Influence of the lens effect in a sample with large dielectric constant ina loop-gap resonator on the EPR signal intensity at 700 MHz

The influence of the lens effect on the electron paramagnetic resonance (EP A) signal intensity was investigated in a loop-gap resonator (LGR) with an inner diameter of 41 mm. The Q-value and EPR signal intensity were measured when the phantoms containing 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-1-y loxy dissolved in sodium chloride aqueous solutions were put in the LGR. Th e Q-value and signal intensity reduced with increasing concentrations of so dium chloride in the phantom, indicating that the imaginary part of the die lectric constant is larger in the phantom with the higher concentration of sodium chloride. However, relationships between Q-values of the resonator a nd EPR signal intensities were not proportional and signal intensities were relatively higher compared with the Q-values. These findings suggest that the signal reduction due to low Q is slightly compensated by the lens effec t in the sample with the large real part of the dielectric constant. In the distribution of the signal intensities of a pinpoint sample made of diphen ylpicrylhydrazyl in the agar medium containing sodium chloride in the LGR, it was found that the signal intensity decreased according to the distance from the center and the difference in the signal intensity within 10 mm fro m the center was about 20%, indicating the inhomogeneity of the alternating magnetic field at the center and marginal region in the sample with the la rge dielectric constant caused by the lens effect.