Laser Separation of Isotopes

In nature most elements are mixtures of isotopes: they are made up of at­ oms that all have the same number of protons and electrons but that differ in number of neutrons. Ordinarily the fact that an element is a mixture of sev­ eral ·kinds of atoms matters little be­ cause the various isotopes are all but indistinguishable in most of their prop­ erties. For that very reason. however. when we do want to separate the iso­ topes of an element. the task can be ex­ ceedingly difficult. Some recent investi­ gations into the absorption of laser light by the isotopes of elements and by com­ pounds containing those elements now promise to make the separation both easier and cheaper. In isolating an element from a com­ pound or a mixture of elements it is common to rely on differences in the chemical properties of atoms. These properties are determined almost entire­ ly by the electron clouds that surround the nucleus; the atoms of each element. of course. have a characteristic number of electrons. The isotopes of a single ele­ ment. on the other hand. have virtually identical electron clouds: the electrons are identical in number and differ only slightly in their geometric configuration. As a result the separation of isotopes by single-step chemical extraction has not proved practical. For light elements isotope separation by repeated chemical extraction is possi­ ble. but for all those heavier than oxy­ gen only physical methods have been developed. The physical methods all de­ pend ultimately on the small discrepan­ cies in mass per atom that result from variations in the number of neutrons in the nucleus. The best-known of these methods is gaseous diffusion. in which isotopes are distinguished by the slightly different rates at which they diffuse through a porous barrier; lighter atoms pass through the barrier somewhat fast­ er than heavier ones. Gaseous centrifu­ gation. multiple distillation and electro­ magnetic separation take advantage of other properties that depend on atomic mass. All these procedures are tedious and cumbersome. with the result that