ADIABATIC INVERSION FOR SELECTIVE EXCITATION

System for changing the energy state of particles of one type of isotope in a medium containing several types of isotope, and more especially U235 in a mixture of U-isotopes, comprising (i) means for generating energetic radiation e.g. laser, in a sufficiently narrow band to be absorbed by the one type of particle without absorption by the other types (ii) means for causing the frequency of energetic radiation to change smoothly through a range of frequencies which contains an absorption line for the particular type of particle, so as to change its energy state selectively without altering that of the other particles of different type, such change of frequency of the energetic radiation being so defined that it produces the required change of energy state in the particular type of particle. In particular, the change in energy state is from a lower to a higher level, the variation of frequency operating so as to produce an adiabasic inversion in the energy states, which means are provided for photo-ionising the excited isotope particles and for separating them from the medium by magneto-hydrodynamic forces. Provides an improved technique for nuclear fuel enrichment particularly in the vapour phase. The system theoretically permits 100% ecitation of all the particles illuminated of the desired type. It can also be applied to laser amplifiers.