The profile, density, and temperature of a supersonic rare‐gas jet was determined applying methods of nuclear physics and optical spectroscopy. The jet was produced in the windowless gas target facility RHINOCEROS and the heavy‐ion particle beams were provided by the Stuttgart DYNAMITRON accelerator. Maximum densities in the range of 1017to some 1018particles/cm3could be achieved in a gas jet of about 2.6 mm diam (FWHM). In a supersonic helium jet a cooling of the gas down to 45 K was observed. These results were compared with theoretical calculations.