Laser Separation of Isotopes The isotopes of an element, ordinarily indistinguishable, can be sorted out in the monochromatic light of a laser. The process may make isotopes plentiful for medicine, research and nuclear power by Richard N. Zare n nature most elements are mixtures the products of separation-isotopically or other electromagnetic radiation. It is of isotopes: they are made up of at­ pure or enriched elements--are ex­ only a matter of convenience whether I oms that all have the same number tremely costly. The enriched uranium the radiation is regarded as a stream of of protons and electrons but that differ that serves as the fuel for nuclear fission particles or as a system of waves; the in number of neutrons. Ordinarily the reactors is the most important of these two expressions are formally equiva­ fact that an element is a mixture of sev­ products; other isotopes. employed in lent. The particles. called photons. each eral ·kinds of atoms matters little be­ small quantities in research. in medicine have a definite energy. inversely propor­ cause the various isotopes are all but and in other fields. are many times more tional to the wavelength of the corre­ indistinguishable in most of their prop­ expensive. sponding wave. Photon energy (and erties. For that very reason. however. The separation of isotopes by laser wavelength) is unaffected by the intensi­ when we do want to separate the iso­ light differs fundamentally from other ty of a light source; increasing the inten­ topes of an element. the task can be ex­ methods. It distinguishes between atoms sity simply increases the number of pho­ ceedingly difficult. Some recent investi­ of different isotopes or between com­ tons. Within the visible portion of the gations into the absorption of laser light pounds containing different isotopes not spectrum the energy or wavelength of a by the isotopes of elements and by com­ on the basis of mass but through sub­ photon is perceived by the eye as color. pounds containing those elements now tle differences in electronic structure. Light can be absorbed by matter only promise to make the separation both These differences. even though they are when the energy of a photon corre­ easier and cheaper. small. affect the wavelengths of light ab­ sponds to the difference in energy be­ In isolating an element from a com­ sorbed by a substance; each isotope ab­ tween two of an atom's allowed states; pound or a mixture of elements it is sorbs light of a slightly different color. photons of other energies simply fail to common to rely on differences in the Because a laser emits light of very pure interact with the atoms and the light chemical properties of atoms. These color it can be employed to "tag" the is transmitted. When a photon is ab­ properties are determined almost entire­ atoms or compounds of one isotope sorbed. it induces a change in the atom's ly by the electron clouds that surround while leaving all others undisturbed. electronic structure: a single electron is the nucleus; the atoms of each element. promoted to a state of higher energy and of course. have a characteristic number ne of the first principles of quantum the atom is said to have entered an ex­ of electrons. The isotopes of a single ele­ O mechanics is that an atom or a cited state. On the average the excited ment. on the other hand. have virtually molecule can absorb energy only in dis­ electron is farther from the nucleus. identical electron clouds: the electrons crete units. That is because the atom has Additional photons (of the appropriate are identical in number and differ only only a finite number of discrete energy energy or wavelength) can elevate the slightly in their geometric configuration. states. each one representing a particu­ electron to still higher energy levels. As a result the separation of isotopes by lar configuration of its electron clouds. and as the energy increases so does the single-step chemical extraction has not By absorbing or radiating away energy density of the allowed states. The ulti­ proved practical. the atom can move abruptly from one mate level is the ionization limit. where For light elements isotope separation state to another. but intermediate ener­ the electron is torn loose from the atom; by repeated chemical extraction is possi­ gies are forbidden. ionization can be regarded as a contin­ ble. but for all those heavier than oxy­ Atoms can make these abrupt tran­ uum of allowed states. gen only physical methods have been sitions by absorbing or emitting light After an atom has absorbed a photon 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 SELECTIVE LASER EXCITATION of two isotopes of molecular iodine (12) is demonstrated the nucleus. The best-known of these in an experiment couducted by Douglas M. Brenner aud Saswati Datta iu the author's labora­ methods is gaseous diffusion. in which tory at Columbia University. The laser, part of which is visible at the right, emits light in a nar­ isotopes are distinguished by the slightly row band of wavelengths, but its spectrum is broad euough for the light to be absorbed by both different rates at which they diffuse isotopes of iodiue. The laser beam passes through two glass cells, one containing molecules in through a porous barrier; lighter atoms which both iodine atoms have an atomic mass of 127 (le!t), the other containing iodine mole­ pass through the barrier somewhat fast­ cules made up of atoms with a mass of 129 (center). In the top photograph the light is absorbed by the gases in both cells. Each molecule that absorbs a photon, or light quantum, is promoted er than heavier ones. Gaseous centrifu­ to an excited state, then dissipates its energy by reemitting photons, which are observed as flu­ gation. multiple distillation and electro­ orescence, a streak of soft orange light through the middle of each cell. In the bottom photo­ magnetic separation take advantage of graph an additional cell (right) containing molecular iodine 127 has been inserted in the laser other properties that depend on atomic cavity, where it suppresses the output of the laser at just those wavelengths that are absorbed mass. All these procedures are tedious by iodine-127 molecules. As a result the iodiue 127 iu the external cell is not stimUlated; only and cumbersome. with the result that the iodine 129 absorbs the laser light and fluoresces. The photographs were made by Fritz Goro. 86 © 1977 SCIENTIFIC AMERICAN, INC 87 © 1977 SCIENTIFIC AMERICAN, INC "With TDR* cable testing at American Airlines, we're finding cable disturbances long before they cause system failures." Aubrey Thomas, American Airlines Maintenance and Engineering Center, Tulsa, Oklahoma Aubrey Thomas checks a cable and antenna by looking at their signatures on 'Time Domain Reflectometer the screen of a TEKTRONIX TOR cabre tester in a 747's electronic equipment bay, from which most routine avionics troubleshooting can now be done. Not only can he spot subtle changes in sy_stems-if there is a problem, he quickly knows what it is and precisely where. Aubrey Thomas points out that "any­ Back in 1971 Aubrey Thomas and two body who has had to pull panels other people. at American Airlines were ' throughout an airplane to find a cable looking for a better way to test the fault will understand why we appreci­ miles of cable that carry radio, naviga­ ate the TOR's* ability to quickly identi­ tion, and instrumentation information, fy the nature of the problem and to tell as well as power for complex basic where it is-often to the inch, or electrical systems. "I knew that the TV closer. industry used time domain reflectome­ "But even more impressive is a situa­ try a lot," Aubrey says, "and couldn't cable produces Crimped cable this characteristic ture. One example 01 tion where, let's say, you might have to see why it wouldn't work on airplanes." signature. Detectable a potential problem figure out why a pilot was getting They started by using a TEKTRONIX by other methods, but virtually impossible to strong navigation signals close to a oscilloscope and plug-in TOR unit, on a TOR gives the pre­ detect by conven­ cise location. tional methods. station, but not farther away. If you a roll-around cart. But because work used a meter and it showed continuity quarters on aircraft are often either What an avionics tech sees in the system, you might waste hun­ cramped or relatively inaccessible, A time domain reflectometer (TOR) dreds of man-hours and change thou­ what they really needed was some­ works in a manner similar to radar. sands of dollars worth of components, thing much smaller and battery pow­ It generates repetitive pulses of en­ and still not be any better off. ered. ergy that are sent down a cable and "But by using the TOR in the equip­ Recognizing these needs, Tektronix displayed on a cathode-ray tube ment bay, you would know immediate­ had been developing TOR units that screen. Any cable faults (impedance ly that the problem was, let's say, six were not only small, portable, and bat­ changes) cause pulse reflections. feet of corroded cable shield, starting tery operated-but were also simple The shape of the reflection (the sig­ at the vertical-fin disconnect and com­ to use, had Chart-recording capabili­ nature) allows the type of problem to ing back toward the inside fuselage.