Potassium in the deep Earth: Radioactivity under pressure Kanani K. M. Lee DOANOW, March 23-25, 2007...

Potassium in the deep Earth: Radioactivity

under pressure

Kanani K. M. LeeDOANOW, March 23-25, 2007

[email protected]

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

Lamb & Sington (1994)

Earth’s Deep Interior

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

Heat Dynamics

SOURCES

Primordial:

• accretion

• differentiation

Radioactivity:

• K, U, Th

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?!?

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

Fe-K alloying at high P/T

Diamond-Anvil Cell experiments

Ab-initio QM calculations


Heat Dynamics

Up to 20% of the Earth’s power generated from 40K decay in the core!!

•Geodynamo•Mantle convection

•Long-lived magma ocean?•Core-Mantle boundary reactions?

Very HOT early Earth.

Earth’s current P/T conditions

300 K

~2000 K

~3000 K ?

~6000 K ?

or greater?

€

n→ p+ e−

p→ n + e+

p+ e− → n

- decay

+ decay

e- capture

radioactive decay schemes

•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)

A bit of Electron Capture History

€

δλec =ρ e,totalρ e,total ,0

−1 ⎡

⎣ ⎢

⎤

⎦ ⎥λ ec,0€

λec ∝ total electron charge density ρ e,total

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

e.g., Bukowinski, 1979

Electron orbitals

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

6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

5d

4d

3d

En

erg

y

Relative energies of atomic orbitals

In collaboration with Gerd Steinle-Neumann, BGI

•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

Computational Method

1/2 ~ 53.3 days100% electron capture decay

= 0.478 MeVBe, BeO, BeCl2

7Be

€

7Be→7Li + γ

1.0

0.8

0.6

0.4

0.2

0.0

percent difference,

λec

50403020100

( )Pressure GPa

, Be hcp , BeO hcp BeCl2 , orth

., 1963Gogarty et al ., 1973Hensley et al & , 2000Liu Huh

7Be under pressure

Prediction: ~0.1 days decrease in 1/2 at 50 GPa forBe, BeO in hcp structure and BeCl2 in orth structure

€

7Be→7Li + γ

7Be


Heat Dynamics

SOURCES

Primordial:

• accretion

• differentiation

Radioactivity:

• K, U, Th

1940K +e−→18

40Ar +γ

1940K→ 20

40Ca+e− +γ

12total~1.25 billion years!!!

Decay energy and concentration relevant to the Earth

40K long-lived radioactive decay

Electron capture

- decay

1940K +e−→18

40Ar +γ

40K long-lived radioactive decay

Electron capture

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

With pressure a 4s 3d

electronic transition makes K, an alkali metal, more like a

transition metal (Bukowinski, 1976)

Transition metals

s d electronic transition in K

6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

5d

4d

3d

En

erg

y


6s

5s

4s

3s

2s

1s

5p

4p

3p

2p

5d

4d

3d


K

En

erg

y

1/2,total ~ 1.25 Gyr1/2,EC ~ 11.9 Gyr1/2,- ~ 1.4 Gyr

~11% electron capture decay

= 1.461 MeVK, K2O, KCl

40K

€

40K→40Ar + γ

40K under pressure

0.02

0.01

0.00

percent difference,

λec

50403020100

( )Pressure GPa

, K bcc , K fcc K2 , O fcc

, 1KCl B , 2KCl B

5395.0

5394.5

5394.0

5393.5

total electron charge density,

ρe

80604020

(Volume Å3)

1.0

0.8

0.6

0.4

0.2

0.0

- number of d orbital electrons

05102550 ( )Pressure GPa

40K under pressure

sdelectronictransition

fcc K

5395.0

5394.5

5394.0

5393.5


ρe

80604020

(Volume Å3)

1.0

0.8

0.6

0.4

0.2

0.0

- number of d orbital electrons

05102550 ( )Pressure GPa

40K under pressure

fcc K

start of sd electronic transition: ~1% of electrons are in d orbital

40K

€

40K→40Ar + γ

Prediction: ~3 Myr decrease in 1/2,ec at 25 GPa for K and

~0.6 Myr decrease for K2O and KCl

sd transition matters!

Are these changes measurable?

7Be: Yes! ~40 billion decays/min

40K: Probably not. ~40 decays/day

Periodic Table of Elements

Comparable EC system: 22Na

1/2,total ~ 2.6 yr1/2,EC ~ 27.7 yr1/2,- ~ 2.8 yr

~9.4% electron capture decay = 1.275 MeVNa, Na2O, NaCl

~32 billion decays/day!!

22Na under pressure0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

percent difference,

λec

100806040200 ( )Pressure GPa

, Na bcc , Na fcc , 1NaCl B , 2NaCl B Na2 , O fcc

22Na under pressure

0.3

0.2

0.1

0.0

number of d-orbital electrons

40302010

Volume (Å3)

893

892

891

890

889

888


ρe

010251001000

, Na fcc total electron density 4 - th order polynomial fit - d orbital occupancy

( )Pressure GPa

0.25

0.20

6543

892

891

1000

22Na under pressure

0.3

0.2

0.1

0.0


40302010

Volume (Å3)

893

892

891

890

889

888


ρe

010251001000


( )Pressure GPa

sdelectronic

transition!?!

22Na under pressure

0.3

0.2

0.1

0.0


40302010

Volume (Å3)

893

892

891

890

889

888


ρe

010251001000


( )Pressure GPa

start of sd electronic transition: ~2% of electrons are in d orbital

Are these changes measurable?

7Be: Yes! ~40 billion decays/min

22Na: Yes! ~32 billion decays/day

40K: Probably not. ~40 decays/day

DIAMOND ANVIL CELL

Diamond• Strength• Transparency

=

ForceArea

Pressure

Ge

detector

Ge

detector

measurements under high P

1400

1200

1000

800

600

400

200

0

counts

300025002000150010005000

Energy (keV)

1-day background spectraof empty diamond cell

1400

1200

1000

800

600

400

200

0

counts

300025002000150010005000

Energy (keV)

511 keV e+ emission

1275 keV 22Na -ray emission: 3+ billion

counts per day!!!

1-day expected spectra

1400

1200

1000

800

600

400

200

0

counts

300025002000150010005000

Energy (keV)

511 keV e+ emission

1275 keV 22Na -ray emission: 3+ billion

counts per day!!!

661 keV: 137Cs 1461 keV:

40K2615 keV: 232Th208Tl

•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

Conclusions

Funded by:

Alexander von Humboldt Foundation

Bayerisches Geoinstitut (Bayreuth)

CDAC (Department of Energy)

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)

Potassium in the deep Earth: Radioactivity under pressure Kanani K. M. Lee DOANOW, March 23-25, 2007...

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Transcript of Potassium in the deep Earth: Radioactivity under pressure Kanani K. M. Lee DOANOW, March 23-25, 2007...