thorium spectroscopy

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



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

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

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

thorium spectroscopy

Documents