A Portable Ultrasensitive SERF Atomic Magnetometer for Biomagnetic Measurements R Wyllie, 1 Z Li, 2...
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Transcript of A Portable Ultrasensitive SERF Atomic Magnetometer for Biomagnetic Measurements R Wyllie, 1 Z Li, 2...
![Page 1: A Portable Ultrasensitive SERF Atomic Magnetometer for Biomagnetic Measurements R Wyllie, 1 Z Li, 2 R Wakai, 3 N Proite, 1 P Cook, 1 T Walker 1 1 – Department.](https://reader035.fdocuments.in/reader035/viewer/2022070409/56649e995503460f94b9c574/html5/thumbnails/1.jpg)
A Portable Ultrasensitive SERF Atomic Magnetometer for
Biomagnetic Measurements
R Wyllie,1 Z Li,2 R Wakai,3 N Proite,1 P Cook,1 T Walker1
1 – Department of Physics, UW-Madison
2 – Center for Clinical Neurosciences, UT-Houston Health Science Center
3 – Department of Medical Physics, UW-Madison
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Requirements for Biomagnetic Measurements
• Sensitivity• Bandwidth~100Hz• Portability
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Atomic SERF Magnetometer
'
Response StateSteady
yzx SS
Romalis et. al., Nature, 422, 596, 2003
Note: Spin-exchange collisions do not affect ´
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Experimental Concerns• For full SERF sensitivity, precession rate<<spin
relaxation rate• Magnetic shielding, careful nulling using Helmholtz coils
RB
R
fTkI rms
Brms
1~
4
• Noise Sources– Nonmagnetic, technical noise (e.g. vibrations, thermal fluctuations, etc.)– Johnson noise (thermal electron motion)
nT30 gives )12(
221
B
I
BBF
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Technical Noise from an hot air heated cell scheme
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Z-Mode
• Goal: use lock-in technique to extract signal from nonmagnetic noise
• Apply a large, oscillating ~kHz magnetic field in z-direction (along pump)
• For best sensitivity, Larmor frequency~parametric frequency, sets Bz=430nT
• Use lock-in detection of signal• Two detection directions
– Z1, Sy signal oscillates at z
– Z2, Sx signal oscillates at 2z
• Zero transverse fields produce no signal—background free
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Z-Mode Results• Significantly reduces nonmagnetic noise
• Retains sensitivity and bandwidth of normal SERF magnetometer operation
z
zx
zzx qutuJuJSS
0
10 where)sin()()(2'
)(ySensitivit Z1
'
:ySensitivit DC
to Z1Compare
yzx SS
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Current Setup
• Rb 87 with N2 (100T) buffer gases
• Circularly polarized pump at 795nm
• Linearly polarized probe at ~780nm
• Cell heated to 180 C
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Technical Improvements• Plastic, ceramic, and Teflon parts reduce Johnson noise, improve portability• Matched resistive film heaters create little magnetic field, allow smaller
apparatus, less noise (similar to Kitching et al heating scheme)• RF heating, atoms not affected by high frequency (MHz) fields• Insulation allows subject to be 1cm away from cell• Easy conversion to gradiometer with pump tube
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Adult MCG
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Acknowledgments
This work funded by a grant from the NIH
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Collisional Mixing and Pumping
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Equations
fT
rad104~
ΔOD
Δ
1
3[Rb]
7
2
23
23
23
x
x
SW
θ
Smc
leθ
22
22222
222
222
'
'
)'()'('2
)'()'('2
)'()'('2
Response StateSteady z
zxyzx
zz
zyxy
zxyx
SS
sRS
sRS
sRS
Limits on SS equations
nT34 gives )12(
221
B
I
BBF
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Notes
• Sensitivity Bandwidth product independent of species except slowing down factor
• Two requirements for ambient fields in SERF scheme– Spin temperature limit – Hyperfine states not well resolved, lots of
collisions in this timeframe– High sensitivity limit – spin’s do not precess far before relaxing
• Noise reduction using Z-mode• Current Setup• Adult MCG
• Sensitivity Bandwidth product independent of species, ~1/q (slowing down factor)
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Extra
1 2 3 4u
0.2
0.4
0.6
2 J0uJ1u