A 280 MHz electron spin resonance spectrometer

Electron spin resonance spectrometers operating at low frequencies are reviewed and the construction of a 280 MHz spectrometer is described. The instrument employs a reflection cavity consisting of a quarter-wave coaxial line and uses superheterodyne detection at 15 MHz. A novel frequency control system locks the cavity to the oscillator frequency and spectra are displayed on an XY recorder. The sensitivity is 2.3 × 1014 spins per gauss linewidth, measured at room temperature using a 1 sec time constant and 1 mw power. This figure is compared with the theoretical value, and with the experimental and theoretical sensitivities of other low frequency spectrometers with similar cavities.