The new method is based on the interaction of fast beam electrons with electrons in the outer shells of atoms, which generates the characteristic X-ray radiation. The intensity of X-rays is proportional to the concentration of the atoms in investigated gas.The gasdynamic structures of rarefied argon jets were studied in the range of stagnation temperatures up to 5200 K using the this X-ray method. The experimental studies of the present work were performed in a low-density gasdynamic chamber. The arc heater was used as a gasdynamic source. The changes in the structure of a jet of monatomic gas beyond a sonic nozzle with variation in the rarefaction and the temperature factor have been clarified as a result of the experiments and generalisations performed.