Simple Atom, Extreme Nucleus: Laser Trapping and Probing of He-8 Zheng-Tian Lu Argonne National Laboratory University of Chicago Funding: DOE, Office of Nuclear Physics
Helium Atom fm Å e - Ionization Energy of Helium Atom Level 2 3 S 1 Calculation 1 152 842 741 ± 6 MHz Experiment 1 152 842 743 MHz Gordon Drake, Phys. Scripta (1999) 2
Effective Model of Nuclear Interaction Two-body potential: Argonne V18 H = i K i + i < j v γ ij + v π ij + v R ij EM 1-π short-range Coupling parameters fit to NN scattering data Problem: binding energy of most light nuclei too small q q Meson QCD exchange N q π q N q q Three-body potential: Illinois-2 V = V + V + ijk 2π 3π ijk ijk V R ijk Coupling parameters fit to energy levels of light nuclei Pieper & Wiringa. Ann. Rev. Nucl. Part. Sci. (2001) 3
Quantum Monte Carlo Calculations of Light Nuclei Pieper & Wiringa. Ann. Rev. Nucl. Part. Sci. (2001) 4
Halo Nuclei 6 He and 8 He Isotope Half-life Spin Isospin Core + Valence He-6 807 ms 0 + 1 α + 2n He-8 119 ms 0 + 2 α +4n Quantum Monte Carlo calculation 4 He 6 He 8 He Borromean Nucleus Borromean Rings Neutron Proton 5
Hadronic Probe: Scattering of 6 He & 8 He Beams 18 O 9 Be 6 He 1 12 HC 6 He 6 He θ Tanihata et al, Phys Lett (1985) LBNL Alkhazov et al, Nucl Phys (2002) GSI Elastic and inelastic collision: He on C, B Elastic collision: He on H, 700 MeV/u Matter distribution, matter radii 6
E&M Probe of Nuclear Charge Distribution dσ dσ 4 dω dω He electron θ 2 2 ( ) exp = ( ) Mott F( q ) 1 F q = q < r > + q 2 2 2 ( ) 1 charg e 6 mean-square radius root-mean-square radius ( ) < >= r 2 ρ r r 2 dv 2 < r > 4 He rms charge radius = 1.676 (8) fm [I. Sick Phys Lett B (1982)] Proton rms charge radius = 0.895(18) fm [I. Sick Phys Lett B (2003)] 7
Atomic Energy Levels of Helium He energy level diagram Cooling & Trapping at 1083 nm 3 3 P 0,1,2 100 ns 1.6 MHz Single photon kick k 01 0.1 m/s Transition rate ~ 4 x 10 6 /s 389 nm Acceleration ~ 4 x 10 5 m/s 2 100 ns 2 3 P 0,1,2 Spectroscopy at 389 nm 1083 nm Single photon kick 0.3 m/s 2 3 S 1 Doppler shift 400 khz 19.82 ev 1 1 S 0 8
Atomic Isotope Shift Isotope Shift δν = δν MS + δν FS Mass shift: due to nucleus recoil δν MS m e me 1 m e 1+ M N m M e N Field shift: due to nucleus size δν FS Ζ Δ[Ψ(0)] 2 <r 2 > 9
Field (Volume) Shift E δν FS = 2π Ze 3 r 2 Δ Ψ(0) 2 δ r 2 AA s p V ~ - 1/r Isotope Shift, G Hz 100 10 1 0.1 0.01 1E-3 1 10 100 Atomic number, Z 10
Atomic Theory of Helium Drake, Can. J. Phys (2006); Pachucki & Sapirstein, J Phys B (2002) 3 P J Non-relativistic wave functions from variational calculations Perturbation theory for relativistic corrections, QED, finite nuclear mass and nuclear charge radius QED terms cancel in isotope shift 3 S 1 For 2 3 S 1 3 3 P 2 transition @ 389 nm: 6 He - 4 He : δν 6,4 = 43196.207(15) MHz + 1.008 (<r 2 > He4 -<r 2 > He6 ) MHz/fm 2 8 He - 4 He : δν 8,4 = 64702.409(74) MHz + 1.008 (<r 2 > He4 -<r 2 > He8 ) MHz/fm 2 8,4 He4 He8 100 khz error in IS ~ 1% error in radius 11
Laser Cooling and Trapping Technical challenges: Short lifetime, small samples (<10 6 atoms/s available) Metastable efficiency ~ 10-5 Precision requirement (~100 khz) Magneto-Optical Trap (MOT) Cooling: Temperature~ 1 mk, avoid Doppler shift / width Long observation time: 100 ms Spatial confinement: trap size < 1 mm single atom sensitivity Selectivity: no isotopic / isobaric interference 12
8 He: The Most Neutron-Rich Nucleus # of protons # of neutrons 13
He-8 @ GANIL 75 MeV/u, 0.4 pμa 13 C beam on 12 C target 14
8 He @ GANIL by Antonio Villari et al. He-8: 5 x 10 5 s -1 Salle D2 ECR Ion Source 1 GeV, 400 pna 13 C He-6: 1 x 10 8 s -1 Mass separator 8 He + @ 20 kev 8 He thermal 5 m 1.65 MOT 2.15 3.15 0.75 1.85 Laser System 15
Atom Trapping of 6 He & 8 He at GANIL ~1x10 86 He + /s ~5x10 58 He + /s Xe Atom Trap Setup Transverse cooling Zeeman slower 389 nm MOT 1083 nm He level scheme 3 3 P 2 Spectroscopy 389 nm 2 3 P 2 Trap 1083 nm 2 3 S 1 1 1 S 0 RF - Discharge 1.2 One trapped 6 He atom PMT Source Capture efficiency Trap 6 He ~ 5x10 7 /s 1x10-7 6 He ~ 5 /s 8 He ~ 1x1010 5 /s 8 He ~ 1x1010-2 /s Photon countrate e/ khz 1.0 0.8 0.6 0.4 02 0.2 0.0 0 5 10 15 20 Time (s) 16
He-8 Trapped! Count ts per Channe el 500 400 300 200 100 First He-8 8Atom June 15 th 2007 f 6 He 8 He t 50 khz s per Channe el Count 100 80 60 40 20 110 khz 60 atoms 0 0-10 -8-6 -4-2 0 2 4 6 8 10-10 -8-6 -4-2 0 2 4 6 8 10 Rel. Laser Frequency, MHz Rel. Laser Frequency, MHz ~30 6 He atoms/s ~30 8 He atoms/hr 17
Laser Setup - 389 nm (778 nm) Diode Laser 1 @ 778 nm Lock to I 2 Measure beat frequency Diode Laser 2 @ 778 nm Tapered Amplifier 3 He I 2 2 4 He 6 He Frequency doubling (LBO) 2mW @ 389 nm -44.6-2.5 0 40.6 2 3 S 1-3 3 P 2 Frequency (GHz) 18
3 1 P 1 3 3 P 0,1,2 389 nm J=1 J=0 J=1 J=2 2 3 S 1 J=1 Isotope Shift and Field Shift : J - Dependence? MHz 8,4-64701, δν 8 194, MHz δν 6,4-43 0.8 0.6 0.4 0.2 0.0-0.2 0.9 0.7 (a) (b) 8 He 6 He Field Shift, MHz -0.6-0.8-1.0 0.8-1.2 0.6 0.5-1.4 04 0.4-1.6 16 J = 0 J = 1 J = 2 (c) 6 He 8 He J = 0 J = 1 J = 2 Wang 04 Argonne 19
6 He & 8 He Charge Radii 6 He 8 He Field Shift, MHz -1.464(34) -0.916(95) RMS R CH, fm 2.068(11) 1.929(26) Total Uncertainty 0.5 % 1.3 % - Statistical 0.1 % 0.6 % - Trap Systematics 0.3 % 0.6 % - Mass Systematics 0.2 % 1.0 % - He-4: 1.676(8) fm 0.3 % 0.4 % Recoil Correction E = γ E + int P 2 γ 2M atom 20
Charge Radius vs. Point-Proton Radius r c R p Experiment: mean square charge radius <r c2 > Theory: mean square point-proton radius <r p2 > r p R n Z<r c2 > = Z<r p2 > + Z(<R p2 >+0.75/M p2 ) + N<R n2 > Mean square charge radii of nucleons: Neutron <R n2 > = - 0.116(2) fm 2 Proton <R p2 > = 0.769(12) fm 2 (I. Sick) Darwin-Foldy 0.75/M p2 = 0.033 fm 2 (J. Friar) 21
6 He & 8 He RMS Point Proton and Matter Radii Wang et al., PRL (2004) Mueller et al., PRL (2007) 22
He-6 Collaboration P. Mueller, L.-B. Wang, K. Bailey, J.P. Greene, D. Henderson, R.J. Holt, R. Janssens, C.L. Jiang, Z.-T. Lu, T.P. O Conner, R.C. Pardo, K.E. Rehm, J.P. Schiffer, X.D. Tang - Physics, Argonne G. W. F. Drake - Univ of Windsor, Canada He-8 Collaboration P. Mueller, K. Bailey, R. J. Holt, R. V. F. Janssens, Z.-T. Lu, T. P. O'Connor, I. Sulai - Physics, Argonne; M.- G. Saint Laurent, J.-Ch. Thomas, A.C.C. Villari - GANIL, Caen, France G. W. F. Drake - Univ of Windsor, Canada L.-B. Wang Los Alamos Lab 23