1

Dis

co

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,accele

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13/03/2019

Phil Levy

Research Scientist

Accelerator Division

Nuclear-polarized Beams

What is a nuclear-spin polarized beam?

Short answer:

Most types of nuclei have angular momentum (“spin”) and

can be pictured as spinning tops.

e.g. the simplest nucleus is the proton in a hydrogen atom. If all

the spin axes of the protons in a hydrogen beam point in the

same direction, the beam is 100% nuclear-spin polarized in that

direction.

3

More precisely …

A majority of nuclei have angular momentum Î , determined by quantum number I.

|Î |= 𝐼 𝐼 + 1 ħ

The largest possible measured component of

the nuclear angular momentum is I ħ. We say,

“ The nucleus has spin I ”

The allowed values of measured components

along a given direction z are quantized:

IZ = mI ħ with mI = I, I – 1,…-I

z

mI

2

1

0

-1

-2

Polarization of beam in direction z : P = Σ fi mi / I

fi = fraction of nuclei in substate mi

8Li I = 2

• All radioactive ion beams (RIBs) at ISAC are created

unpolarized, at low energy (20-40 keV). They must be

actively polarized.

8Li decays with a half-life of 838 ms by emitting a - particle.

8Li → 8Be + - + ν 8Be → 2

• Each is preferentially emitted along the direction of the 8Li

nuclear spin axis. It has a maximum energy of 13 MeV and

is easily detected.

We use polarized 8Li a lot

We polarize other beta emitters too

• We are interested in polarizing isotopes that decay by

anisotropic beta-particle emission.

• Angular probability distribution of emitted beta rays:

W(θ) ~ 1 + AP cos θ

1.3310.670.330

8Li

A = -1/3P = 1

θ

Asymmetry parameter

Emission angle w.r.t.

polarization direction

Polar plot

W

Polarization

6

Presently, there are two main applications at TRIUMF:

1. MATERIALS SCIENCE Beta-NMR (beta-detected nuclear magnetic

resonance)

The spinning nuclei act as tiny magnets. Their orientation is affected by magnetic

fields within materials. We can follow the time-evolution of the polarization by

measuring the angular distribution of beta particles after implanting the beam into

a target of interest → probe magnetic environment of different sites within

crystals, thin films, interfaces.

2. NUCLEAR STRUCTURE Determination of excited-state spins in

daughter nuclei by measuring A

The asymmetry parameter A has discrete values that depend on spins of parent

and daughter states. Example – Measuring the beta-decay asymmetry of

transitions involved in the decay of polarized Na isotopes to Mg gives us firm spin

values for excited nuclear states in Mg (OSAKA group campaign).

Usefulness of polarized beta emitters

BNMR with 8Li

8Li in Ge

Polarization decay

BNMR

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)

Frequency (kHz)

Time (s)

Asymmetry = (F-B)/(F+B)

8Li+

H1cos(ωt)

H0Backward

Forward

Asym

metr

yR

ela

tive a

sym

metr

y

MgO

Ag 19 nm17645 17650 17655

0.50

0.75

1.00

17645 17650 176550.25

0.50

0.75

1.00

Frequency (kHz)

T = 155 K, E ~ 1kV

T = 155 K, E ~ 30kV

OS

8Li+

0.41 nm

S

O

1 kHz

Study of Nuclear Structure

INa IMg A

28Na2 +0.5

1 1 -0.5

0 -1.0

29,31Na5/2 +0.6

3/2 3/2 -0.4

1/2 -1.0

30Na3 +0.67

2 2 -0.33

1 -1.0

βγ

Na

Mg

Asymmetry parameter A can take 3 possible values depending on spin values.

Low-field

BNMR

High-field

BNMRPOLARIZER

Laser beam

Low energy

radioactive ion beam

▪ Collinear polarized light interacts with atom/ion beam to produce nuclear-spin polarized

beams (longitudinal or transverse polarization)

▪ Magnetic coils (light blue) provide ~10 gauss field along Polarizer axis

▪ Coils (red) downstream of Polarizer preserve polarization in case of paramagnetic ions,

whose electronic magnetic moment strongly couples nuclear spin to outside world.

OSAKA

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Beamline layout

Polarized 8Li

• Probe used by CMMS (materials science) group since 2000 to carry out β-NMR

on thin layers and interfaces.

• Recent MTV (Mott-polarimetry time violation) fundamental symmetries experiment

has finished data-taking.

• High yields, high polarization, nuclear spin = 2, half-life = 0.838 s

• The Li atom has a strong electronic transition suitable for polarizing by

optical pumping. Requires charge-exchange to produce neutral atoms, followed by

re-ionization for transport to experiment.

other polarized alkali-metal atoms

• The other alkali-metals are identical in concept. We have polarized 9,11Li and

most sodium isotopes, the latter for external users. Currently developing polarized 32Na for S1596 (OSAKA), the latest in a series of experiments revealing detailed

nuclear structure of magnesium isotopes.

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12

High-field

BNMR

OSAKA

Polarimeter

GRIFFIN

Early Photo

Laser Spectroscopy

13

December 2018

High-field

BNMR

Low-field

BNMR

OSAKA

Optical pumping of 8Li atoms

mF

s+

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

2P1/2

2S1/2

F = I+J

5/2

3/2

5/2

3/2

44 MHz

382 MHz

671 nm

Hyperfine structure… …showing magnetic substates

Electro-optic modulator (EOM) puts 381 MHz sidebands on laser

frequency, and so both ground state hyperfine levels are pumped.14

Rb-vapour-jet charge-exchange cell,

biased to Doppler-tune atoms into

resonance with laser

Ion deflector

Deceleration

electrodes

Fluorescence

monitor

Cooled He gas

Configuration for polarizing atoms

He cell

589 nm(fixed wavelengths)

Polarized 29Na+

29Na+

Faraday cup

Configuration for polarizing atoms

Rb Cell He Cell Cryo-coolerDeflection plates

Large turbopumpFaraday cup

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Doppler broadening

The ion beam has an energy spread

dU ≈1.5 eV leaving the ion source. During the

neutralization process

Li+(beam) + Alkali (vapour) → Li + Alkali+

and other long-range interactions, energy

transferred to the vapour atoms decreases the

kinetic energy of the forward scattered lithium

atoms and increases dU, creating a low-energy

tail on the absorption profile of the beam.

Typically 50% neutralization efficiency is used.

804 MHz

Laser-induced 7Li D2 fluorescence

observed with Na vapour cell at 435 °C

(upper) and 385 °C (lower) with

estimated neutralization efficiencies of

80% and 30%, respectively. Four peaks

seen due to scanning 2 laser modes

across 2 hyperfine transitions.

Beam deceleration voltage

Doppler width = (q/2mc2U)0.5 f dU

U beam energy [eV]

f transition frequency

Matching laser to absorption

Key to high polarization is matching laser profile to absorption.

Use electro-optic phase modulators.

19 MHz EOM 28 MHz EOM

Single mode

laser

0

0.1

0.20.2

0

INTENSITY

100100− FREQUENCY

1.0

100 0 1000

0.2

0.4

0.5

0

j

100100− f

Re

lative

po

we

r

Laser-sideband frequency offset (MHz)

To Polarizer

Maximizing polarization with EOMs• Pump both hyperfine ground states

• Match laser bandwidth to Doppler – broadened absorption

bandwidth of beam

• We use EOMs (electro-optic phase modulators) for both

purposes when polarizing 8Li.

La

se

r In

ten

sity

HFS

381 MHz

8Li natural linewidth = 6 MHz

Laser-sideband frequency offset (MHz)19

9 - 10 MHz spacing

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Optical pumping parameters

One optical pumping cycle consists of absorption of one photon followed by spontaneous

emission back to the ground state. Stimulated emission has no pumping or scattering effect.

Optical pumping rate γP of two-level system by narrow-band light is given by:

𝑠0γ/2

1+ 𝑠0+ ൗ2δ

γ2

γ = Τ1τ atomic transition rate

δ detuning from resonance 𝑠0 = Τ𝐼 𝐼𝑠 saturation parameter

𝐼 laser intensity

𝐼𝑠 saturation intensity = ൗ𝜋ℎ𝑐𝛾3𝜆3

Maximum possible optical pumping rate is γ/2

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8Li optical pumping – rough estimates

Transition rate γ = 37 MHz

Saturation intensity Is ~ 3 mW cm-2

Laser intensity per mode I ~ 3 mW cm-2

Therefore optical pumping rate γP ~ γ/4 = 9.3 MHz on resonance.

Speed of 30 keV 8Li atom 850 km s-1

Distance travelled by atom before re-ionization ~188 cm

Enough time for ~20 optical pumping cycles

Polarization is heavily saturated.

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Measured polarization ~ 80%

The polarization is measured at the BNMR experimental apparatus.

Why isn’t it over 95%, as predicted by rate equation calculations?

• Implantation loss?

• Impact ionization in He cell?

• Imperfect spatial overlap between particle beam and laser?

• Not enough laser broadening?

• Coherent atomic effects? [see S.J. Park et al, J. Opt. Soc. Am. B, 31, 2278 (2014)]

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Alkali-vapour jet charge-exchange cell for neutralizing ion beams

• Simple design, very low maintenance, compact

• Target thickness ~ 2 x 1014 atoms cm-2

• Works especially well with Rb

• Vapour is well confined to the cell

• Condensed metal runs down walls back to reservoir

• Stainless steel

• Heater cable potted with braze in Cu cup

• Loads in situ at top

• Reservoir temperature ~ 240 °C with Rb

• Cooling fluid ~ 60 °C

• Constant heater power.

Reservoir

Heater

cable

Liquid cooling

Ion beam

aperture

Wick

Viewport

10 cm

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Self-regulating with constant heater power

• Mass flow of vapour to target region is determined by

conductance, Rb liquid surface area and temperature.

• The reservoir is mostly cooled by evaporation.

• The vapour pressure is not at equilibrium.

• If conductance is reduced by partial blockage → vapour

pressure rises → temperature increases → mass flow is

stabilized.

• If Rb liquid surface area decreases → temperature

increases → evaporation rate per cm2 increases →

mass flow is stabilized.

• In above examples, trying to keep the temperature

constant by actively reducing the heater power makes

operation more unstable, not less.

Ion beam

Rb vapour

~ 240°C

~ 40 W

Rb liquid

reservoir

Condensation

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Choice of charge-exchange vapour

Sodium vapour was original choice

• Maximum 95% efficiency in neutralizing Li+ beam

However, Na had very poor efficiency in neutralizing Fr+ beams, so for that we switched to Rb.

Discovered that Rb cell operation was more stable and cleaner than Na, and works well for

neutralizing Li+ beams. [Cs vapour has lowest ionization potential, but produces severe broadening]

Li+

Navap

Li+

Rbvap Navap

Fr+ Ionization

limit

✓ ✓

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2-stage Cryopump

1800 liter s-1 Turbopump

Bellows

He feed

Fixed points

Helium re-ionizer cell

He ~ 1.5 sccm

5∙10-6 torrIris

• Gas target impact-ionizes polarized atoms

• He gas minimizes scatter

• Differential pumping, large turbopump

• Gas load reduced by cooling

• 13 K target tube 200 mm x 9 mm ID

• 70 K heat shield

• Target tube moves ~100 μm vertically

when cooled from room temperature

Laser

~ 2∙1015 atoms cm-2

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• Horizontal and vertical dimensions are shown separately to scale.

• Telescope consists of two f = 200-mm lenses.

• Approximate beam diameter at lenses = 15 mm.

• Fibre optic would improve beam quality and characterization, but not for UV.

Laser beam transport

He tubeAlkali cell

Power monitor

Telescope

Laser

10.0 m

NBM

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Laser table

532 nm pump laser

Dye laser

673 nm, 280 mW

EOMs

Stabilized HeNe laser

300 MHz F.S.R.

scanning interferometer

Wavemeter fibre coupler

2 GHz F.S.R.

scanning

interferometer

Optical diode

Frequency doubler

λ/4 plate

λ/2

ISAC Hall

5W

Optical diode

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Frequency locking

• Light from the dye and HeNe lasers overlap in a 300

MHz free-spectral-range scanning interferometer.

• The interferometer is temperature stabilized to 0.1 °C

and hermetically sealed.

• The interferometer output is analyzed by computer to

determine the fringe positions and separation

• Feedback to a tuning element in the dye laser keeps

the fringe separation constant.

• Major cause of laser drift is air pressure variation.

• Long-term dye laser frequency stability is +/- 5 MHz,

determined by the HeNe reference laser stability and

scan non-linearities.

Fringe separation

(arb. units)

Dye

HeNe

polarized beams & laser spectroscopy 2016

Pump laser Frequency doubler HeNe reference laser

Scanning interferometers

Dye laserEOMs

Laser table

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31

Launch from table

Half-waveplate

Polarizing beamsplitter cube

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Electro-optic phase modulators

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Direct polarization of ions

34

Basic configuration

V

Ion beamPolarized

ion beam

Laser

Deceleration

electrodes

Re-acceleration

electrodesDrift tube

• Many elements have suitable polarizing transition only in the ion

• No neutralization and re-ionization is required

• Doppler-tune by adjusting voltage V on drift tube

• Problem – transition is often in the ultra-violet, where EOMs are

expensive and difficult to obtain, so only single frequency light is available.

• Solution - divide the beam into discrete energies, not the laser.

Optical pumping of 31Mg+ ions

3082 MHz

530 MHz

280 nmσ+

-1 10 mF

2P1/2

2S1/2

F

0

1

0

1

• Nuclear spin ½ ̶ a purely magnetic probe, of interest to Materials and Life Sciences

Polarizing 31Mg+ ions without EOMs

• Natural absorption width = 41 MHz

• Doppler shift on D1 transition at 28 keV = 27 MHz/eV

Not necessary to compensate for Doppler broadening

• Hyperfine ground state splitting = 3082 MHz

equivalent to beam ΔE = 115 eV

• Pumping from hyperfine ground states alternates between F= 1 and F= 0

We can pump all 31Mg+ ions with single frequency light

F=1

0

1

V V + 115

31Mg+ 280 nm

VV V + 115

37

Drift tubes for optical pumping of ions

38Transport of Polarized Ion BeamsTwo classes of polarized ions:

▪ Zero electronic magnetic moment (J=0), for example Li+. In this case the very small nuclear magnetic moment does not significantly precess during the few microseconds of beam transport through small ambient magnetic fields between the Polarizer and the experiment.

▪ Finite electronic magnetic moment, for example Mg+. In this case, the nuclear magnet is coupled to the much stronger electronic magnet, which precesses in ambient fields and reduces or destroys nuclear polarization. A guide field must be applied along the entire beam path to prevent spin precession.

F

J

I Total spin F =

Electronic spin J + Nuclear spin I

Mg+ is paramagnetic

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Low-field

BNMR

High-field

BNMRPOLARIZER

3 large rectangular coils maintain up to 14 gauss guide field to experiments.

Maximum deviation of field direction from polarization direction < 3 degrees.

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Laser table (Mg+)

532 nm pump laser

Dye laser

560 nm, 800 mW

EOMs

Stabilized HeNe laser

300 MHz F.S.R.

scanning interferometer

Wavemeter fibre coupler

2 GHz F.S.R.

scanning

interferometer

Optical diode

Frequency doubler

λ/4 plate

λ/2

ISAC Hall

10W

Optical diode

280 nm, 60 mW

41

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