Optimal dielectric and cavity configurations for improving the efficiency of electron paramagnetic resonance probes

An electron paramagnetic resonance (EPR) spectrometer's lambda efficiency parameter (Lambda) is one of the most important parameters that govern its sensitivity. It is studied for an EPR probe consisting of a dielectric resonator (DR) in a cavity (CV). Expressions for Lambda are derived in terms of the probe's individual DR and CV components, Lambda(1) and Lambda(2) respectively. Two important cases are considered. In the first, a probe consisting of a CV is improved by incorporating a DR. The sensitivity enhancement depends on the relative rather than the absolute values of the individual components. This renders the analysis general. The optimal configuration occurs when the CV and DR modes are nearly degenerate. This configuration guarantees that the probe can be easily coupled to the microwave bridge while maintaining a large Lambda. It is shown that for a lossy CV with a small quality factor Q(2), one chooses a DR that has the highest filling factor, eta(1), regardless of its Lambda(1) and Q(1). On the other hand, if the CV has a large Q(2), the optimum DR is the one which has the highest Lambda(1). This is regardless of its eta(1) and relative dielectric constant, epsilon(r). When the quality factors of both the CV and DR are comparable, the lambda efficiency is reduced by a factor of root 2. Thus the signal intensity for an unsaturated sample is cut in half. The second case is the design of an optimum shield to house a DR. Besides preventing radiation leakage, it is shown that for a high loss DR, the shield can actually boost Lambda above the DR value. This can also be very helpful for relatively low efficiency dielectrics as well as lossy samples, such as polar liquids. (C) 2014 Elsevier Inc. All rights reserved.