Simple Atom, Extreme Nucleus: Laser Trapping and Probing of He-8. Zheng-Tian Lu Argonne National Laboratory University of Chicago

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1 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

2 Helium Atom fm Å e - Ionization Energy of Helium Atom Level 2 3 S 1 Calculation ± 6 MHz Experiment MHz Gordon Drake, Phys. Scripta (1999) 2

3 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

4 Quantum Monte Carlo Calculations of Light Nuclei Pieper & Wiringa. Ann. Rev. Nucl. Part. Sci. (2001) 4

5 Halo Nuclei 6 He and 8 He Isotope Half-life Spin Isospin Core + Valence He ms α + 2n He ms α +4n Quantum Monte Carlo calculation 4 He 6 He 8 He Borromean Nucleus Borromean Rings Neutron Proton 5

6 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

7 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 ( ) 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 = (8) fm [I. Sick Phys Lett B (1982)] Proton rms charge radius = 0.895(18) fm [I. Sick Phys Lett B (2003)] 7

8 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 m/s Transition rate ~ 4 x 10 6 /s 389 nm Acceleration ~ 4 x 10 5 m/s 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 ev 1 1 S 0 8

9 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

10 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 E Atomic number, Z 10

11 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 P nm: 6 He - 4 He : δν 6,4 = (15) MHz (<r 2 > He4 -<r 2 > He6 ) MHz/fm 2 8 He - 4 He : δν 8,4 = (74) MHz (<r 2 > He4 -<r 2 > He8 ) MHz/fm 2 8,4 He4 He8 100 khz error in IS ~ 1% error in radius 11

12 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

13 8 He: The Most Neutron-Rich Nucleus # of protons # of neutrons 13

14 GANIL 75 MeV/u, 0.4 pμa 13 C beam on 12 C target 14

15 8 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 Laser System 15

16 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 S 0 RF - Discharge 1.2 One trapped 6 He atom PMT Source Capture efficiency Trap 6 He ~ 5x10 7 /s 1x He ~ 5 /s 8 He ~ 1x /s 8 He ~ 1x /s Photon countrate e/ khz Time (s) 16

17 He-8 Trapped! Count ts per Channe el First He-8 8Atom June 15 th 2007 f 6 He 8 He t 50 khz s per Channe el Count khz 60 atoms Rel. Laser Frequency, MHz Rel. Laser Frequency, MHz ~30 6 He atoms/s ~30 8 He atoms/hr 17

18 Laser Setup nm (778 nm) Diode Laser 778 nm Lock to I 2 Measure beat frequency Diode Laser 778 nm Tapered Amplifier 3 He I He 6 He Frequency doubling (LBO) 389 nm S P 2 Frequency (GHz) 18

19 3 1 P 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, , δν 8 194, MHz δν 6, (a) (b) 8 He 6 He Field Shift, MHz J = 0 J = 1 J = 2 (c) 6 He 8 He J = 0 J = 1 J = 2 Wang 04 Argonne 19

20 6 He & 8 He Charge Radii 6 He 8 He Field Shift, MHz (34) (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

21 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 > = (2) fm 2 Proton <R p2 > = 0.769(12) fm 2 (I. Sick) Darwin-Foldy 0.75/M p2 = fm 2 (J. Friar) 21

22 6 He & 8 He RMS Point Proton and Matter Radii Wang et al., PRL (2004) Mueller et al., PRL (2007) 22

23 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