thorium spectroscopy
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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
Outline
Why is nuclear laser spectroscopy difficult? -‐ -‐229
-‐229 as a precise optical nuclear clock • Application search for
Energy scales: Photon in optical range:
eV 2
Nucleus: bound nucleon (rest energy of proton: 938 MeV)
m 105 15x
MeV 83,0x)2(
2
2
pmpx
Atomic shell: bound electron (rest energy of electron: 0,51 MeV)
m 10 10x
eV 8,3x)2(
2
2
empx
Visible light not matched to energy scales in nucleus
204 rqE
e-‐ Shell: Nucleus:
I:
202
1LcEI
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
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
area of ampl. medium transition cross section
multipole-‐radiation of order l: (antenna length = 5 ×10-‐15 m)
Long-‐ e.g. Ta-‐180: natural isomer,
l =8) at 75.3 keV, half time > 1015 a !
(Jackson, Classical Electrodynamics)
eV 1 at s 100 ) 1 (
10 ) ( ) (
1 8 2
E
l
E
r r P l
Mößbauer-‐spectrum of 93.3 keV resonance of Zn-‐67
Q x Potzel et al., J. Phys., Colloq. 37, 691 (1976)
Nuclear spectroscopy still holds record in resolution
-‐99
Hg-‐201 W-‐183 Energies on the order U-‐235 of excitation energy -‐229 of electronic shell
2150 eV 1561 eV 544 eV 73 eV
7.8 eV
Outline
Why is nuclear laser spectroscopy difficult? -‐ -‐229
-‐229 as a precise optical nuclear clock • Application search for
actinides
- from 233U -decay - half-life 7880 years
229Th:
Nuclear structure of thorium-‐229
K. Gulda et al., Nuclear Physics A 703, 45 (2002)
-‐lying band-‐heads: ground state and isomer
Nilsson state classification
since 1970s!
Some History
and in the range of outer shell electronic transitions.
Studied by C.W. Reich et al. at INL since the 1970s, from -‐spectroscopy: 3.5 ± 1.0 eV, published in 1994
isomer lifetime, coupling to electronic excitations ( )
-‐233 decay chain in 1997/98
Proposal of nuclear laser spectroscopy and nuclear clock
Unsuccessful search for optical nuclear excitation or decay
More precise energy measurement from -‐spectroscopy at LLNL: 7.6 ± 0.5 eV, published in 2007
2011: still no direct detection of the optical transition;
-‐229 isomer
-‐ from the 71.82-‐keV-‐
98, 142501 (2007)
Isomer energy: Difference of the doublet splittings: 7.6 ± 0.5 eV (corr.: 7.8 ± 0.5 eV, LLNL-‐Proc-‐415170)
-‐UV at about 160 nm
Why is nuclear laser spectroscopy difficult? -‐ -‐229
-‐229 as a precise optical nuclear clock • Application search for
A high-‐precision nuclear clock
can be smaller than in an (electronic) atomic clock. e.g. Zeeman shifts…
µN = 5 x 10-‐27
µB = 9 x 10-‐24
[633] 5 _ + 2
3 _ + 2
[631]
E=7.8 eV M1 transition
s
229
229m
=0.4 N Q=3.1·∙10-‐28 e·∙m2
=-‐0.08 N ·∙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)
Dehmelt et al. 1986
Cycling transition for detection Clock transition to
-‐229 nuclear clocks: Laser-‐ 3+ in an ion trap 2
Experimental problem:
not a system for high resolution spectroscopy yet.
+ -‐doped crystals 3+ ions
UCLA / LANL: -‐doped crystals -‐doped crystals
….
3+ -‐)
can be laser-‐cooled using diode lasers &
electronic and nuclear resonances are separated in energy
229 3+
Campbell et al., Phys. Rev.Lett 106, 223001 (2011)
3+
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)
3+
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 299 3+ 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-‐ +
Th+
Optical Mössbauer Spectroscopy -‐ions in a solid
! -‐ -‐ no impurities / color centers -‐ symmetric -‐ diamagnetic 2 Crystal doped with 1 nucleus per 3: 1014 ions per cm3
- simple fluorescence detection is possible - initial broadband excitation experiment with synchrotron light
Doped solid-‐ +
Th4+
Optical Mössbauer Spectroscopy -‐ions in a solid
! First experiments at ALS in Berkeley: -‐ -‐ -‐ 232 -‐ Measured fluorescence background from -‐decay
0.1 nm!
Doped solid-‐ n+
Th4+
Rellergert et al., Phys. Rev. Lett. 104, 200802 (2010)
-‐15 electric crystal field shifts may be » 10-‐15 (e.g. contact interaction nucleus / e-‐ cloud)
4 (tetragonal): Vzz = 5×1021 V/m2
-‐ ! use cubic crystal symmetry
Rellergert et al., Phys. Rev. Lett. 104, 200802 (2010)
-15
work at cryogenic temperature to freeze out lattice fluctuations
Search for nuclear resonance in 229 +
-‐
Electron Bridge Processes
from the electron shell to the nucleus Excitation of the shell in a 2-‐photon process
Excitation rate may be strongly enhanced at
+
hyperfine structure
nucleus
electrons
atomic resonance line at 402 nm tunable laser to search for nuclear resonance
N E1
10 s-‐1 laser parameters
Excitation rate as a function of nuclear resonance frequency (elect. levels from ab-‐initio calculations)
-‐photon electron bridge excitation rate
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
search range only
density expected
±1
402 nm
3 x 800 nm
Why is nuclear laser spectroscopy difficult? -‐ -‐229
-‐229 as a precise optical nuclear clock • Application search for
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
tRyt
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
lnln;lnlnln FA
tA
tRy
tf
ff
Dzuba et al. PRL 82 (1999)
Laboratory Tests
Sensitivity factor A of different atomic transitions to a potential drift of
229Th A ~ 10,000 . . .
! lnln;lnlnln FA
tA
tRy
tf
ff
Scaling of the 229Th transition frequency in terms of 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
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
• 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
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