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The GEMTEQ is a gaseous detector with Gas Electron Multipliers for signal amplification and Timepix ASICS as highly pixelated readout. It is based on the GEMPix detector and designed for microdosimetry. This paper describes the vacuum chamber developed to house the three versions of the GEMTEQ and to operate the detector in sealed-mode and at low pressure. A sealed version avoiding the need of a gas system offers clear advantages for its transport and use, e.g., for measurements at a clinical particle therapy centre. Operating the detector at a pressure lower than atmospheric increases the spatial resolution, one of the key parameters in microdosimetry. A gas-tight detector housing, a pumping system, and a pressure and temperature monitoring system were developed and tested in operational conditions. These extensive tests have proven the vacuum chamber to be gas tight and well suited for the foreseen microdosimetric measurements: the GEMTEQ can be operated for several weeks at 1000 mbar without noticeable degradation of the detector signal. First measurements have been carried out successfully at pressure values as low as 200 mbar. Further tests are planned to reach the design goal of 10 mbar. Overall, the vacuum chamber fulfils the requirements for a sealed-mode and low-pressure operation of the GEMTEQ. Measurement campaigns using this setup for example for LET measurements in hadron beams have already been carried out and will be discussed in forthcoming papers. [ABSTRACT FROM AUTHOR]