nuclear-polarized beams 1 - triumf...rb-vapour-jet charge-exchange cell, biased to doppler-tune...
TRANSCRIPT
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Dis
co
very
,accele
rate
d
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.
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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
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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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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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Launch from table
Half-waveplate
Polarizing beamsplitter cube
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Electro-optic phase modulators
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Direct polarization of ions
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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
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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
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