Use of Long Cavities for Aqueous Samples in an Electron Spin Resonance Spectrometer

Abstract

IN a recent communication1 Cook and Mallard have described a cavity 5 wave-lengths long for use with aqueous samples in an X-band spectrometer. Feher2 has shown that the highest sensitivity of such a spectrometer is obtained by maximizing the product of the filling factor ζ and the loaded Q of the cavity, this condition being fulfilled when the sample thickness is chosen so as to reduce the Q to two-thirds of its unloaded value. This value of reduction factor follows from three basic assumptions: (a) the cavity is of fixed length; (b) the sample is centred at a nodal plane of electric field; (c) it is always electrically thin. For a sample of fixed thickness there is no maximum in sensitivity as the cavity length is increased, the reduction in the proportion of the total electric field with which the sample interacts being balanced by a corresponding reduction in the interaction with the total magnetic field, so that when the increased power loss in the walls is taken into account, it is found that the filling factor decreases faster than the loaded Q increases.