Thorium Spectroscopy - Quantummetrology
Transcript of Thorium Spectroscopy - Quantummetrology
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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
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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
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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
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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 .
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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
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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
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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
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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
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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
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actinides
- from 233U α-decay - half-life 7880 years
229Th:
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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!
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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
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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
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• 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
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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
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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)
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Analogon: observation of quantum jumps in single ion
Dehmelt et al. 1986
Cycling transition for detection Clock transition to
metastable level
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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 ….
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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+
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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)
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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:
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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):
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Doped solid-state crystals with Th+
Th+
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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+
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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)
√ √
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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
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Search for nuclear resonance in 229Th+
-
Electron Bridge Processes
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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
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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)
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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
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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
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Th+ Level Scheme
• Levels in the search range only incompletely known
• Exponential increase of level density expected
±1σ
402 nm
3 x 800 nm
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• 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
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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?
=
=
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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
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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
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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
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• 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
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Piet Schmidt
Ekkehard Peik
T.E.M.
Optical Clock Groups at PTB:
Christian Tamm Uwe Sterr