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Scalable Atomic Magnetometers for Magnetoencephalography

Cort Johnson, Sandia National Laboratories

(Session : Thursday from )

Abstract. Magnetoencephalography (MEG) is a noninvasive technique used to spatially locate and temporally resolve neuronal currents in the brain. These currents produce weak magnetic fields (10 fT – 1 pT) which are typically detected by highly sensitive superconducting quantum interference devices (SQUIDS). Standard MEG systems employ many (>100) SQUID channels arrayed around the head in order to achieve spatial localization on the order of a few millimeters. The cryogenic system required to maintain SQUIDS at superconducting temperatures are complex and costly. Recently, atomic magnetometers requiring no cryogenics have been developed with sensitivities comparable to SQUIDS. In these magnetometers an alkali vapor is spin polarized by a circularly polarized pump laser. The ensemble precesses in the presence of a magnetic field, causing a change in the vapor’s index of refraction. The resulting optical rotation of a linearly polarized probe beam is detected to measure the field strength. When operated at high densities and near zero magnetic field, the spin exchange collision rate is higher than the Larmor precession frequency. In this spin-exchange relaxation free (SERF) regime, decoherence due to spin exchange collisions does not occur and sensitivities less than 1 fT/rtHz have been achieved. Although SERF magnetometers have been used to detect brain currents, little effort has been made to develop a scalable, multi-channel architecture comparable to state of the art SQUID systems. We present the design of a fiber optics based sensor with a 50mm square footprint on the head (comparable to a SQUID gradiometer) with the sensing element approximately 1 cm from the surface of the head. The architecture is scalable in a straightforward manner, opening up the possibility of developing a whole-head MEG system based upon atomic magnetometers. A test of the sensor on human subjects is planned for fall 2009 at the MIND Research Network’s MEG facility in Albuquerque, NM.