Method and apparatus for the separation of isotope mixtures

In apparatus for the separation of an isotope substance from a vaporous mixture of isotope substances involving selective excitation, dissociation or ionization of one substance by adiabatically decompressing the vaporous mixture to cool it to a temperature below 100.degree. K. and irradiating the cooled vaporous mixture by an electromagnetic wave selectively absorbed by an isotope substance, the improvement which comprises a diffuser in which the vaporous mixture, after cooling and irradiation, flows at a velocity greater than the speed of sound and in which shock wave occurs and there is a transition from supersonic to reduced velocity flow, cooling members disposed in the path of flow of the substances, partition members to separate the flow of substances into a flow containing most of the substances which have been subjected to irradiation from substances of which most have not been subjected to irradiation and outlet means for separately discharging said two separated flows.