PURPOSE: To take out a single isotope by introducing an atomic or molecular gaseous mixture contg. an isotope and a scavenger into a chamber and irradiating this mixture with specific radiation to form a scavenger compd.

CONSTITUTION: The atomic or molecular gaseous mixture is admitted into the chamber 4. The mixture is rotated at a high velocity by an electromagnetic motor 16 to blow and plash the heavy isotope toward the inside wall surface 30 of the chamber 4. The light isotope is maintained within the region between the inside wall surface 30 of the chamber and a lamp 22. This gaseous mixture is irradiated with the radiation of the wavelength or frequency corresponding to the absorption characteristic of the specific isotope. The collected isotope is made to flow through the inside hole 38 of a scooping member and a conduit 36 to a suitable receiver, by which the sepn. is executed. As a result, the long- time operation is made possible.