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 Walker1 1 – Department of Physics, UW-Madison 2 – Center for Clinical Neurosciences, UT-Houston Health Science Center 3 – Department of Medical Physics, UW-Madison

2. Requirements for Biomagnetic Measurements • Sensitivity • Bandwidth~100Hz • Portability

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

4. 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)

5. Technical Noise from an hot air heated cell scheme

6. 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 wz • Z2, Sx signal oscillates at 2wz • Zero transverse fields produce no signal—background free

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

8. Current Setup • Rb 87 with N2 (100T) buffer gases • Circularly polarized pump at 795nm • Linearly polarized probe at ~780nm • Cell heated to 180 C

9. 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

10. Adult MCG

11. Acknowledgments This work funded by a grant from the NIH

12. Collisional Mixing and Pumping

13. Equations Limits on SS equations

14. 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)

15. Extra