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Potassium in the deep Earth: Radioactivity under pressure

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Potassium in the deep Earth: Radioactivity under pressure. Kanani K. M. Lee. DOANOW, March 23-25, 2007. [email protected] http://www.physics.nmsu.edu/~kanani. Earth’s Deep Interior. Lamb & Sington (1994). Heat  Dynamics. SOURCES Primordial: accretion differentiation

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slide1

Potassium in the deep Earth: Radioactivity under pressure

Kanani K. M. Lee

DOANOW, March 23-25, 2007

[email protected]

http://www.physics.nmsu.edu/~kanani

slide2

Earth’s Deep Interior

Lamb & Sington (1994)

slide3

Heat  Dynamics

  • SOURCES
  • Primordial:
    • accretion
    • differentiation
  • Radioactivity:
    • K, U, Th

www.gridclub.com/fact_gadget/ 1001/earth/earth/99.html

slide4

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?!?
slide5

Fe-K alloying at high P/T

Diamond-Anvil Cell experiments

Ab-initio QM calculations

Lee & Jeanloz, GRL, 2003; Lee et al., GRL, 2004

slide6

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

earth s current p t conditions
Earth’s current P/T conditions

300 K

~2000 K

~3000 K ?

~6000 K ?

or greater?

slide8

 radioactive decay schemes

- decay

+ decay

e- capture

slide9

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)
slide10

e.g., Bukowinski, 1979

EC decay is dependent on pressure, temperature, chemistry, ionization, etc.

slide11

Electron orbitals

cougar.slvhs.slv.k12.ca.us/. ../firstsemass.html

slide12

5d

4d

3d

6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

Energy

Relative energies of atomic orbitals

slide13

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

slide14

7Be

  • 1/2 ~ 53.3 days
  • 100% electron capture decay
  • = 0.478 MeV

Be, BeO, BeCl2

slide16

7Be

Prediction: ~0.1 days decrease in 1/2 at 50 GPa for

Be, BeO in hcp structure and BeCl2 in orth structure

slide17

Heat  Dynamics

  • SOURCES
  • Primordial:
    • accretion
    • differentiation
  • Radioactivity:
    • K, U, Th

www.gridclub.com/fact_gadget/ 1001/earth/earth/99.html

slide18

40K long-lived radioactive decay

Electron capture

- decay

t1/2,total~1.25 billion years!!!

Decay energy and concentration  relevant to the Earth

slide19

40K long-lived radioactive decay

Electron capture

Decay is dependent on pressure, temperature, chemistry, ionization, etc.

slide20

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)

slide21

5d

4d

3d

6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

Energy

Relative energies of atomic orbitals

slide22

5d

4d

3d

6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

Energy

K

Relative energies of atomic orbitals

slide23

5d

4d

3d

6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

Energy

K

Relative energies of atomic orbitals

slide24

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

slide26

sd

electronic

transition

40K under pressure

fcc K

slide27

40K under pressure

start of sd electronic transition:

~1% of electrons are in d orbital

fcc K

slide28

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!

slide29

Are these changes measurable?

7Be: Yes! ~40 billion decays/min

40K: Probably not. ~40 decays/day

slide31

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!!

slide34

sd

electronic

transition!?!

22Na under pressure

slide35

22Na under pressure

start of sd electronic transition:

~2% of electrons are in d orbital

slide36

Are these changes measurable?

7Be: Yes! ~40 billion decays/min

22Na: Yes! ~32 billion decays/day

40K: Probably not. ~40 decays/day

slide37

Pressure

DIAMOND ANVIL CELL

  • Strength
  • Transparency

Diamond

slide38

 measurements under high P

Ge

detector

Ge

detector

slide39

 measurements under high P

Ge

detector

Ge

detector

slide40

1-day background  spectra

of empty diamond cell

slide41

511 keV e+ emission

1275 keV 22Na -ray emission: 3+ billion counts per day!!!

1-day expected  spectra

slide42

511 keV e+ emission

1275 keV 22Na -ray emission: 3+ billion counts per day!!!

661 keV: 137Cs

1461 keV: 40K

2615 keV: 232Th208Tl

slide43

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
slide44

Funded by:

Alexander von Humboldt Foundation

Bayerisches Geoinstitut (Bayreuth)

CDAC (Department of Energy)

slide45

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)

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