THERMAL NOISE IN BIOMAGNETIC MEASUREMENTS

Studies of weak magnetic fields are generally influenced by magnetic n oise emanating from thermal agitation of electric charge (Johnson nois e) in electrically conducting materials surrounding the magnetic-field sensor. In this article, the thermal magnetic noise fields generated by slabs with high electric conductivity (copper, aluminum) or high ma gnetic permeability (mu metal) are studied. The analysis is based both on a previously published phenomenological model and on measurements with an ultrasensitive superconducting magnetometer. Both the spectral densities and spatial correlations of the magnetic field fluctuations are evaluated. The computed correlation coefficients are utilized to develop a practical method for estimating the thermal noise due to thi n conducting foils, such as thermal radiation shields in a cryogenic m easurement Dewar. Also experiments to reduce the Dewar noise are repor ted. Finally, estimations are presented for the thermal noise fields a rising in the walls of a magnetically shielded room. In practice, ther mal magnetic noise, particularly due to the superinsulation in cryogen ic Dewars, can be the limiting factor of sensitivity in measurements o f weak biomagnetic signals arising in the human heart and brain. The r esults are useful in the estimation and minimization of the contributi on of thermal noise. (C) 1996 American Institute of Physics.