, measurements on chemical environment effect on 7Be decay • 1963: first pressure-dependent measurement on 7Be decay (Gogarty et al., ONR) • 1970’s: more theory (40K, e.g., Bukowinski, 1976), another P measurement (7Be, Hensley et al., 1973) • 2000’s: more theory, another P measurement (Liu et al., 2000)

10. e.g., Bukowinski, 1979 EC decay is dependent on pressure, temperature, chemistry, ionization, etc.

11. Electron orbitals cougar.slvhs.slv.k12.ca.us/. ../firstsemass.html

12. 5d 4d 3d 6s 5s 4s 3s 2s 1s 5p 4p 3p 2p Energy Relative energies of atomic orbitals

13. Computational Method • Structures fully relaxed using VASP • All-electron method, full potential (LAPW), Wien2k • Both GGA and LDA approximations to many-body interactions VXC • Energy convergence to ~1 meV/atom In collaboration with Gerd Steinle-Neumann, BGI

14. 7Be • 1/2 ~ 53.3 days • 100% electron capture decay • = 0.478 MeV Be, BeO, BeCl2

15. 7Be under pressure

16. 7Be Prediction: ~0.1 days decrease in 1/2 at 50 GPa for Be, BeO in hcp structure and BeCl2 in orth structure

17. Heat  Dynamics • SOURCES • Primordial: • accretion • differentiation • Radioactivity: • K, U, Th www.gridclub.com/fact_gadget/ 1001/earth/earth/99.html

18. 40K long-lived radioactive decay Electron capture - decay t1/2,total~1.25 billion years!!! Decay energy and concentration  relevant to the Earth

19. 40K long-lived radioactive decay Electron capture Decay is dependent on pressure, temperature, chemistry, ionization, etc.

20. s  d electronic transition in K Transition metals With pressure a 4s  3d electronic transition makes K, an alkali metal, more like a transition metal (Bukowinski, 1976)

21. 5d 4d 3d 6s 5s 4s 3s 2s 1s 5p 4p 3p 2p Energy Relative energies of atomic orbitals

22. 5d 4d 3d 6s 5s 4s 3s 2s 1s 5p 4p 3p 2p Energy K Relative energies of atomic orbitals

23. 5d 4d 3d 6s 5s 4s 3s 2s 1s 5p 4p 3p 2p Energy K Relative energies of atomic orbitals

24. 40K 1/2,total ~ 1.25 Gyr 1/2,EC ~ 11.9 Gyr 1/2,- ~ 1.4 Gyr ~11% electron capture decay  = 1.461 MeV K, K2O, KCl

25. 40K under pressure

26. sd electronic transition 40K under pressure fcc K

27. 40K under pressure start of sd electronic transition: ~1% of electrons are in d orbital fcc K

28. 40K Prediction: ~3 Myr decrease in 1/2,ec at 25 GPa for K and ~0.6 Myr decrease for K2O and KCl sd transition matters!

29. Are these changes measurable? 7Be: Yes! ~40 billion decays/min 40K: Probably not. ~40 decays/day

30. Periodic Table of Elements

31. Comparable EC system: 22Na 1/2,total ~ 2.6 yr 1/2,EC ~ 27.7 yr 1/2,- ~ 2.8 yr ~9.4% electron capture decay  = 1.275 MeV Na, Na2O, NaCl ~32 billion decays/day!!

32. 22Na under pressure

33. 22Na under pressure

34. sd electronic transition!?! 22Na under pressure

35. 22Na under pressure start of sd electronic transition: ~2% of electrons are in d orbital

36. Are these changes measurable? 7Be: Yes! ~40 billion decays/min 22Na: Yes! ~32 billion decays/day 40K: Probably not. ~40 decays/day

37. Pressure DIAMOND ANVIL CELL • Strength • Transparency Diamond

38.  measurements under high P Ge  detector Ge  detector

39.  measurements under high P Ge  detector Ge  detector

40. 1-day background  spectra of empty diamond cell

41. 511 keV e+ emission 1275 keV 22Na -ray emission: 3+ billion counts per day!!! 1-day expected  spectra

42. 511 keV e+ emission 1275 keV 22Na -ray emission: 3+ billion counts per day!!! 661 keV: 137Cs 1461 keV: 40K 2615 keV: 232Th208Tl

43. Conclusions • Pressure and chemistry DO have an effect on electron capture radioactive decay, although small • 7Be predictions are compatible with previous experiments, although lower • Na and K as pure metals are predicted to show more P-dependence than respective simple oxides and chlorides • Pressure, chemical environment effects are measurable for longer-lived isotope systems

44. Funded by: Alexander von Humboldt Foundation Bayerisches Geoinstitut (Bayreuth) CDAC (Department of Energy)

45. Special thanks to: Gerd Steinle-Neumann (BGI) Sofia Akber-Knutson (UCSD) Ron Nelson (LANL) Bob Rundberg (LANL) Boris Kiefer (NMSU) Allen Knutson (UCSD) David Dolejs (BGI) Innokenty Kantor (BGI) Artem Oganov (ETH)