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 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 Romalis et. al., Nature, 422, 596, 2003 Note: Spin-exchange collisions do not affect G ´

Experimental Concerns For full SERF sensitivity, precession rate<<spin relaxation rate Magnetic shielding, careful nulling using Helmholtz coils Noise Sources –Nonmagnetic, technical noise (e.g. vibrations, thermal fluctuations, etc.) –Johnson noise (thermal electron motion)

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 B z =430nT Use lock-in detection of signal Two detection directions –Z1, S y signal oscillates at w z –Z2, S x signal oscillates at 2 w z Zero transverse fields produce no signal— background free

Z-Mode Results Significantly reduces nonmagnetic noise Retains sensitivity and bandwidth of normal SERF magnetometer operation

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 Limits on SS equations

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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