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

Å

e-Ionization Energy of Helium Atom

Level 2 3S1

Calculation 1 152 842 741 ± 6 MHz

Experiment 1 152 842 743 MHz

Gordon Drake, Phys. Scripta (1999)

Helium Atom

fm

3

QCD

q

q

qq

qq

N

N

MesonexchangeN

N

π

Effective Model of Nuclear Interaction

∑ ∑<

+++=i ji

Rijijiji vvvKH πγ

EM 1-π short-range

Coupling parameters fit to NN scattering data

Problem: binding energy of most light nuclei too small

Rijkijkijkijk VVVV ++= ππ 32

Coupling parameters fit to energy levels of light nuclei

Pieper & Wiringa. Ann. Rev. Nucl. Part. Sci. (2001)

Two-body potential: Argonne V18

Three-body potential: Illinois-2

4

Quantum Monte Carlo Calculations of Light Nuclei

Pieper & Wiringa. Ann. Rev. Nucl. Part. Sci. (2001)

5

Halo Nuclei 6He and 8He

Isotope Half-life Spin Isospin Core + Valence

He-6 807 ms 0+ 1 α + 2n

He-8 119 ms 0+ 2 α + 4n

Borromean RingsBorromean Nucleus

4He

8He

6He

QuantumMonte Carlocalculation

Proton

Neutron

6

Hadronic Probe: Scattering of 6He & 8He Beams

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

18O9Be

6He12C

6Heθ1H

6He

7

E&M Probe of Nuclear Charge Distribution

22exp( ) ( ) ( )Mott

d d F qd d

σ σ= ⋅

Ω Ω

2 2 2arg

1( ) 16 ch eF q q r= − < > + ⋅ ⋅ ⋅

( )2 2r r r d vρ< > = ⋅∫mean-square radius

2r< >root-mean-square radius

4He 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)]

electron

q

θ4He

8

Atomic Energy Levels of Helium

23S1

11S0

389 nm

1083 nm

23P0,1,2

19.82 eV

33P0,1,2

He energy level diagram

100 ns

100 ns 1.6 MHz• Single photon kick 0.1 m/s

• Transition rate ~ 4 x 106 /s

• Acceleration ~ 4 x 105 m/s2

Cooling & Trapping at 1083 nm

• Single photon kick 0.3 m/s

• Doppler shift 400 kHz

Spectroscopy at 389 nm

Isotope Shiftδν = δνMS + δνFS

9

10 100 1000 10000

-150

-100

-50

0

δνM

S R

el. t

o M

ass

∞ (G

Hz)

Mass, amu

Mass Shift & Search for Helium-Like Strangelets

1s2s 3S1metastable

1s2p 3P2

1083 nm (1 eV)

1s1s 1S

20 eV

u d s

Strangelet

P. Mueller, Z-T Lu et al., PRL (2004)

Witten, PRD (1984)Jaffe, Busza, Wilczek, Sandweiss, RMP (2000)

1 /e

MSe N

mm M

ν ∝+4He

3He

6He

8He

10

10 100 1000 1000010-20

10-18

10-16

10-14

10-12

10-10

10-8

10-6

IsotopicKlein et al.

SolarVandegriff et al.

SolarThis Work

TerrestrialThis Work

Li

mits

on

Abu

ndan

ce

Mass (amu)

IsotopicThis Work Isotopic “per 4He in

Earth’s atmosphere”

Terrestrial “per atom of all elements on Earth”

Solar “per atom ofall elements in Sun”

Limits on the Abundance of Anomalously Heavy Helium

References:This work -- P. Mueller et al., PRL 92, 022501 (2004)Klein et al. in Proc. of Symp. on Accelerator Mass Spectrometry, ANL/PHY-81-1 (1981)Vandegriff et al., Phys. Lett. B365, 418 (1996)

11

Field (Volume) Shift

AA

FS rZe′

⋅ΨΔ⋅−= 222 )0(3

2 δπδν

1 10 1001E-3

0.01

0.1

1

10

100

| Iso

tope

Shi

ft |,

GH

zAtomic number, Z

rE

s

p

V ~ - 1/r

12

Drake, Can. J. Phys (2006);Pachucki & Sapirstein, J Phys B (2002)

Non-relativistic wave functions fromvariational calculations

Perturbation theory for relativistic corrections, QED,finite nuclear mass and nuclear charge radius

QED terms “cancel” in isotope shift

Atomic Theory of Helium

3PJ

3S1

For 23S1 – 33P2 transition @ 389 nm:6He - 4He : δν6,4 = 43196.207(15) MHz + 1.008 (<r2>He4 - <r2>He6) MHz/fm2

8He - 4He : δν8,4 = 64702.409(74) MHz + 1.008 (<r2>He4 - <r2>He8) MHz/fm2

100 kHz error in IS ~ 1% error in radius

13

Laser Cooling and Trapping

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

Technical challenges:

Short lifetime, small samples (<106 atoms/s available)

Metastable efficiency ~ 10-5

Precision requirement (~100 kHz)

14

8He: The Most Neutron-Rich Nucleus

# of neutrons

# of

pro

tons

15

He-8 @ GANIL

75 MeV/u, 0.4 pμA 13C beamon 12C target

16

8He @ GANIL by Antonio Villari et al.

2.15 3.15

1.85 0.75

1.65

5 m

Salle D213C

• He-8: 5 x 105 s-1

• He-6: 1 x 108 s-1

8He+

@ 20 keV

8Hethermal

Laser System

MOT

ECR Ion Source

Massseparator

1 GeV, 400 pnA

17

Atom Trapping of 6He & 8He at GANIL

~1x108 6He+/s~5x105 8He+/s

PMT

Zeemanslower

MOTTransverse

cooling 389 nm1083 nm

Atom Trap Setup

RF -Discharge

Xe

Spectroscopy389 nm

23S1

11S0

2 3P2

3 3P2

Trap1083 nm

He level scheme

Capture efficiency1x10-7

Source6He ~ 5x107/s8He ~ 1x105/s

Trap6He ~ 5 /s8He ~ 1x10-2/s

0 5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

1.2

Pho

ton

coun

trate

/ kH

zTime (s)

One trapped 6He atom

18

He-8 Trapped!

First He-8 AtomJune 15th 2007

t

f

-10 -8 -6 -4 -2 0 2 4 6 8 100

100

200

300

400

500

Cou

nts

per C

hann

el

Rel. Laser Frequency, MHz

-10 -8 -6 -4 -2 0 2 4 6 8 100

20

40

60

80

100

Cou

nts

per C

hann

el

Rel. Laser Frequency, MHz

50 kHz

6He

~30 6He atoms/s

110 kHz60 atoms

8He

~30 8He atoms/hr

19

0.4

0.5

0.6

0.7

0.8

0.9

-0.2

0.0

0.2

0.4

0.6

0.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

δν6,

4 - 4

3194

, MH

z

(b)

(a)

δν8,

4 - 6

4701

, MH

z

J = 0 J = 1 J = 2

(c)

6He8He

Fiel

d S

hift,

MH

z

J = 0 J = 1 J = 2

8He

6He

Isotope Shift and Field Shift : J - Dependence?

2 3S1

389

nm

3 3P0,1,2

J=2J=1

J=0

J=1

Wang 04Argonne

J=13 1P1

20

6He & 8He Charge Radii6He 8He

Field Shift, MHz -1.464(34) -0.916(95)

RMS RCH, 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 %

2

int 2 atom

PE E

Mγ

γ = +Recoil Correction

21

Charge Radius vs. Point-Proton Radius

Theory:mean square

point-proton radius<rp

2>

Experiment:mean squarecharge radius

<rc2>

rp

Z<rc2> = Z<rp

2> + Z(<Rp2>+0.75/Mp

2) + N<Rn2>

Mean square charge radii of nucleons:

Neutron <Rn2> = - 0.116(2) fm2

Proton <Rp2> = 0.769(12) fm2 (I. Sick)

Darwin-Foldy 0.75/Mp2 = 0.033 fm2 (J. Friar)

Rp

rc

Rn

22

6He & 8He RMS Point Proton and Matter Radii

Wang et al., PRL (2004)Mueller et al., PRL (2007)

23

He-6 CollaborationP. 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, ArgonneG. W. F. Drake - Univ of Windsor, Canada

He-8 CollaborationP. 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, FranceG. W. F. Drake - Univ of Windsor, Canada L.-B. Wang – Los Alamos Lab