NTHU Physics Colloquium, 2/27/2008

Laser Spectroscopy of Exotic Helium Laser Spectroscopy of Exotic Helium IsotopesIsotopes

王立邦

清華大學物理系

Neutron HaloNeutron Halo

Discovery of neutron halo change our common view of nuclear structureHow is it discovered?

neutron proton stable nucleus3,4He, 6,7Li, 40,48Ca

neutron halo6,8He, 11Li, 14Be

proton halo8B, 17Ne(?)

Nuclear Interaction Cross SectionNuclear Interaction Cross Section

Interaction cross section σI =π(Rtarget+Rbeam)2

First observation of “halo nuclei” by Tanihata, 1985

σI(6He) ~ σI(4He) + σ-2n(6He)

target

target

Radioactivebeam

Target nuclei

3.0

2.5

2.0

1.5

1.0

Inte

ract

ion

Rad

ius

(fm)

876543

Helium Mass Number A

Nuclear ChartNuclear Chart

Neutron number

Pro

ton

num

ber

stable nuclei

unstable nuclei

NeutronNeutron--rich rich 66He and He and 88HeHe

Charge radius probes “valence neutron correlation”

Isotope Half-life Spin Isospin Core + Valence

He-6 807 ms 0+ 1 α + 2n

He-8 119 ms 0+ 2 α + 4n

Methods of Measuring Charge RadiiMethods of Measuring Charge Radii

( )

2sin4

)(42

222

θασ

E

cZdd

Rutherfordh

=Ω

222exp )(*2

cos*)()( qGdd

dd

ERutherfordθσσ

Ω=

Ω

02

22

arg2

2

)(6=

−=><q

Eech dq

qdGr h

Electron scattering

Nuclei with finite size:

R=1.21×A1/3 fm for non-deformed nucleiCan NOT apply to unstable (short-lived) nuclei

e beam

qpi

pf

θ

NeutronNeutron--rich rich 66He and He and 88HeHe

Charge radius probes “valence neutron correlation”

Isotope Half-life Spin Isospin Core + Valence

He-6 807 ms 0+ 1 α + 2n

He-8 119 ms 0+ 2 α + 4n

Nuclear ForceNuclear Force

Fundamental theory QCD not calculable in low-energy regime

Modern nuclear models use “effective potential”

Long range (~2 fm) attraction: one-pion exchange

Intermediate range (~1 fm) attraction: two-pion exchange

Short range repulsion: hard-core effect, quarks are fermions

QCD

q

q

qq

qq

N

N

Mesonexchangeq

qq

q

qq

N

N

π

qq

Neutron Halo Nuclei 6He and 8HeNeutron Halo Nuclei 6He and 8HeIsotope Half-life Spin Isospin Core + Valence

He-6 807 ms 0+ 1 α + 2n

He-8 119 ms 0+ 2 α + 4n

4He

8He

6He

QuantumMonte Carlocalculation

Proton

NeutronPieper & Wiringa (2006)

Charge Radius of Helium IsotopesCharge Radius of Helium Isotopes

Charge radii 4He < 6He ~ 8HeElectron scattering, not possible for 6He, 8HeElastic proton scattering involves strong interaction, cannot separate proton and neutronNo “Poisson equation” for QCD to relate strong force to nucleon distributionAtomic isotope shift, measure the difference

??1.673(1)1.964(1)AtomicIsotope shift

1.80(7)1.56(6)

2.09(9)1.95(8)1.79(4)

Protonscattering

1.676(8)1.96(3)Electron scattering

1.99(1)2.05(1)1.662(5)1.954(5)Theory

8He6He4He3He

Atomic Isotope ShiftAtomic Isotope Shift

Mass shift: due to nucleus recoil

Isotope Shift δν = δνMS + δνFS

Field shift: due to nuclear size

δνMS ∝ ''

AAAA −

δνFS∝ [Ψ(0)]2 × δ< rc2 >

Nuclear mass precisely known Atomic structure Ψ(0) calculable,H, He, Li.

Field (Volume) ShiftField (Volume) ShiftAA

FS rZe′

⋅ΨΔ⋅−= 222 )0(3

2 δπδν

1 10 1001E-3

0.01

0.1

1

10

100

| Iso

tope

Shi

ft |,

GH

zA tom ic num ber, Z

Mass shift

Field

shift

rE

s

p

V ~ - 1/r

Gordon W.F. Drake (Can. J. Phys 84, 83 2006)

Non-relativistic wave functions fromvariational calculationsPerturbation theory for relativistic corrections, QED, finite nuclear mass and nuclear charge radiusQED terms “cancel” in isotope shift

Atomic Theory of HeliumAtomic Theory of Helium

Experimental confirmationTotal transition frequency4He Fine structure splitting3He-4He Isotope shift + HFS

3He 4He

33PJ

23S1

Atomic Isotope ShiftAtomic Isotope Shift

Mass shift: due to nucleus recoil

Isotope Shift δν = δνMS + δνFS

Field shift: due to nuclear size

δνMS ∝ ''

AAAA −

δνFS∝ [Ψ(0)]2 × δ< rc2 >

Nuclear mass precisely known Atomic structure Ψ(0) calculable,H, He, Li.

For 23S1 – 33P2 transition @ 389 nm:

6He - 4He : δν6,4 = 43196.202(16) MHz + 1.008 (He4 - He6) MHz/fm28He - 4He : δν8,4 = 64702.410(70) MHz + 1.008 (He4 - He8) MHz/fm2

G.W.F. Drake, Univ. of Windsor, Nucl. Phys. A737c, 25 (2004)

100 kHz error in IS ~ 1% error in radius

Helium Energy LevelHelium Energy Level

For noble gas atom, first excited state too high for laser excitation, (VUV)Exp. on metastable stateRF discharge by electron collision

A glow discharge He gas cell

23S1

11S0

389 nm

1083 nm

23P0,1,2

19.82 eV

33P0,1,2

He energy level

τ=106 ns

τ=98 ns

Spectroscopy of Spectroscopy of 66HeHeTechnical challenges:

Short lifetime, small samples (~ 106 atom/s available)Difficult spectroscopy (Metastable efficiency ~ 10-5)Precision requirement (100kHz = Doppler shift @ 0.04 m/s)

Solution: Spectroscopy with cooled atoms in a magneto-optical trap

High resolution: cold atoms, Doppler width largely reducedHigh sensitivity: capable of detecting a single atom

66He ProductionHe Production

Stripping reaction at ATLAS: 12C(7Li,6He)13N6He extraction rate ~ 3×106 /s

12C Target

7Li3+ 100 pnA60 MeV

β Counter

750 ºC

6He

Turbopump 6He→ 6Li+β-+ν

0 1 2 3 410

100

1000

Rat

e (c

ount

s/se

c)

Time (sec)

t1/2 ~ 800 mst1/2 ≈ 800 ms

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01

10

100

Cou

nts

Energy (MeV)

3.5 MeV

Experimental SetupExperimental Setup

6He from ATLASTransport time ~ 1 sec

PhotonCounter

RF discharge

4He, Kr

6He

He* Zeeman Slower MOT

TransverseCooling

389 nm 1083 nm

Mixingchamber

389 nm

Single Single 66He Atom DetectionHe Atom Detection

0 5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

1.2

Pho

ton

coun

t rat

e (k

Hz)

Time (s)

Capture efficiency ~ 2×10−8single atom detection necessary!

Imaging system and reducing background6He capture rate ~ 150 per hour

Trap

Detector

Aperture

Filter

Single

6He atom!

60 70 80 90

0

1

2

3

4

5

6

Pho

ton

coun

t rat

e (k

Hz)

Time (s)

4He test

Spectroscopy of Trapped AtomsSpectroscopy of Trapped Atoms

Light shift and Zeeman shiftHeating and cooling effect: asymmetric line shape

single γ recoil: 440 kHz for 6He, 330 kHz for 8HeProbing laser power must be very low!M. Zhu, C. Oates, J. Hall, Opt. Lett., 18,1186 (1993)

-15 -10 -5 0 5 10 15 20

0

2

4

6

8

10

Phot

odet

ecto

r sig

nal (

a.u.

)

R e la tive frequency (M H z)

6uw 150uw 650uw

Test using 4He trap

Frequency Scanning StrategyFrequency Scanning Strategy

off

onoffon

8μs2μs

~ 12 μs

Frequency scan 12MHz

Trapping light

Probe laser

Fast switch & scan

Identification50 ms

Spectroscopy for 1 s

Two frequency-detuning trap (hot and cold)Balance between the two probe beams

Systematic Effect StudySystematic Effect Study

PMT

liquid N2 cooledRF discharge

He*

transversecooling

He + Kr

4He fine structure splitting as a testAtomic Beam ExperimentVery different systematic effect

2 3S1

3 3PJ

389 nm

J=0

J=1

J=2ν12

ν01ν02

4He level

44He Fine StructureHe Fine Structure

ReferencesLamb ‘57: I. Wieder and W.E. Lamb, Jr., Phys. Rev. 107, 125 (1957)Pipkin ‘78: P. Kramer and F. Pipkin, Phys. Rev. A18, 212 (1978)Metcalf ‘86: D.-H. Yang, P. McNicholl, and H. Metcalf, Phys. Rev. A33, 1725 (1986)This work: P. Mueller, L.-B. Wang, G.W.F. Drake, K. Bailey, R.J. Holt,

Z.-T. Lu, and T.P. O’Connor, Phys. Rev. Lett. 94, 133001(2005)

8113.6 8114.0658.4 658.8 8772.4 8772.8

Metcalf '86

Pipkin '78

Lamb '57

Drake '04

This work(trap)

ν12 ν01

Fine structure splitting, MHz

ν02

This work(atomic beam)

66He SingleHe Single--Atom SpectroscopyAtom Spectroscopy

~150 6He atomsin one hour

IS = 43 194.772(56) MHz

1/2 = 2.054(14) fm (0.7%)Phys. Rev. Lett. 93, 142501 (2004)

0 2 4 6 8 10 12 14 16 18 2043194.0

43194.4

43194.8

43195.2

43195.6

Isot

ope

Shift

(MH

z)

# of Measurement

April Run May Run

Frequency (MHz)

Res

idua

l

- 8 -6 -4 -2 0 2 4 6 8-4 0

0

4 0

-8 -6 -4 -2 0 2 4 6 8-1 0 0

0

1 0 0

50

100

150

200

250

300

Pho

ton

coun

ts

0

300

600

900

1200

Phot

on c

ount

s 6He4He

Res

idua

l

Frequency (MHz)

(a) (b)

Point Proton RadiusPoint Proton Radius

Theory:ms point-proton radius

rp

Experimental mean square charge radii:Proton 1/2 = 0.895(18) fm, [Sick, 2003]Neutron = −0.120(5) fm2, [Kopecky, 1997]

Experiment:ms charge radius

Rp

rc

Rn

>< 22

222 143 n

pppc RZ

NM

Rrr

Comparison with TheoriesComparison with Theories

Neutron-halo structure confirmedUnderestimate in cluster modelsDisagree with nuclear collision

Reaction collision

Elastic collision

Isotope shift

Clustermodels

No-core shell

Quantum MC

1.8 1.9 2.0 2.1 2.2 2.3 2.4

Nuclear charge radius of 6He (fm)

[Tanihata, 1992]

[Alkhazov, 1997]

This work

[Csoto, 1993]

[Funada, 1994]

[Varga, 1994]

[Wurzer, 1997]

[Esbensen, 1997]

[Navratil, 2001]

[Pieper, 2005]

AIP Physics News Update, # 702-3, Sep. 2004

Where to find Where to find 88He?He?

GANILCaen, France

Other possible sites:

TRIUMF in CanadaISOLDE @ CERNNSCL @ MSU

88He @ GANILHe @ GANIL

2.15 3.15

1.85 0.75

1.65

5 m

Salle D2

6,8He+@ 20 keV

6,8Hethermal

Laser System

MOT

ECR Ion Source

Massseparator

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

Atom Trapping of Atom Trapping of 66He & He & 88He at GANILHe at GANIL

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

PMT

Zeemanslower

MOTTransverse

cooling 389 nm 1083 nm

Atom Trap Setup

RF -Discharge

Xe

Capture efficiency1x10-7

Source~5x107 He-6/s,~1x105 He-8/s

Trap~5 He-6/s,

~30 He-8/hr

Jan. 26th 2007

… June 2007

June 15June 15thth -- HeHe--8 Trapped!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 10

0

20

40

60

80

100

Cou

nts

per C

hann

el

Rel. Laser Frequency, MHz

50 kHz

6He

~ 5 6He atoms/s

110 kHz60 atoms

8He

~ 30 8He atoms/hr

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 Isotope Shift and Field Shift : J -- Dependence?Dependence?

2 3S1

389

nm

3 3P0,1,2

J=2J=1

J=0

J=1

Wang 04Argonne

Experimental Uncertainties and CorrectionsExperimental Uncertainties and Corrections

74 kHz15 kHzNuclear Mass

97 kHz35 kHzTOTAL

45 kHz30 kHzZeeman Shift

15 kHz0 kHzProbing Power Shift

24 kHz2 kHzReference Laser Drift

12 kHz2 kHzLaser Alignment Drift

32 kHz8 kHzPhoton Counting

8He6He

Statistical {

Systematic {

-2(1) kHz-14(3) kHzNuclear Polarization

+165(0) kHz+110(0) kHzRecoil EffectCorrections

66He & He & 88He RMS Point Proton RadiiHe RMS Point Proton Radii

0.4 %0.3 %- He-4: 1.460(8) fm1.0 %0.2 %- Mass Systematics0.6 %0.3 %- Trap Systematics

0.6 %0.1 %- Statistical1.3 %0.5 %Total Uncertainty

1.809(28)1.926(11)RMS Rpp, fm-0.916(95)-1.464(34)Field Shift, MHz

8He6He

PRL 99, 252501 (2007)

66He & He & 88He Charge RadiiHe Charge Radii

AIP Physics News Update, # 851-2, Dec. 2007

ConclusionConclusion

Hypothesis of neutron-halo structure confirmed model-independentlyDemonstrated trap spectroscopy with single atom sensitivity

OutlookTo further improve the precision and compare with theories:Nuclear polarizability and MEC (meson exchange current) correction necessary at this level of precisionNeed more precise value of proton radius Need more precise value of helium-4 radius See also recent laser spectroscopy work on Li-11 at GSI/TRIUMFNew He-8 mass measurement in Penning trap @ TRIUMF in Dec. ‘07

CollaborationCollaborationK. Bailey, J.P. Greene, D. Henderson, R.J. Holt, R.

Janssens, C.L. Jiang, Z.-T. Lu, P. Mueller, T. O’Conner, R.C. Pardo, K.E. Rehm, J.P. Schiffer, I.

Sulai, X.D. Tang Argonne National Laboratory, USA

M.-G. Saint Laurent, J.-Ch. Thomas, A.C.C. Villari et al.- GANIL, Caen, France

G. W. F. Drake - University of Windsor, Canada

L.-B. Wang - Los Alamos National Laboratory, USA

P. Mueller L.-B. Wang I. Sulai Z.-T. Lu

GFMC GFMC –– What happens to the Core?What happens to the Core?

MesonMeson--Exchange Current CorrectionExchange Current Correction

Mesons mediating strong force carry electric chargeExpected to be different among 4He, 6He and 8HeIn H−D isotope shift, MEC correction ~0.2% of deuteron charge radiusSmall but NOT negligible at this precision

⇒ further theoretical investigation necessary

MECnp

ppc rRZN

MRrr >< 222

222 143

Cluster Models for Cluster Models for 66HeHe

Assume α core not affected much by the two neutronsEmpirical form of n-n, n-α potential, parameterized by scattering phase shiftCluster modelsTriton + triton channel used

α

C.M.

n n

r1R1C.M.

α

n n

r2 R2

α

n n

R3

r3

t

t

R4

(a) (d)(c)(b)

MuonicMuonic Atom XAtom X--ray Spectroscopyray Spectroscopy

mμ/me ~ 200

Bohr radius

Energy level

Wave function

Energy shift due to nuclear size

~

Sensitivity ~ (mμ/me)3

muon decay μ νν− −→ e

nucleus μ-em

a 1~0

2~ nmE en −

Muonic 4He+ Energy Level

δν(2S1/2-2P3/2)=1.813 −0.102 eV811.68(15)nm ⇒ 1/2(4He)=1.673(1) fm

Carboni et al., Nucl. Phys. A278, (1977)

Muonic hydrogen in PSI ⇒ proton radius1 S1/2

2 P3/2

8.2 keVX ray

2 S1/2

1.5 eV 812 nm

4% in metastable state

( ) 220 rδΨΔ

( ) 0/2/30~ arear −−Ψ

Atomic Theory of HeliumAtomic Theory of Helium

Solve 3-body Schrödinger Equation

2×10-96×10-31×10-78×10-85×10-47×10-49×10-44

Z4 (RN/ao)2Z4 α3ln αZ4 α2 μ/MZ2(μ/M)2Z2μ/MZ4 α3Z4α2Z2Magnitude

Relativistic correctionAnomalous magnetic moment

Nonrelativistic energy

Second-order mass polarizationMass polarization (SMS)

Finite Nuclear SizeQED correction (Lamb shift)Finite mass correction (NMS)

Contribution

Error

Binding Energy of Light NucleiBinding Energy of Light Nuclei

α+2n 6He+2n8He+2n

S. Pieper and R. Wiringa, Annu.Rev. Nucl. Part. Sci. 51, 53(2001)

Monte Carlo CalculationMonte Carlo Calculation

Argonne v18 two-body potential:

∑ ∑<

+++=i ji

Rijijiji vvvKH

πγ

EM OPE TPE+HC

• Coupling strength to fit NN scattering data• Problem: binding energy of most light nuclei too small

Illinois 3-body potential:

Rijkijkijkijk VVVV ++=

ππ 32

Urbana IX Illinois 1-5

•In nuclei A>3, excitation of the excited state of nucleons•Parameters to fit 3H, 3He binding energy• Isospin dependant, Neutron-rich light nuclei, e.g. 6He

S. Pieper and R. Wiringa, Annu.Rev. Nucl. Part. Sci. 51, 53(2001)

NuclearNuclear--Medium CorrectionMedium Correction

Electron does NOT see the bare nucleus ⇒ Nuclear polarizabilityThe energy shift in the helium 23S1−33P2 transition:

)()()( 087 2

46

HαHeαHeαkHz.δν

E

EE −×=

∑≠ −

=0 0

20

32

N NE EE

DNαα ∫∞

=+0

22

)(21 ω

ωωσ

πβα γ dME

G.W.F. Drake, private communication

αE(6He) = 0.68 − 0.74 fm3 by Esbensen αE(6He) ~ 0.8 fm3 by BaccaαE(2H) = 0.63 fm3 by Friar

10 kHz shift expectedIncrease 6He 1/2 by 0.1%

Baldin-Lapidus sum rule

389 nm Spectroscopy Laser Setup389 nm Spectroscopy Laser Setup

389 nmoutput

Diode Laser 1@ 778 nm

Diode Laser 2@ 778 nm

lock to I2 transition

TaperedAmplifier

measure beatfrequency

I2 spectrum near 777.946 nm

2S-3P 389 nm Frequency (GHz)40.6-44.6 0

3He I2×24He 6He

-2.5

Frequencydoubling

0 200 400 600 800 1000 1200 1400 1600

-3

-2

-1

0

1

2

3

Sign

al (a

rb. u

nit)

Relative Frequency (MHz)

used for laser locking

AOM

Fast frequency scan for spectroscopy