title: Optimal dielectric and cavity configurations for improving the efficiency of electron paramagnetic resonance probes.
abstract: An electron paramagnetic resonance (EPR) spectrometer's lambda efficiency parameter (Λ) 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 Λ are derived in terms of the probe's individual DR and CV components, Λ1 and Λ2 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 Λ. It is shown that for a lossy CV with a small quality factor Q2, one chooses a DR that has the highest filling factor, η1, regardless of its Λ1 and Q1. On the other hand, if the CV has a large Q2, the optimum DR is the one which has the highest Λ1. This is regardless of its η1 and relative dielectric constant, ɛr. When the quality factors of both the CV and DR are comparable, the lambda efficiency is reduced by a factor of 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 Λ above the DR value. This can also be very helpful for relatively low efficiency dielectrics as well as lossy samples, such as polar liquids.
authors.affiliation: Department of Electrical and Computer Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3 Canada.
authors.forename: Sameh Y
authors.initials: SY
authors.lastname: Elnaggar
authors.affiliation: Department of Electrical and Computer Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3 Canada.
authors.forename: Richard
authors.initials: R
authors.lastname: Tervo
authors.affiliation: Department of Chemistry and Centre for Laser, Atomic and Molecular Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada. Electronic address: mattar@unb.ca.
authors.forename: Saba M
authors.initials: SM
authors.lastname: Mattar
mesh_terms: D004578:Electron Spin Resonance Spectroscopy / Q000295:instrumentation*; D004867:Equipment Design; D020342:Finite Element Analysis; D008872:Microwaves; D012680:Sensitivity and Specificity; D059629:Signal-To-Noise Ratio
keywords: Coupled mode theory; Coupled modes; Coupling coefficients; Dielectric resonators; Efficiency parameter; Electron paramagnetic resonance; Field distributions; Filling factor; Finite element methods; Quality factor; Resonant cavity; Resonator modes; Signal-to-noise ratio; Spectrometer sensitivity