250 GHZ EPR SPECTROMETER

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We have begun the design and construction of a 250.2 GHz EPR spectrometer that will operate in both pulsed and CW. The spectrometer will operate in reflection mode, using a circulator, to facilitate low temperature studies. Ferrite circulators are not available at these high frequencies; instead, a quasi-optical approach will be used. Quasi-optical circulators have been employed previously at 245 GHz, and have low insertion loss and are relatively inexpensive. To obtain a high Q cavity, either a cylindrical or Fabry-Perot resonator will be employed. Many of the biological systems that will be studied have very long relaxation times at low temperatures, making it difficult to obtain unsaturated absorption spectra. Our spectrometer will use a quadrature detection scheme to allow for detection of both absorption and dispersion signals. Saturated dispersion signals are very similar in appearance to direct detection absorption. The advantages of pulsed over CW techniques have been discussed above and make it desirable to have a pulse-forming capability. However, high frequency microwave switches are currently not available at 2S0 GHz. Instead, switching will be accomplished using a low frequency pin switch to switch on and off the 13.9 GHz injection-locking signal to a "quenched oscillator". Quenched oscillators act as reflection amplifiers and output microwaves only in the presence of an input injection lock signal. The advantage of this method is that there is no insertion loss associated with the switching. Furthermore, the isolation will be very high as a minimum input power of 4 mW is required to drive the frequency tripler to 250.2 GHz. Finally, the magnet for the 2S0.2 GHz EPR spectrometer will have a center field of 8.9 T (37S MHz 1H, 2S0.2 GHz electron) and a field sweep range of q1.0 T. This sweep range is sufficient for most biological samples of interest.