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

Requirements for Biomagnetic Measurements

• Sensitivity• Bandwidth~100Hz• Portability

Atomic SERF Magnetometer

'

Response StateSteady

yzx SS

Romalis et. al., Nature, 422, 596, 2003

Note: Spin-exchange collisions do not affect ´

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

Technical Noise from an hot air heated cell scheme

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

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

Current Setup

• Rb 87 with N2 (100T) buffer gases

• Circularly polarized pump at 795nm

• Linearly polarized probe at ~780nm

• Cell heated to 180 C

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

Adult MCG

Acknowledgments

This work funded by a grant from the NIH

Collisional Mixing and Pumping

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

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)

Extra

1 2 3 4u

0.2

0.4

0.6

2 J0uJ1u