[en] The University of Washington Model FN Tandem accelerator [1] is used for Accelerator Mass Spectrometry (AMS) of ¹⁰Be and ¹⁴C. This paper describes our basic system, our methods for rare-isotope normalization, final ion detection, and sample preparation, and the general problem of adapting an existing accelerator to meet the stringent stability requirements of precision AMS measurements while retaining human and technical compatibility with other users and uses of the accelerator. Recent preliminary data obtained on ¹⁴C in thin sequential sections of a single Sitka spruce tree ring (1963) are presented