The problems to control radioactive contaminations in water or air gain in public importance. Solid state nuclear track detectors (SSNTDs) are used to register alpha-active or fissioning nuclides. On-line control of water or gas streams by means of SSNTDs with high efficiency is difficult. For this purpose a multi-layer chamber with SSNTDs was developed. This paper describes the construction of this chamber and gives the results of test measurements.