Thorium Spectroscopy

Tanja E. Mehlstäubler

Center for Quantum Engineering and Space Time Research Leibniz Universität Hannover

Physikalisch-Technische Bundesanstalt, Braunschweig Department of Time & Frequency

Physics with Trapped Charged Particles – Les Houches, 19 January 2012 Outline

ͻ Why is nuclear laser spectroscopy difficult? ͻdŚĞůŽǁ-ĞŶĞƌŐLJŝƐŽŵĞƌŝĐƐƚĂƚĞŝŶdŚ-229 ͻdŚ-229 as a precise optical nuclear clock • Application search for D Visible light not matched to energy scales in nucleus

Energy scales: Photon in optical range: !Z | 2 eV

Nucleus: bound nucleon 'x | 5˜1015 m 2 ǁŝƚŚ 'x ˜'p o ! 0,83 MeV ! 2('x)2 m p (rest energy of proton: 938 MeV)

10 Atomic shell: bound electron 'x |10 m 2 ǁŝƚŚ 'x ˜'p o ! 3,8 eV ! 2('x)2 m e (rest energy of electron: 0,51 MeV) Electric field scales inside atom / nucleus

q E 2 4SH 0r

e- Shell: 10 11 r 10 m E S 1. 4 ˜ 10 V/m

15 19 Nucleus: r 5 ˜ 10 m E N 5. 8 ˜ 10 V/m

ůĞĐƚƌŝĐĨŝĞůĚŽĨĞůĞĐƚƌŽŵĂŐŶĞƚŝĐǁĂǀĞŽĨŝŶƚĞŶƐŝƚLJI:

1 E E o I 2. 5 ˜ 1015 W/cm 2 I H cE 2 L S 2 0 L 32 2 E L E N o I 4. 5 ˜ 10 W/cm Maximum intensity of short-pulse laser

Intensity Limit:

ŐĂŝŶďĂŶĚǁŝĚƚŚ ƉŚŽƚŽŶĞŶĞƌŐLJϭͬ;ŵŝŶ͘ǁĂŝƐƚͿ

Q 2 I | N ˜ h Q ˜ 'v˜ max Ph c 2 | area of ampl. medium | 12 N Ph transition cross section 10

24 2 I max | 10 W/cm

e- shell-field strength: reachable nuclear electr. field strength: far beyond Mourou et al., Phys. Today 51, 22 (1998) EƵĐůĞƵƐŝƐŶŽƐƵŝƚĂďůĞĂŶƚĞŶŶĂĨŽƌǀŝƐŝďůĞůŝŐŚƚ

>ŝĨĞƚŝŵĞĨŽƌƌĂĚŝĂƚŝǀĞĚĞĐĂLJǀŝĂĞůĞĐƚƌŝĐ multipole-radiation of order l: (antenna length = 5 ×10-15 m)

1 P r 2 l r  v Z ˜ ( ) | 10 8

W E (l ) ! Z O O

W E (1 ) | 100 s at 1 eV (Jackson, Classical Electrodynamics)

Long-ůŝǀĞĚĞdžĐŝƚĞĚƐƚĂƚĞƐ͗ŝƐŽŵĞƌƐ e.g. Ta-180: natural isomer, ĚĞĐĂLJƐǀŝĂϴƌĂĚŝĂƚŝŽŶ;l =8) at 75.3 keV, half time > 1015 a ! Nuclear spectroscopy still holds record in resolution

Mößbauer-spectrum of 93.3 keV resonance of Zn-67

Q 'QQ x

Potzel et al., J. Phys., Colloq. 37, 691 (1976) EƵĐůĞŝǁŝƚŚŝƐŽŵĞƌŝĐƐƚĂƚĞƐĂƚůŽǁĞŶĞƌŐŝĞƐ

dĐ-99 2150 eV Hg-201 1561 eV W-183 544 eV Energies on the order U-235 73 eV of excitation energy dŚ-229 7.8 eV of electronic shell

Outline

ͻ Why is nuclear laser spectroscopy difficult? ͻ dŚĞůŽǁ-ĞŶĞƌŐLJŝƐŽŵĞƌŝĐƐƚĂƚĞŝŶdŚ-229 ͻdŚ-229 as a precise optical nuclear clock • Application search for D 229Th: - from 233U D-decay - half-life 7880 years

actinides Nuclear structure of thorium-229

since 1970s!

Nilsson state classification

dǁŽĐůŽƐĞ-lying band-heads: ground state and isomer

K. Gulda et al., Nuclear Physics A 703, 45 (2002)

Some History dŚĞŽŶůLJŬŶŽǁŶŝƐŽŵĞƌǁŝƚŚĂŶĞdžĐŝƚĂƚŝŽŶĞŶĞƌŐLJŝŶƚŚĞŽƉƚŝĐĂůƌĂŶŐĞ and in the range of outer shell electronic transitions.

ͻ Studied by C.W. Reich et al. at INL since the 1970s, ĞƐƚĂďůŝƐŚĞĚƚŚĞůŽǁĞŶĞƌŐLJŝƐŽŵĞƌ͕ from J-spectroscopy: 3.5 ± 1.0 eV, published in 1994

ͻ dŚĞŽƌĞƚŝĐĂůǁŽƌŬďLJ͘s͘dŬĂůLJĂ͕&͘&͘<ĂƌƉĞƐŚŝŶ͕ĂŶĚŽƚŚĞƌƐ isomer lifetime, coupling to electronic excitations (W ΕĨĞǁϭϬϬϬƐ)

ͻ &ĂůƐĞĚĞƚĞĐƚŝŽŶƐŽĨŽƉƚŝĐĂůĞŵŝƐƐŝŽŶŝŶƚŚĞh-233 decay chain in 1997/98

ͻ Proposal of nuclear laser spectroscopy and nuclear clock ͘WĞŝŬĂŶĚŚƌ͘dĂŵŵ͕ƉƵďůŝƐŚĞĚŝŶϮϬϬϯ

ͻ Unsuccessful search for optical nuclear excitation or decay

ͻ More precise energy measurement from J-spectroscopy at LLNL: 7.6 ± 0.5 eV, published in 2007 ͻ 2011: still no direct detection of the optical transition; ĞdžƉĞƌŝŵĞŶƚĂůĞĨĨŽƌƚƐŝŶƐĞǀĞƌĂů ŐƌŽƵƉƐǁŽƌůĚǁŝĚĞ DĞĂƐƵƌĞŵĞŶƚŽĨƚŚĞĞŶĞƌŐLJŽĨƚŚĞdŚ-229 isomer

ĞĐŬĞƚĂů͘;>>E>Ϳ͕WŚLJƐ͘ZĞǀ͘>Ğƚƚ͘98, 142501 (2007)

ƒJ-ƐƉĞĐƚƌŽƐĐŽƉLJŽĨƚǁŽĚĞĐĂLJĐĂƐĐĂĚĞƐ from the 71.82-keV-ůĞǀĞů

ƒ Isomer energy: Difference of the doublet splittings: 7.6 ± 0.5 eV (corr.: 7.8 ± 0.5 eV, LLNL-Proc-415170)

Ϯϵ<ĞsůŝŶĞƐ ϰϮ<ĞsůŝŶĞƐ

'ƌŽƵŶĚƐƚĂƚĞїŝƐŽŵĞƌ͗ƚƌĂŶƐŝƚŝŽŶŝŶƚŚĞǀĂĐƵƵŵ-UV at about 160 nm ǁĂǀĞůĞŶŐƚŚ ͻ Why is nuclear laser spectroscopy difficult? ͻdŚĞůŽǁ-ĞŶĞƌŐLJŝƐŽŵĞƌŝĐƐƚĂƚĞŝŶdŚ-229 ͻ dŚ-229 as a precise optical nuclear clock • Application search for D A high-precision nuclear clock

EƵĐůĞĂƌŵŽŵĞŶƚƐĂƌĞƐŵĂůů͘&ŝĞůĚŝŶĚƵĐĞĚƐLJƐƚĞŵĂƚŝĐĨƌĞƋƵĞŶĐLJƐŚŝĨƚƐ can be smaller than in an (electronic) atomic clock. e.g. Zeeman shifts…

-27 µN = 5 x 10 :ͬd µ = 9 x 10-24 :ͬd 229mdŚ/ƐŽŵĞƌ B + _3 [631] 2 P=-0.08 PN YуϮ·10-28 e·m2

' E=7.8 eV M1 transition WуϭϬϬϬ s

P=0.4 PN _5 + Q=3.1·10-28 e·m2 2 [633] 229dŚ'ƌŽƵŶĚ^ƚĂƚĞ A high-precision nuclear clock

ƒ Frequency shifts that only depend on |n,L,S,J> are common in both levels and do not change the transition frequency

ƒ For structureless point-like nucleus ground and excited state shifts are identical

Campbell et al., arXiv:1110.2490v1 (2011) Peik et al., EPL 61, 181 (2003)

ŶĂůŽŐŽŶ͗ŽďƐĞƌǀĂƚŝŽŶŽĨƋƵĂŶƚƵŵũƵŵƉƐŝŶƐŝŶŐůĞŝŽŶ

Cycling transition for detection Clock transition to ŵĞƚĂƐƚĂďůĞůĞǀĞů

Dehmelt et al. 1986 WŽƐƐŝďůĞƌĞĂůŝnjĂƚŝŽŶƐŽĨdŚ-229 nuclear clocks: ͻ Laser-ĐŽŽůĞĚdŚ3+ in an ion trap

ͻ dŚŝŽŶƐĂƐĚŽƉĂŶƚŝŶĂƚƌĂŶƐƉĂƌĞŶƚĐƌLJƐƚĂů;ůŝŬĞĂ&2͕>ŝ&ĞƚĐ͘Ϳ

Experimental problem: dƌĂŶƐŝƚŝŽŶĞŶĞƌŐLJŬŶŽǁŶŽŶůLJƚŽуϭϬйƵŶĐĞƌƚĂŝŶƚLJ͕ not a system for high resolution spectroscopy yet.

džƉĞƌŝŵĞŶƚĂůƉƌŽũĞĐƚƐ͗

Wd͗ ƚƌĂƉƉĞĚdŚ+ ŝŽŶƐ͖dŚ-doped crystals 'ĞŽƌŐŝĂdĞĐŚ͗ ƚƌĂƉƉĞĚdŚ3+ ions UCLA / LANL: dŚ-doped crystals dhsŝĞŶŶĂ͗ dŚ-doped crystals

:LJǀćƐŬLJůćͬDĂŝŶnj ZĞƐŽŶĂŶĐĞŝŽŶŝnjĂƚŝŽŶƐƉĞĐƚƌŽƐĐŽƉLJŽĨdŚƌĞĐŽŝůŶƵĐůĞŝ ….

EƵĐůĞĂƌĐůŽĐŬǁŝƚŚůĂƐĞƌĐŽŽůĞĚ229dŚ3+

ƒ dŚ3+ ƉŽƐƐĞƐƐĞƐĂŵƵĐŚŵŽƌĞƐŝŵƉůĞůĞǀĞůƐĐŚĞŵĞ ;ƐŝŶŐůĞǀĂůĞŶĐĞĞ-) ƒ can be laser-cooled using diode lasers & ĚĞƚĞĐƚĞĚǀŝĂƌĞƐŽŶĂŶĐĞĨůƵŽƌĞƐĐĞŶĐĞŝŶƚŚĞƌĞĚŽƌE/Z ƒ electronic and nuclear resonances are separated in energy dƌĂƉƉŝŶŐĂŶĚůĂƐĞƌĐŽŽůŝŶŐŽĨdŚ3+

ƒ Loading via laser ablation with ns pulsed Nd:YAG (tripled) ƒ Trap L = 188 mm r = 3.3 mm, taylored for efficient loading of ablation plume ƒ Trapping and cooling 103 – 104 Th3+ ions (Th-229 & Th-232) (enhanced loading efficiency with initial buffer gas cooling)

Campbell et al., Phys. Rev.Lett 106, 223001 (2011) dƌĂƉƉŝŶŐĂŶĚůĂƐĞƌĐŽŽůŝŶŐŽĨdŚ3+

Low lying energy levels in 229Th3+ :

ƒ cooling on 1088 nm line to tens of K ƒ cooling to tens of mK on lambda scheme ƒ sympathetic cooling on even isotope (no HF!) for lowest temperatures

229Th3+ Laser cooled ion crystals:

232Th3+

Campbell et al., Phys. Rev.Lett 106, 223001 (2011) Ground state in 299dŚ3+ for clock spectroscopy?

Clock transition from ground state (5F5/2):

With laser cooled and trapped ion fractional frequency inaccuray as low as 10-19 should be possible!

or metastable S-state: Campbell et al., arXiv:1110.2490v1 (2011) Peik et al., EPL 61, 181 (2003) Doped solid-ƐƚĂƚĞĐƌLJƐƚĂůƐǁŝƚŚdŚ+

Th+ Doped solid-ƐƚĂƚĞĐƌLJƐƚĂůƐǁŝƚŚdŚ+

Optical Mössbauer Spectroscopy

ƒ >ĂƐĞƌĞdžĐŝƚĂƚŝŽŶŽĨdŚ-ions in a solid Th4+ їĐŽŵƉĂĐƚŽƉƚŝĐĂůĨƌĞƋƵĞŶĐLJƐƚĂŶĚĂƌĚ!

ƒ ,ŽƐƚĐƌLJƐƚĂůŵƵƐƚďĞͬŚĂǀĞ͗ - ůĂƌŐĞďĂŶĚŐĂƉїƚƌĂŶƐƉĂƌĞŶƚ - no impurities / color centers - symmetric - diamagnetic

WŽƐƐŝďůĞĐĂŶĚŝĚĂƚĞƐ͗Ă&2͕>ŝ&͕ĞƚĐ͙

ƒ Crystal doped with 1 nucleus per O3: 1014 ions per cm3 - simple fluorescence detection is possible - initial broadband excitation experiment with synchrotron light

Doped solid-ƐƚĂƚĞĐƌLJƐƚĂůƐǁŝƚŚdŚn+

Optical Mössbauer Spectroscopy

ƒ >ĂƐĞƌĞdžĐŝƚĂƚŝŽŶŽĨdŚ-ions in a solid Th4+ їĐŽŵƉĂĐƚŽƉƚŝĐĂůĨƌĞƋƵĞŶĐLJƐƚĂŶĚĂƌĚ!

ƒ First experiments at ALS in Berkeley:

- ^LJŶĐŚƌŽƚƌŽŶƉƌŽǀŝĚĞƐƚƵŶĂďůĞůŝŐŚƚ;ϱ-ϯϬĞsͿŽĨůŝŶĞǁŝĚƚŚϬ͘ϭϳϱĞs - >ŝ&ĐƌLJƐƚĂůĚŽƉĞĚǁŝƚŚ232dŚ ¥ - Measured fluorescence background from D-decay ¥

їŶĂƌƌŽǁĚŽǁŶƌĞƐŽŶĂŶĐĞ– 0.1 nm!

Rellergert et al., Phys. Rev. Lett. 104, 200802 (2010) &ŝĞůĚƐŚŝĨƚƐŝŶƐŝĚĞĐƌLJƐƚĂů

ŽŵŝŶĂŶƚĐƌLJƐƚĂůĨŝĞůĚƐŚŝĨƚ͗ůĞĐƚƌŝĐƋƵĂĚƌƵƉŽůĞƐŚŝĨƚ

21 2 Ğ͘Ő͘ĨŝĞůĚŐƌĂĚŝĞŶƚŝŶdŚ4 (tetragonal): Vzz = 5×10 V/m ĺdŚ-ϮϮϵŶƵĐůĞĂƌŐƌŽƵŶĚƐƚĂƚĞƋƵĂĚƌƵƉŽůĞƐŚŝĨƚуϭ',nj! їuse cubic crystal symmetry

dĞŵƉĞƌĂƚƵƌĞĚĞƉĞŶĚĞŶĐĞŽĨůŝŶĞǁŝĚƚŚĂŶĚĨƌĞƋƵĞŶĐLJƐŚŝĨƚƐ͗ ͻ ƌĞůĂƚŝǀŝƐƚŝĐŽƉƉůĞƌƐŚŝĨƚ͗ϭϬ-15 ͬ< ͻ electric crystal field shifts may be » 10-15 ͬ< (e.g. contact interaction nucleus / e- cloud)

ĺ)RUKLJKSUHFLVLRQEH\RQG-15 work at cryogenic temperature to freeze out lattice fluctuations

Rellergert et al., Phys. Rev. Lett. 104, 200802 (2010) WĞŝŬĞƚĂů͕͘WƌŽĐ͘ϳƚŚ^LJŵƉ͘ŽŶ&ƌĞƋƵĞŶĐLJ^ƚĂŶĚĂƌĚƐĂŶĚDĞƚƌŽůŽŐLJ;Ăƌyŝǀ͗ϬϴϭϮ͘ϯϰϱϴͿ Search for nuclear resonance in 229dŚ+

-

Electron Bridge Processes ^ĞĂƌĐŚĨŽƌŶƵĐůĞĂƌĞdžĐŝƚĂƚŝŽŶǀŝĂĞůĞĐƚƌŽŶďƌŝĚŐĞƉƌŽĐĞƐƐ

ͻ Ed;EƵĐůĞĂƌdžĐŝƚĂƚŝŽŶďLJůĞĐƚƌŽŶdƌĂŶƐŝƚŝŽŶͿ͗dƌĂŶƐĨĞƌŽĨĞdžĐŝƚĂƚŝŽŶ from the electron shell to the nucleus

ͻ Excitation of the shell in a 2-photon process їŶŽƚƵŶĂďůĞůĂƐĞƌĂƚϭϲϬŶŵƌĞƋƵŝƌĞĚ

ͻ Excitation rate may be strongly enhanced at ƌĞƐŽŶĂŶĐĞďĞƚǁĞĞŶĞůĞĐƚƌŽŶŝĐĂŶĚŶƵĐůĞĂƌ ƚƌĂŶƐŝƚŝŽŶĨƌĞƋƵĞŶĐLJ ĺǀĞƌLJůŝŬĞůLJŝŶƚŚĞĚĞŶƐĞůĞǀĞůƐƚƌƵĐƚƵƌĞŽĨdŚ+

ͻ ĞƚĞĐƚŝŽŶŽĨƚŚĞŶƵĐůĞĂƌĞdžĐŝƚĂƚŝŽŶǀŝĂĨůƵŽƌĞƐĐĞŶĐĞŽƌĐŚĂŶŐĞŝŶ hyperfine structure dǁŽ-photon electron bridge excitation rate

&ĞLJŶŵĂŶĚŝĂŐƌĂŵ Zatomicresonance line at 402 nm Ztunable laser to search for nuclear resonance electrons  Z Z Z E1 N  Dϭ,&^ nucleus Excitation rate as a function of nuclear resonance frequency (elect. levels from ab-initio calculations)

їĞdžĐŝƚĂƚŝŽŶƌĂƚĞŽĨĂƚůĞĂƐƚ 10 s-1 ǁŝƚŚĐŽŶǀĞŶƚŝŽŶĂů laser parameters

WŽƌƐĞǀĞƚĂů͕͘WŚLJƐ͘ZĞǀ͘>Ğƚƚ͘105, 182501 (2010) Laser spectroscopy of trapped Th+ ions at PTB

- Linear Paul trap for buffer gas cooled clouds of Th+ (N >105)

- Laser ablation loading (N2-Laser, now Nd:YAG laser) - Fluorescence detection in several spectral channels Laser spectroscopy of trapped Th+ ions

Decay channels for the 402 nm resonance line

- Laser excitation in Th+ leads to population of many metastable levels - These are quenched by collisions or emptied with repumper lasers Th+ Level Scheme

ͻ >ĞǀĞůƐŝŶƚŚĞsearch range only ±1V ŝŶĐŽŵƉůĞƚĞůLJŬŶŽǁŶ

ͻ džƉŽŶĞŶƚŝĂůŝŶĐƌĞĂƐĞŽĨůĞǀĞů density expected 3 x 800 nm

402 nm ͻ Why is nuclear laser spectroscopy difficult? ͻdŚĞůŽǁ-ĞŶĞƌŐLJŝƐŽŵĞƌŝĐƐƚĂƚĞŝŶdŚ-229 ͻdŚ-229 as a precise optical nuclear clock • Application search for D Are fundamental constants really constant?

Equivalence Principle: fundamental constants need to be constant in time

=

=

Reinhold et al., PRL 96, 151101 (2006) Murphy et al., Mon. Not. R. Astron. Soc. 345, 609 (2003) Laboratory Tests

Sensitivity factor A of different x f wln f wln Ry wlnD wln F atomic transitions to a potential  A ; A { drift of D f wt wt wt wlnD

Al+/Hg+ Hg+

Yb+

w lnD (2.4 r 2.7) ˜1017 yr1 wt w ln Ry (0.0 r 3.2) ˜1016 yr-1 Present status: wt Dzuba et al. PRL 82 (1999) Laboratory Tests

Sensitivity factor A of different x atomic transitions to a potential f wln f wln Ry wlnD wln F  A ; A { drift of D f wt wt wt wlnD

A ~ 10,000 229Th ! . . .

Dzuba et al. PRL 82 (1999) Th-229: most sensitive probe in a search for D

Scaling of the 229Th transition frequency Z in terms of D and quark masses: V. Flambaum et al., Phys. Rev. Lett. 97, 092502 (2006)

105 enhancement in sensitivity results from near perfect cancellation of O(MeV) contributions to nuclear level energies

But: it depends a lot on nuclear structure! > 10 theory papers 2006 - 2009 See for example: Hayes et al., Phys. Rev. C 78, 024311 (2008) (|A| } 103) Litvinova et al., Phys. Rev. C 79, 064303 (2009) (|A| } 4×104)

Solution: measure isomer shift (') and get better estimate for change in Coulomb energy! J. C. Berengut et al., PRL 102, 210808 (2009) To Do List for Thorium Trappers

ͻ locate transition at 160 – 10 nm ͻŵĞĂƐƵƌĞŝƐŽŵĞƌƐŚŝĨƚїƐĞŶƐŝƚŝǀŝƚLJŽŶD ͻůŝĨĞƚŝŵĞŽĨŝƐŽŵĞƌŝĐƐƚĂƚĞ͍ • evaluate clock systematics  Optical Clock Groups at PTB:

Ekkehard Peik

Christian Tamm Uwe Sterr

Piet Schmidt T.E.M.