Cryogenics

Abstract

The word “cryogenics” means the production and effects of very low temperatures. Cryogenics is relevant to biomagnetic measurements for one general and several specific reasons. The general reason is that the sensitivity of sensors of all kinds, including magnetic sensors, are inherently limited by equilibrium thermodynamic fluctuations (commonly called “thermal fluctuations,” or in some contexts “Nyquist noise”) of whatever mechanism is involved in the sensing process, and this is diminished by cooling the device. The most important specific reason relevant to biomagnetism is that superconducting sensors such as the SQUID must be operated below a specific temperature (called the “superconducting transition temperature”) for the phenomenon of superconductivity to exist. In a sense, thermodynamic fluctuations disrupt superconductivity if the temperature is too high. In this chapter we shall examine the specific effects of these fluctuations and explain how an appropriate low temperature environment can be established for magnetic field sensors. For a general discussion of cryogenic techniques, the reader may consult the book of Lounasmaa (1974).