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 | 51015 m 2 ǁŝƚŚ ! 'x 'p o 0,83 MeV ! 2 2('x) mp (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 hQ '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) J-ƐƉĞĐƚƌŽƐĐŽƉLJŽĨƚǁŽĚĞĐĂLJĐĂƐĐĂĚĞƐ from the 71.82-keV-ůĞǀĞů 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 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) 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+ dŚ3+ ƉŽƐƐĞƐƐĞƐĂŵƵĐŚŵŽƌĞƐŝŵƉůĞůĞǀĞůƐĐŚĞŵĞ ;ƐŝŶŐůĞǀĂůĞŶĐĞĞ-) can be laser-cooled using diode lasers & ĚĞƚĞĐƚĞĚǀŝĂƌĞƐŽŶĂŶĐĞĨůƵŽƌĞƐĐĞŶĐĞŝŶƚŚĞƌĞĚŽƌE/Z electronic and nuclear resonances are separated in energy dƌĂƉƉŝŶŐĂŶĚůĂƐĞƌĐŽŽůŝŶŐŽĨdŚ3+ 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) dƌĂƉƉŝŶŐĂŶĚůĂƐĞƌĐŽŽůŝŶŐŽĨdŚ3+ Low lying energy levels in 229Th3+ : 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 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 >ĂƐĞƌĞdžĐŝƚĂƚŝŽŶŽĨdŚ-ions in a solid Th4+ їĐŽŵƉĂĐƚŽƉƚŝĐĂůĨƌĞƋƵĞŶĐLJƐƚĂŶĚĂƌĚ! ,ŽƐƚĐƌLJƐƚĂůŵƵƐƚďĞͬŚĂǀĞ͗ - ůĂƌŐĞďĂŶĚŐĂƉїƚƌĂŶƐƉĂƌĞŶƚ - no impurities / color centers - symmetric - diamagnetic WŽƐƐŝďůĞĐĂŶĚŝĚĂƚĞƐ͗Ă&2͕>ŝ&͕ĞƚĐ͙ 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 >ĂƐĞƌĞdžĐŝƚĂƚŝŽŶŽĨdŚ-ions in a solid Th4+ їĐŽŵƉĂĐƚŽƉƚŝĐĂůĨƌĞƋƵĞŶĐLJƐƚĂŶĚĂƌĚ! 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