Abstract The increasing demand for measuring long-lived radionuclides with small AMS machines at energies below 1 MeV per nucleon raises the need for compact detectors which still have a decent energy resolution and allow for a clear identification of the incident particles. Based on a design by the AMS group at the ETH Zurich a compact gas ionization chamber was built and installed at the 3 MV tandem AMS facility VERA (Vienna Environmental Research Accelerator). The main challenge in AMS is the detection of rare isotope species in the presence of strong isotopic and isobaric interferences. The task of the ionization chamber is the suppression of the unwanted isobar by separating the ions via their different stopping powers. Measurements of 36Cl at VERA showed an achieved suppression of the unwanted stable isobar 36S of 3 × 10−4 and measurements of 10Be showed an achieved suppression of 10B of at least 3 × 10−6. Additional suppression of the isobaric ions can be achieved by a degrader foil technique applied to 10Be measurements by G.M. Raisbeck. In combination with the new ionization chamber the achieved suppression of 10B is at least 10−10. Measurements of blank samples at VERA show that the background for AMS with 10Be is below 2 × 10−15.