thorium spectroscopy - quantummetrology

Thorium Spectroscopy

Center for Quantum Engineering and Space Time Research Leibniz Universität Hannover

Physikalisch-Technische Bundesanstalt, Braunschweig

Department of Time & Frequency

Tanja E. Mehlstäubler

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

Outline

• Why is nuclear laser spectroscopy difficult? • The low-energy isomeric state in Th-229 • Th-229 as a precise optical nuclear clock • Application search for α

Energy scales: Photon in optical range:

eV 2≈ω

Nucleus: bound nucleon with (rest energy of proton: 938 MeV)

m 105 15−⋅≈∆x

MeV 83,0x)2(

2

2

=∆

→=∆⋅∆pm

px

Atomic shell: bound electron with (rest energy of electron: 0,51 MeV)

m 10 10−≈∆x

eV 8,3x)2(

2

2

=∆

→=∆⋅∆em

px

Visible light not matched to energy scales in nucleus

204 r

qEπε

=

e- Shell: Nucleus:

Electric field of electromagnetic wave of intensity I:

202

1LcEI ε=

Electric field scales inside atom / nucleus

2 32

2 15

W/cm 10 5 . 4 W/cm 10 5 . 2

⋅ = → = ⋅ = → =

I E E I E E

N L

S L

V/m 10 8 . 5 m 10 5 19 15 ⋅ = ⋅ = − N E r

V/m 10 4 1 m 10 11 10 ⋅ = = − S E r .

Intensity Limit:

e- shell-field strength: reachable nuclear electr. field strength: far beyond

gain bandwidth photon energy 1/(min. waist)

Maximum intensity of short-pulse laser

Mourou et al., Phys. Today 51, 22 (1998)

2 24 max

12

2

2

max

W/cm 10

10

≈

≈ ≈

⋅ ⋅ ⋅ ≈

I

N

c ∆v h N I

Ph

Ph ν

ν

area of ampl. medium transition cross section

Lifetime for radiative decay via electric multipole-radiation of order l: (antenna length = 5 ×10-15 m)

Long-lived excited states: isomers e.g. Ta-180: natural isomer, decays via E8 radiation (l =8) at 75.3 keV, half time > 1015 a !

(Jackson, Classical Electrodynamics)

Nucleus is no suitable antenna for visible light

eV 1 at s 100 ) 1 (

10 ) ( ) (

1 8 2

≈

≈ ⋅ ∝ = −

E

l

E

r r P l

τ

λ λ ω

ω τ

Mößbauer-spectrum of 93.3 keV resonance of Zn-67

Q = 8.3 1014 , ∆ν/ν=1.2 x 10−15

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

Nuclear spectroscopy still holds record in resolution

Tc-99 Hg-201 W-183 Energies on the order U-235 of excitation energy Th-229 of electronic shell

2150 eV 1561 eV 544 eV 73 eV

7.8 eV

Nuclei with isomeric states at low energies

Outline

• Why is nuclear laser spectroscopy difficult? • The low-energy isomeric state in Th-229 • Th-229 as a precise optical nuclear clock • Application search for α

actinides

- from 233U α-decay - half-life 7880 years

229Th:

Nuclear structure of thorium-229

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

Two close-lying band-heads: ground state and isomer

Nilsson state classification

since 1970s!

Some History The only known isomer with an excitation energy in the optical range and in the range of outer shell electronic transitions.

• Studied by C.W. Reich et al. at INL since the 1970s, established the low energy isomer, from γ-spectroscopy: 3.5 ± 1.0 eV, published in 1994

• Theoretical work by E.V. Tkalya, F.F. Karpeshin, and others isomer lifetime, coupling to electronic excitations (τ ~ few 1000 s)

• False detections of optical emission in the U-233 decay chain in 1997/98

• Proposal of nuclear laser spectroscopy and nuclear clock E. Peik and Chr. Tamm, published in 2003

• Unsuccessful search for optical nuclear excitation or decay

• More precise energy measurement from γ-spectroscopy at LLNL: 7.6 ± 0.5 eV, published in 2007

• 2011: still no direct detection of the optical transition; experimental efforts in several groups worldwide

Measurement of the energy of the Th-229 isomer

γ-spectroscopy of two decay cascades from the 71.82-keV-level

Beck et al. (LLNL), Phys. Rev. Lett. 98, 142501 (2007)

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

Ground state → isomer: transition in the vacuum-UV at about 160 nm wavelength

29 KeV lines 42 KeV lines

• Why is nuclear laser spectroscopy difficult? • The low-energy isomeric state in Th-229 • Th-229 as a precise optical nuclear clock • Application search for α

A high-precision nuclear clock

Nuclear moments are small. Field induced systematic frequency shifts can be smaller than in an (electronic) atomic clock. e.g. Zeeman shifts…

µN = 5 x 10-27 J/T

µB = 9 x 10-24 J/T

[633] 5 _ + 2

3 _ + 2

[631]

∆ E=7.8 eV M1 transition τ ≈1000 s

229Th Ground State

229mTh Isomer

µ=0.4 µN Q=3.1·10-28 e·m2

µ=-0.08 µN Q≈2·10-28 e·m2

A high-precision nuclear clock

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

Analogon: observation of quantum jumps in single ion

Dehmelt et al. 1986

Cycling transition for detection Clock transition to

metastable level

Possible realizations of Th-229 nuclear clocks: • Laser-cooled Th3+ in an ion trap • Th ions as dopant in a transparent crystal (like CaF2, LiCAF etc.)

Experimental problem: Transition energy known only to ≈ 10% uncertainty, not a system for high resolution spectroscopy yet.

Experimental projects: PTB: trapped Th+ ions; Th-doped crystals Georgia Tech: trapped Th3+ ions UCLA / LANL: Th-doped crystals TU Vienna: Th-doped crystals Jyväskylä/Mainz Resonance ionization spectroscopy of Th recoil nuclei ….

Th3+ possesses a much more simple level scheme (single valence e-) can be laser-cooled using diode lasers &

detected via resonance fluorescence in the red or NIR electronic and nuclear resonances are separated in energy

Nuclear clock with laser cooled 229Th3+

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

Trapping and laser cooling of Th3+

Loading via laser ablation with ns pulsed Nd:YAG (tripled) Trap L = 188 mm r = 3.3 mm, taylored for efficient loading of ablation plume 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)

Trapping and laser cooling of Th3+

Low lying energy levels in 229Th3+ :

229Th3+

232Th3+

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

Laser cooled ion crystals:

Campbell et al., arXiv:1110.2490v1 (2011)

Ground state in 299Th3+ for clock spectroscopy?

or metastable S-state: Peik et al., EPL 61, 181 (2003)

With laser cooled and trapped ion fractional frequency inaccuray

as low as 10-19

should be possible!

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

Doped solid-state crystals with Th+

Th+

Optical Mössbauer Spectroscopy Laser excitation of Th-ions in a solid → compact optical frequency standard ! Host crystal must be / have: - large band gap → transparent - no impurities / color centers - symmetric - diamagnetic Possible candidates: CaF2, LiCAF, etc… Crystal doped with 1 nucleus per λ3: 1014 ions per cm3

- simple fluorescence detection is possible - initial broadband excitation experiment with synchrotron light

Doped solid-state crystals with Th+

Th4+

Optical Mössbauer Spectroscopy Laser excitation of Th-ions in a solid → compact optical frequency standard ! First experiments at ALS in Berkeley: - Synchrotron provides tunable light (5-30 eV) of linewidth 0.175 eV - LiCAF crystal doped with 232Th - Measured fluorescence background from α-decay → narrow down resonance 0.1 nm!

Doped solid-state crystals with Thn+

Th4+

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

√ √

Temperature dependence of linewidth and frequency shifts: • relativistic Doppler shift: 10-15 / K • electric crystal field shifts may be » 10-15 / K (e.g. contact interaction nucleus / e- cloud)

Dominant crystal field shift: Electric quadrupole shift e.g. field gradient in ThB4 (tetragonal): Vzz = 5×1021 V/m2

→ Th-229 nuclear ground state quadrupole shift ≈ 1 GHz ! → use cubic crystal symmetry

Rellergert et al., Phys. Rev. Lett. 104, 200802 (2010) Peik et al., Proc. 7th Symp. on Frequency Standards and Metrology (arXiv:0812.3458)

Field shifts inside crystal

→ For high precision beyond 10-15

work at cryogenic temperature to freeze out lattice fluctuations

Search for nuclear resonance in 229Th+

-

Electron Bridge Processes

Search for nuclear excitation via electron bridge process

• NEET (Nuclear Excitation by Electron Transition): Transfer of excitation from the electron shell to the nucleus

• Excitation of the shell in a 2-photon process → no tunable laser at 160 nm required • Excitation rate may be strongly enhanced at resonance between electronic and nuclear transition frequency → very likely in the dense level structure of Th+

• Detection of the nuclear excitation via fluorescence or change in hyperfine structure

Feynman diagram

nucleus

electrons

ω1: atomic resonance line at 402 nm ω: tunable laser to search for nuclear resonance

ωN = ω1 + ω E1

M1 HFS

→ excitation rate of at least 10 s-1 with conventional laser parameters

Excitation rate as a function of nuclear resonance frequency (elect. levels from ab-initio calculations)

Two-photon electron bridge excitation rate

Porsev et al., Phys. Rev. Lett. 105, 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

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

Decay channels for the 402 nm resonance line

Th+ Level Scheme

• Levels in the search range only incompletely known

• Exponential increase of level density expected

±1σ

402 nm

3 x 800 nm

• Why is nuclear laser spectroscopy difficult? • The low-energy isomeric state in Th-229 • Th-229 as a precise optical nuclear clock • Application search for α

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

Equivalence Principle: fundamental constants need to be constant in time

Are fundamental constants really constant?

=

=

1-16

117

yr10)2.30.0(ln

yr10)7.24.2(ln

−

−−

⋅±=∂

∂

⋅±−=∂

∂

tRy

tα

Dzuba et al. PRL 82 (1999)

Hg+ Al+/Hg+

Yb+

Present status:

Laboratory Tests

Sensitivity factor A of different atomic transitions to a potential drift of α

αα

lnln;lnlnln

∂∂

≡∂

∂+

∂∂

=∂

∂=

•

FAt

AtRy

tf

ff

Dzuba et al. PRL 82 (1999)

Laboratory Tests

Sensitivity factor A of different atomic transitions to a potential drift of α

229Th A ~ 10,000 . . .

! α

αlnln;lnlnln

∂∂

≡∂

∂+

∂∂

=∂

∂=

•

FAt

AtRy

tf

ff

Scaling of the 229Th transition frequency ω in terms of α 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

Th-229: most sensitive probe in a search for α

Solution: measure isomer shift (∆<r²>) and get better estimate for change in Coulomb energy! J. C. Berengut et al., PRL 102, 210808 (2009)

But: it depends a lot on nuclear structure!

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)

> 10 theory papers 2006 - 2009

• locate transition at 160 10 nm • measure isomer shift → sensitivity on α • life time of isomeric state? • evaluate clock systematics

To Do List for Thorium Trappers

Piet Schmidt

Ekkehard Peik

T.E.M.

Optical Clock Groups at PTB:

Christian Tamm Uwe Sterr