  We have developed and successfully tested a low-pressure Time Projection
Chamber (TPC) with Gas Electron Multiplier (GEM) readout for Accelerator Mass
Spectrometry (AMS).
  AMS facility in Novosibirsk has a problem of separating isobar ions of
different chemical elements that have the same atomic mass. The typical example
is radioactive isotopes 10Be and 10B that are used to date geological objects
at a time scale of ten million years. To solve this problem, a new ion
identification technique, namely that based on measuring both ion track ranges
and ion energies in low-pressure TPCs with GEM readout, has been developed.
This technique is proposed to be applied in AMS for dating geological objects,
namely for geochronology of Cenozoic era.
  In this work, we developed a new larger version of the TPC with a dedicated
thin silicon nitride window for an efficient passage of ions. The TPC
characteristics were studied in isobutane at low pressures using alpha particle
sources. In addition, the use of GEM instead of THGEM has been shown to
substantially improve the energy resolution at a nominal pressure (50 torr).
Using these results and SRIM code simulations, it is shown that isobaric boron
and beryllium ions can be effectively separated at AMS, providing efficient
dating on a scale of ten million years. This technique will be applied in the
AMS facility in Novosibirsk in the near future.
