Polarized radioactive ion beams

XIIth International Workshop on Polarized Sources, Targets & Polarimetry

September 2007

Phil Levy, TRIUMF, Vancouver

Methods of Production

Radioactive ion beam (RIB) production

Projectile fragmentation:

High energy stable beam

Moderate thickness target

Separator

High energy RIB

Radioactive ion beam (RIB) production

ISOL – isotope separation on-line:

Thick target

Ion source

p+ beam500 MeV

Diffusion and effusion of reaction products

“pure” RIB10 – 60 keV

“dirty” ion beam

Mass separator

Radioactive ion beam (RIB) production

Combined:

High energy RIB

Helium gas stopper and ion guide

Low energy RIB

Post acceleration electrodes

Low emittanceNo chemical selectivityFast

Methods of polarization

In-beam

High energy Low energy

Tilted foils Recoil angle andenergy selection

Optical pumping

CollinearTransverse

Atomic beams

Motivation

Usefulness of polarized RIBs

Experimenters want beta emitters • Can monitor/control relaxation and precession of polarization of nuclei implanted in crystals.• Decay asymmetry depends on the transition

1.3310.670.330

Angular distribution W() of emitted beta rays  W() = 1 + (v/c) AasymP cos   Aasym= -1/3

P = 1

Geometry of a β-NMR Experiment

8Li

H1cos(ωt)

H0Backward

Forward

18895 18900 18905 18910-0.30

-0.25

-0.20

-0.15

-0.10

Asy

mm

etry

Frequency (kHz)

0 2 4 6 8 10-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

Asy

mm

etry

Time (s)½= 838 ms

Physics with polarized -emitters

• Fundamental symmetries• spin manipulation with applied rf• comparison of symmetry between -decays in mirror nuclei (G-parity)[Minamisono et al, Hyperfine Interactions 159 (2004) 265]

• High resolution atomic spectroscopy• -detected optical pumping of low intensity beams• hyperfine structure, nuclear moments, isotope shifts, nuclear charge radii [Kluge and Nörtershäuser, Spectrochimica Acta B 58 (2003) 1031]

• -NMR and -NQR• condensed matter physics – ultrathin films, superconductivity [Kiefl et al, NIMB 204 (2003) 682]• nuclear magnetic dipole and electric quadrupole moments

• Nuclear structure• -delayed nuclear spectroscopy• -decay asymmetries measured in coincidence with delayed radiationsto obtain spin and parity of daughter product excited levels [Miyatake et al, Phys. Rev. C 67 (2003) 014306]

Polarizer

•βNQR and low field βNMR•Fundamental symmetries

High-fieldβNMR

Nuclear structure

Collinear spectroscopy

Collinear polarization method

Collinear Optical Pumpingat TRIUMF

Collinear beam line at TRIUMF

Optical pumping of alkali metals

2S1/2

5/2

3/2

F =5/2

2P1/2

3/2

8Li I = 2

Li 671 nmNa 590 K 770 Rb 795 Cs 894 Fr 817

Typical polarization = 60 – 70%

381 MHz

mF-5/2 -3/2 -1/2 1/2 3/2 5/2

Valence electron absorbs light tranfers polarization to nucleus via hyperfine coupling

Advantages of collinear geometry

• Allows long laser interaction time (microseconds) with fast beams• Doppler width of beam is small high resolution low saturation powers [Anton et al., PRL 40 (1978) 642]

EmqU

2

11

Element Transition

wavelength

(nm)

Acceleration

voltage U

(volts)

Doppler width (MHz)

8Li 671 28 000 22

210Fr 817 60 000 2.4

11Be 313 28 000 40

20F 686 40 000 11

For beam energy spread EeV:

Other energy broadening mechanisms

• Collisions in neutralizer cell

Li+(beam) + Na (vapour) Li + Na+

Changes in internal energy change the kinetic energy of the forward scattered lithium atom. This broadens the energy spread of the beam and limits the thickness of Na vapour to less than optimum for neutralization

Typically ~40% neutralization efficiency is used

Collisions have negligible effect on transverse emittance

•Zeeman splitting of hyperfine magnetic substates (unimportant for guide fields < 10 gauss)

•Ion beam and laser divergences (unimportant for typical ~2 mrad)

7Li fluorescence 435 deg 80%

385 deg 30%

500 MHz

Matching laser to absorption

Key to high polarization is matching laser profile to absorption

single-mode laser broadened by electro-optic modulators

19 MHz EOM 28 MHz EOM

Single modelaser

0

0.1

0.20.2

0

INTENSITY

100100 FREQUENCY MHz

1.0

100 0 1000

0.2

0.40.5

0

j

100100 f

Rel

ativ

e p

owe

r

MHz

To collinearbeam line

~300 mW1 MHz bandwidth

Beam emittance

Dashed line – Li+ beam with neutralizer switched off, deflectors off

Solid line – “polarized” ion beam

Shows scattering due to helium, no effect from sodium.

7Li+ @ 29 keV

60% reionization efficiency

Beyond the alkali metals

• Ion has electronic structure similar to neutral alkalis.•Typical polarization = 20% ?

Polarizing wavelength (nm)

Repump wavelength

(nm)

Be 313 none

Mg 280 none

Ca 397 866

Sr 422 1092

Ba 494 650

Ra 468 10792S1/2

2P1/2

2D3/2

Repump

[W. Geithner et al., ISOLDE Collaboration, Phys. Rev. Lett. 83, 3792 (1999)]

Alkaline earths

• Closed transition uses metastable 1s5(J=2) atomic state as lower level [Shimizu, Phys. Rev. A 39 (1989) 2758]• Rare gas+ + alkali rare gas + alkali+ • Typical polarization = 20% ?

Polarizing wavelength (nm)

Binding energy of metastable state (eV)

He 1083 4.77

Ne 640 4.94

Ar 812 4.21

Kr 812 4.08

Xe 882 3.81

Rn 745 3.98

AlkaliRaregas

Groundstate

Groundstate

metastablestate

Positive ion

Ionization

potential (eV)

Na 5.14

K 4.34

Cs 3.89

Rare gases

Other examples of closed transitions

Element Transition Wavelength

(nm)

Energy of metastable state (eV)

Binding energy of metastable state (eV)

O 5S2 – 5P3 777 9.15 4.47

F 4P5/2 – 4D7/2 686 12.70 4.73

Si 3P1 – 3P0 252 0.0095 8.14

Ge 271 0.069 7.81

Sn 304 0.21 7.13

Pb 368 0.97 6.44

Development of polarized 20F

Development of polarized 20F beam

• Required for continuation of spin alignment G-parity studies begun with mirror nucleus 20Na

• Questions • Metastable production efficiency and survival• Hyperfine structure of 20F (I =2)

•Methods• 20F is a precious commodity not yet available, therefore use stable 19F (I =1/2)• Laser induced fluorescence 4P5/2 4D7/2

• Metastable depopulation pumping 4P5/2 4D5/2

J=1/2 3/2 5/2 7/2

1/2 3/2 5/2

1/2

3/2

metastable = 7 s

ground state

3p 4D

3s 4P

2p5 2P

12.7 eV

678 nm (depopulating)

686 nm (polarizing)

[C.D.P. Levy et al., doi.org/10.1016/j.nima.2007.07.013]

Polarizing transition 4P5/2 4D7/2

686 nm

4P5/2 -- 4D7/2

hyperfine spectrum

Laser-induced fluorescence

F+

ion beam

Na vapour neutralizer

:Positive ion

:Neutral atom Doppler tuning

Scaler Photon counter

Laser induced fluorescence

Depopulating transition 4P5/2 4D5/2

678 nm

Chopper

F+

ion beam

Faraday cup

He gas reionizer

Phase-locked amplifier

Voltage to frequency converter

Scaler 4P5/2 ? 4D5/2 hyperfine spectrum

Na vapour neutralizer

Metastable yield

Metastable-derived ion beam fraction at low He flow

= (R0-1) 0.57R0 (-1)

= 0.24 +0.16/-0.03

= ionization cross section ratio = 3.4 [O.B. Firsov, Soviet Phys. JETP 36 (1959) 1076]

Polarization and transmission efficiency

0

0.05

0.1

0.15

0.2

0.25

0

0.005

0.01

0.015

0.02

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Helium flow rate (sccm)

P2 I

PMAX

I = 1 at entrance to He cell

• PMAX assumes 100% polarization of metastables• P2I assumes I =1 at entrance to He cell

Calculated 20F hyperfine structure

1712 MHz

1401

1090 778

2368 MHz

1842

1315

789

F’ = 11/2

9/2

F = 9/2

7/2

7/2

5/2

5/2

3/2

1/2 3/2

4D7/2

4P5/2

+666 +441 -656 +0 +677 MHz Laser frequency

• Electric quadrupole shifts < 5 MHz (extrapolating from other halogens)• 666 MHz EOM covers 80% of population

predicted beam polarization up to 14 - 17%

Near term plans at TRIUMF

• Species such as 20F+ and 11Be+ are paramagnetic. Electron magnetic moment precesses rapidly in the Earth’s field and destroys nuclear polarization

Extend guide magnetic field to experiments

• Doppler tuning of 11Be+ requires drift region for ion beam in optical pumping region. Avoid EOMs by applying ~1 eV energy spread to beam.

30 keV 11Be+

100 V101 V 100 V

W W

Summary

• Many elements can be polarized in a collinear beam line - alkalis - alkaline earths - rare gases - O, F, Si, Ge, Sn, Pb, …..

• Suited to low emittance, low energy beams of beta-emitters, of course applicable to stable isotopes as well

• Collinear beam lines are also useful more generally in high resolution atomic spectroscopy.

• Development can proceed with stable beams

Acknowledgements

Rick BaartmanJohn BehrAtsushi HatakeyamaKeerthi JayamannaMatt PearsonLarry RootGeoff WightDick YuanAnatoli Zelenski

Students

Thomas CocoliosYoshi HirayamaRichard Labbe

Users

Rob Kiefl Andrew MacFarlaneKei MinamisonoTad MinamisonoTadashi Shimoda