Surface Dielectric Resonators for X-band EPR Spectroscopy

Abstract

A new resonator for X-band electron paramagnetic resonance (EPR) spectroscopy, which utilizes the unique resonance properties of dielectric substrates, has been developed using a single crystal of titanium dioxide. As a result of the dielectric properties of the crystal(s) chosen, this novel resonator provides the ability to make in vivo EPR spectroscopy surface measurements in the presence of lossy tissues at X-band frequencies (up to 10 GHz). A double-loop coupling device is used to transmit and receive microwave power to/from the resonator. This coupler has been developed and optimized for coupling to the resonator in the presence of lossy tissues to further enable in vivo measurements, such as in vivo EPR spectroscopy of human fingernails or teeth to measure the dose of ionizing radiation that a given individual has been exposed to. An advantage of this resonator for surface measurements is that the magnetic fields generated by the resonator are inherently shallow, which is desirable for in vivo nail dosimetry.

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Figures

Figure 1.

(a) CAD design of resonator in the machined holder. (b) HFSS visualization showing the magnetic field protruding from the top/bottom sensing surfaces of the dielectric resonator; (c) the HFSS model showing the visually transparent metallic shield; and (d) HFSS visualization of the electric field in the dielectric resonator. For sub-figures (b and d): Higher field values approach white, and lower values approach black.

Figure 2.

Simplified, equivalent circuit diagram of the variable magnetic coupling mechanism of the surface resonator. This mechanism utilizes two balanced loops to transfer energy (M) from the 50-ohm transmission line into the resonator through varying the distance of the coupling loops to the resonator for a wide range of resonator loads or impedances. The dielectric resonator is shown as an ideal LC-resonator. The enclosure of the resonator is shown as a rectangular box around the coupler and resonator, with an 8 mm diameter sensing aperture.

Figure 3.

A two-segment coupling loop designed for high coupling capability to a dielectric resonator measuring lossy samples. The parallel gaps between the wire is then filled with an appropriate capacitor (typically copper clad Teflon) to create the desired impedance (100 ohms).

Figure 4.

The resonator assembly is comprised of the following components: 1, 50-ohm Coaxial cable (rigid); 2, SMA connector; 3, Brass threaded rod; 4, Aluminum body of the resonator; 5, Clamp cap; 6, Clamp ring; 7, Rexolite spacer; 8, Inner dielectric plate holder; 9, Outer dielectric plate holder; 10, Coupling adjustment knob; 11, Spring; 12, Teflon washer; 13, Dielectric substrate; 14, Coupling loop.

Figure 5.

S11 measurement performed on an Agilent E5071C calibrated network analyzer showing a single resonance mode with reasonable Q (~580 unloaded) of the dielectric surface resonator within the desired 9–10 GHz frequency range.

Figure 6.

Measured sensitivity of the resonator performed using the perturbing metallic sphere method described in Reference (5), and measuring the effect of the frequency shift on the resonance mode.

Figure 7.

Approximately 200 ml of two micro-molar solution of TEMPO in a polyethylene test tube, resulting in a SNR of ~7, provides an indication of the sensitivity of this resonator. Spectral fit is overlaid on the EPR data. Twenty-five scans were collected.

Figure 8.

A 2 Gy and a 60 Gy (added dose) extracted human upper incisor tooth (whole) held in a rubber mold pressed against the aperture of the dielectric resonator. Fifty scans were collected on each tooth.

Figure 9.

A 10 mm × 10 mm sample of Kapton35 tape overlaid on the aperture of the resonator. Fifty scans were collected.