The Rare-RI Ring project has been launched at RIKEN. The Rare-RI Ring is a storage ring specially designed for the isochronous mass spectrometry of unstable nuclei. Precise mass measurements (delta m/m similar to 10(-6)) are necessary to reveal the r-process path and, therefore, the ion-optical conditions must be tuned to yield isochronicity of order 10(-6). For this purpose, we employ a highly sensitive resonant Schottky cavity as a probe for single-ion detection. Here, we first explain this technique theoretically and derive the necessary equations. Then, based on the results of off-line tests, we determine the sensitivity of the Schottky pick-up and estimate the intensities of the signals induced inside the cavity.