Experimental:

The EPR spectrometer, for this experiment, was constructed as simply as possible using components that were readily available in order to keep costs down. The basic setup of the spectrometer consists of several crucial parts such as the microwave bridge, the cavity resonator, the magnet, and signal processing system.

Figure 4: Schematic of the EPR spectrometer

The microwave bridge circuit was constructed as shown in figure 4. Continuous wave X-Band microwaves were produced by a reflex klystron and were passed through an isolator. The klystron was water cooled to prevent it from overheating. A frequency meter was used to measure the microwave frequency and tune the klystron near the resonate frequency of the cavity. An attenuator was used to adjust the amplitude and phase of the microwaves. Most microwave bridge circuits use a Hybrid Tee or Hybrid Circle, but since these components were not readily available, a 10 Db directional coupler was substituted for this. The side screw tuner section was used to fine-tune the microwave signal before entering the cavity. All of the bridge components were connected with WR-90 waveguide and lead gaskets were used to prevent the leakage of microwaves at connection points.

Figure 5: Cross section of the cylindrical cavity resonator.

The Cavity Resonator:

The cylindrical cavity, figure 5, used in this experiment was machined out of copper and consisted of a top, side wall, and bottom. The bottom was designed to extend into the cavity to allow for the length to be adjusted, while the diameter remained fixed. In an ideal cavity the side walls would be coated with gold and polished to a mirror finish, unfortunately the inner side wall of the available cavity was uncoated and un-polished. The side wall had a simple cylindrical geometry with a diameter of 3.8024 cm and a length of 3.4872 cm. Two set screws were used to adjust the length of the cavity by up to 0.6318 cm. The microwaves entered the cavity through an iris in the cavity top and a dielectric filler was placed above the iris in the wave guide coupler. Mylar rings were used to electrically isolate the top and bottom from the cavity side wall, and the top was securely screwed to the cavity side wall. The length of the cavity was set such that it would resonate in the TE112 mode. The cavity and microwave frequency were then fine tuned until the cavity resonance frequency was found and a quality factor (Q) above 1000 was obtained. 

The Magnet:

The large magnet used was chosen for its ability to produce a homogeneous magnetic field, to sweep across a range of field strengths and was also the only one available. The magnetic field strength was measured using a Hall-effect probe and set to be around 3300 G. The magnet was powered by a field-regulated power supply which utilized the Hall probe to stabilize the field. The field-regulated power supply was also used to control the magnetic field sweep. Modulation coils, connected in parallel, were placed on both sides of the cavity. A power amplifier and lock in amplifier were used to modulate the magnetic field at 20 KHz with an amplitude of 0.5 V. The field modulation amplitude and frequency were chosen to be smaller than the expected line width of the absorption line to prevent false signals and as large as possible, for a good signal to noise ratio.

Signal Processing:

A piezoelectric detector was attached to the directional coupler and used to output the signal to an oscilloscope during the setup and testing phase. The signal was then transmitted to a computer using an analog to digital data acquisition card and a LabView virtual instrument was used to record the data. The sample of DPPH was used to test and calibrate the spectrometer. To obtain a spectrum the magnetic field was swept across various ranges and recorded. Although the approximate field strength needed to resonate the electrons had been calculated, the initial sweep range was fairly wide. The sweep range was then adjusted accordingly after a strong signal had been observed. Once the EPR spectrometer had been properly tuned, the synthetic ruby with a doping of 0.03% was placed in the cavity and the modulation and lock-in scheme was adjusted until a good signal was obtained. The last sample to be analyzed was the cylindrical ruby. To analyze the effects of orientation on the location of the absorption lines, multiple spectrums were recorded for sample orientations between 0o through 180o at 10o increments.

Samples:

The samples used were 2,2-diphenyl- 1-picrylhydrazyl (DPPH), a synthetic ruby with a 0.03% doping of Cr2O3, and a cylindrical synthetic ruby. The samples were chosen for their availability and signal strength. The samples were mounted to a brass rod that extended to just above the cavity as shown in figure 6. Using Apiezon Grease, the samples were attached to a cube of salt, which was cut to a length that would place the sample in the center of the cavity. The salt cube was glued to the brass rod using superglue. The sample was then encapsulated in a quartz tube and inserted into the center of the cavity from the top. The cavity was retuned into resonance and the frequency meter was then moved well away from the cavity absorption frequency.

Figure 6: Sample mounting configuration