A Portable SERF Magnetometer for Biomagnetic Measurements R

A Portable SERF Magnetometer for Biomagnetic Measurements R Wyllie, R Wakai, T G Walker University of Wisconsin, Madison Theory We have developed a portable atomic magnetometer (AM) suited for biomagnetic measurements. Using this magnetometer, we have been able to collect adult magnetocardiograms (MCG) and present a sensitivity which should enable the detection of fetal MCG. An Alkali vapor cell is optically pumped, creating an atomic alignment. This alignment then precesses in the presence of an orthogonal magnetic field. This precession causes a Faraday rotation of a mutually orthogonal probe beam, which is detected using a balanced polarimeter. For small fields, the signal is proportional to the rotation angle of the probe polarization, qx~ Px , the net atomic polarization. We can solve the rate equation to get Px: Z-Mode Magnetometer Design and Results Pump optics Probe collimation optics Polyamide Thermal Insulation Probe detection optics The lasers are both couples to the optics tubes through optical fibers, and the entire apparatus is inside tri-axial helmholtz coils and is operated either in a small 4 layer mu-metal sheild or a 3 layer mu-metal room. Apply a large oscillating field along the pump direction and use lock-in detection. As shown below, the signal has two frequency components proportional to orthogonal field components. We also retain the same bandwidth as the original magnetometer. To the left, we demonstrate a noise reduction while using Z-Mode modulation. z z Rb cell in ceramic and Teflon oven A Rb-87 cell with 400 Torr N 2 buffer gas is heated to 180 C. A circularly polarized optical pump tuned to the D 1 line provides a net electronic polarization along the pump axis. Another beam detuned from the D 2 line is linearly polarized, and a rotation in probe polarization is detected on the balanced polarimeter. y y x x Biomagnetic Signals Heart signal frequency spectrum from DC-100 Hz Where R is the optical pumping rate, G are spindecoherence rates, and W are Larmor frequencies. If DC residual fields are cancelled, then the polarization and frequency response are Where q is the “slowing down factor, ” which is the ratio of the free electron to net sample precession rates (see box on spin-exchange collisions). If a spin temperature distribution is assumed in the atomic gas, then we can write a closed form for Rb-87 with a polarization P: Spin-Exchange Relaxation Free Atomic magnetometers work by detecting magnetically induced Larmor precession of electronic spins. The minimum detectable field goes as Spin exchange collisions cause a decoherence of the individual spins, and would limit usable densities for detecting individual spins. But generally, AMs are sensitive to net spin, which doesn’t decohere under spin-exchange, so densities can be turned up until other collisional processes become important. Thus, the probe beam polarization will, in this case, be sensitive to oscillating fields in the y-direction, and will have a low-pass filter type frequency response with a characteristic bandwidth determined by atomic species and experimental parameters. Three regimes of precession to collision rate, from Happer and Tang, PRA 16 1977 This work is supported by the National Institutes of Health Adult heart signals have a peak amplitude ~100 p. T. A fetus signal has a peak amplitude of ~1 p. T. To detect fetal MCG thus requires a sensitivity of better than 100 f. T/Hz 1/2 over the 100 Hz bandwidth. The signals below are adult MCG taken in a 3 -layer mu-metal room with our atomic magnetometer operating at about 40 f. T/Hz 1/2 sensitivity.