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 differonly 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 airplan e 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. ties, could be used in hostile environ­ be identified. Distance to the fault That's the beauty of the TOR, it's not ments (rain, snow, extremes in tem­ is determined by measuring the limited to identifying shorts or opens perature and altitude) , and on a wide elapsed time between the pulse and -it points out any disturbances." variety of cable types. reflection. Entire lengths of cable

© 1977 SCIENTIFIC AMERICAN, INC "Cost reductions are a major objective at American Airlines. One way we American Airlines keeps Aubrey saw I was right, and had located the achieve this is through preventive maintenance programs that result in Thomas and George Beyl on 24-hour problem precisely, he looked straight less down-time for airplanes." call, forming a two-man office that acts at me and said, 'What is that machine? John Hill, Supervisor, Aircraft as a liaison between engineering, I want one of them.' " Maintenance Support Group flight, and repair personnel-doing ev­ Tektronix' 1502 TOR Cable Tester erything from identifying avionics ($3,200) is recommended for cable can be examined, and when a dis­ problems to developing and evaluating lengths up to 2,000 feet. For longer turbance is detected, that portion of test equipment and training mainte­ lengths, such as telephone lines, the the cable can be viewed in detail. nance personnel. 1503 TOR Cable Tester ($2,985) is Conventional testing methods re­ Their job is to come up with tools and recommended. The optional, plug-in Y - quire access to a cable at both ends procedures that solve problems quick­ u.s.T sales Chart price Recorder FOB, Beaverton, is priced Oregon at $575. (a watt meter, for example, being ly and eliminate guesswork. George used at one end and a signal source recalls an incident that happened in These are just three of more than 835 at the other). On aircraft, there is the the early days of TOR at American, in­ TEKTRONIX products that serve the sheer physical problem of getting to volving another airline's plane on test, measurement, and information and removing an antenna, for ex­ which American had contracted to do display needs of thousands of cus­ ample, which may be out on a wing, heavy maintenance work. "They'd tomers worldwide, in transportation, on top of the plane, or six stories up been having a VHF problem, and I science, education, health, communi­ a tail fin. Additionally, conventional knew they'd been changing transceiv­ cations, and industry. methods tell if a cable system is ers trying to solve it. The TDR told me For a copy of our annual report or shorted or open, but they cannot lo­ that the problem was in the cable, in product information, write to Tektronix, cate the fault. Nor do they deal with a relatively inaccessible place, 77 feet Inc., P.O. Box 500, Beaverton, Oregon the more difficult situations, such as from the electronic equipment bay. Not 97077. By phone-cali 503-644-0161. gradual impedance changes caused a place you want to go into unless by moisture and corrosion. TOR is you're sure. It took a lot of convincing. also very useful where knowing pre­ And about four hours to get in there. cise cable length is critical. But when the man from the other airline COMMI TTED TO EXCE LLENCE

Knowing the condition of cables is Within minutes, George Beyl (left) and becoming even more important as Sterling Griffin tested the new Omega avionics systems increase in complex­ navigation system cable. "The thing "One oeauty of the TDR is its sim­ ity. Chart recordings provide useful about cables on aircraft, "George says, plicity," says Aubrey Thomas. "I can system histories, records of unusual "is that they're buried. Even when you train a mechanic to use it in about four problems, and can be used as do uncover them, you can't always hours. And since it's battery operated, training aids. look at one and say it's good or bad." we can take it anywhere we need it."

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LASER LIGHT

DEFLECTION OF EXCITED ATOMS is perhaps the most obvious the atoms of one isotope but not those of the other. Excited atoms method for the laser separation of isotopes. The atoms, a mixture of often have electrical and magnetic properties different from those of two isotopes (black dots and colored dots), issue from an oven to form atoms in the lowest energy state, and the excited atoms can be extract­ an atomic beam. They are then irradiated by a laser tuned to excite ed from the beam by an inhomogeneous electric or magnetic field.

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RADIATION PRESSURE of repeated photon absorptions can also brief transit of the atoms through the laser beam. With each photon be employed to de8ect one isotopic species. Again the laser is tuned absorbed the atom acquires a small increment of momentum, and so to excite one isotope, but in this case the excited state decays quickly it is gradually pushed out of the atomic beam; atoms of the other (by 8uorescence), so that many photons can be absorbed during the isotope, which do not absorb the laser photons, are not de8ected.

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LASER LIGHT

RECOIL OF DISSOCIATING MOLECULES represents a third that depend on isotopic composition. The laser excites and dissociates means of separating isotopes by de8ection. The molecules, which are only molecules that include one isotope (color). Part of the energy of made up of one element that has two isotopes (black and colored dots) the broken chemical bond pushes the fragments apart; the lighter frag­ and another with only one isotope (gray dots), have absorption bands ment (color) is de8ected through a larger angle and can be collected. 90

© 1977 SCIENTIFIC AMERICAN, INC it can give up its energy of excitation in from one another. Several methods are An ex periment in the separation of any one of several ways. The simplest then available for actually sorting them isotopes by the deflection of ex cited at­ route is for it simply to emit a photon of into different "bins." oms might begin with a beam of some the same energy as the one it absorbed; Laser light is es sential to the tech­ arbitrary element A, which might con­ this is the process of fluorescence. Some nique; no other light source would do. sist of two isotopes. designated Al and ex cited atoms can decay to a metasta­ Lasers themselves operate by exploiting A2• The beam is irradiated by a laser ble state. which persists for a relatively the quantum -m echanical transitions be­ tuned to some feature of the spectrum of long period before the en ergy is reemit­ tween the energy le vels of atoms and the isotope AI. so that many Al atom s ted. The energy can also be dissipated molecules. In a laser a substantial frac­ make a transition to an ex cited state (de­ through a series of small transitions and tion of all the atoms in a population of noted by AI·) whereas almost all the A2 thereby be converted to random kinetic atoms are promoted to the same excited atoms remain in the ground state. The en ergy. or heat. Ultimately the atom re­ state; the atoms return to a lower energy AI· atoms are thus conspicuou sly la­ turns to its lowest allowed energy state: state by em itting light coherently. or in beled and can be separated from the the ground state. Since an atom can oc­ phase. The coherence is ensured by rest. The separation is accomplished by cupy only designated energy states. it bouncing a portion of the light back and passing the atomic beam through an in­ absorbs and em its light only in narrow forth between two mirrors so that it homogeneous el ectric or magnetic field. lines of precisely defined wavelength. passes through the laser medium repeat­ which deflects those atoms that are ea ch line corresponding to a particular edly. stimulating atoms to radiate as it more readily polarized or have a larger atomic transition. The collection of all passes. The two mirrors form a resonant magnetic moment. The deflected atoms such lines is the spectrum of the atom. cavity. a kind of organ pipe for light. In are then collected. for example by con­ Like atoms. molecules have electroni­ some cases the cavity can be "tuned" in densation on a cooled surface. It should cally excited states. In addition. they order to ad just the wavelength of the be em phasized that the selective step in have vibrational and rotational ene rgy light emitted. this procedure is not the deflection in an states. corresponding to various possi­ Two properties in particular re com­ inhomogeneous field; the field would ble modes of oscillation and rotation of mend the laser for the work of isotope deflect excited A2• atoms as well as AI· the constituent atoms around their com­ separation. First. it can produce a light atoms. The isotopes are separated be­ mon center of mass. Transitions be­ of high intensity. that is. a beam with cause of the selective laser excitation of tween vibrational states are also associ­ a large flux of photons; with dimmer the AI species. ated with the absorption and emission of sources separation would probably be electromagnetic radiation. although the too slow to be practical. Second. laser or this process to be successful two wavelengths involved are generally light can be made highly monochromat­ F conditions must be met: a substan­ longer than those of atomic spectra. ic. so that all the photons em itted have tial fraction of all the Al atoms must be Most transitions between the electronic very nearly the same energy. Emission "pumped" by the laser. and the ex cited energy levels of atoms or molecules are over a narrow range of wave lengths is state must survive long en ough for the in the visible and ultraviolet regions of essential for the ex citation of one iso­ field to cause a measurable deflection. the spectrum. whereas transitions be­ tope to the ex clusion of others. since the Unfortunately these two requirements tween molecular vibrational states are spectral lines of isotopes are generally are contradictory. A large proport io n of in the infrared. close together. the atoms can be made to undergo a The various proposals for laser iso­ particular transition only if that transi­ he characteristic spectra of both at­ tope separation differ mainly in the tion is a highly probable. or favored. Toms and molecules are subtly influ­ method employed to extract the selected one. but in that case the reverse trans i­ en ced by the details of nuclear structure. atoms or molecules after they have been tion to the ground state must also be Electrons are bound to an atom by the excited by the laser. One of the most favored. In other words. if an atom is positive charge of the nucleus. and their straightforward techniques simply ex­ likely to absorb a photon of a given motions are dete rmined primarily by ploits intrinsic physical properties that wavelength. then it is also likely to em it the number of protons in the nucleus. distinguish excited species from those in such a photon. The excited atoms decay Neutrons do have an influence on the the ground state. For example. in the rapidly by fluorescence. electron cloud. however; by changing excited states of many atoms the elec­ One possible way to resolve th is di­ the mass. volume and shape of the nu­ tron cloud is more readily distorted in lemma might be to illuminate the atom­ cleus. changes in neutron number can an electric field or it has a larger magnet­ ic beam with a second laser (or some subtly alter the tra jectory of an electron. ic moment than it does in atoms in the other source of energy) that would con­ In a molecule differences in nuclear ground state. vert the excited AI· atoms into a longe r­ mass can have somewhat larger effects. For experiments in isotope separation lived. metastable species that could be by altering the frequency of the molecu­ it is often convenient to employ an atom­ labeled AI··' The meta stable atom s. lar vibrational states. All these eff ects ic beam: a stream of atoms that all which also commonly bear distinct ive are small and they play no part in most move in the same direction and h ave a electrical and magnet ic traits. could interactions of matter. They are none­ relatively narrow range of speeds. The then be de fle cted. theless crucial for the laser separation of beam is created by evaporating the sub­ Alternatively the short radiative life­ isotopes. They give each isotope of an stance in an oven inside an evacuated time of the exc it ed species A I· might be el ement and each molecule composed chamber; the resulting thermal motions turned to advantage. That could be of different isotopes a different set of of the atoms are random. with the aver­ achieved by actually pushing the atom s energy levels. leading to small shifts in age velocity determined by the tempera­ out of the beam with the radiat ion pres­ absorption and em ission spectra. ture. Atoms that happen to be moving in sure of the la ser light. For this purpo se Several mechanisms for the laser sep­ a particular direction pass through a the la ser must inter sect the atomic beam aration of isotopes have been proposed. small opening in the oven. and the beam at a right angle. Each atom that ab sorb s but all of them have 'as their basis the can be further collimated by passing it a photon then acquires from it a small same principle. A collection of atoms or th ro ugh a series of orifices. although increment of momentum that tend s to molecules containing more than one iso­ only at the cost of reduced intensity. Be­ propel it away from the lase r sou rc e. tope is irr,adiated by a laser whose wave­ cau se the beam travels in a vacuum. col­ The momentum is proportional to the length has been adjusted so that it ex­ lision s that would disturb the stra ight­ energy of the laser photons. but it is al­ cites the atoms of one isotope but has no line motion of the atoms are minimized. ways very small . so that for any appr e­ eff ect on the others. In this way the vari­ The same technique can be employed to ciable defle ct ion each atom must absorb ous species of atoms are distinguished prepare a beam of molecules. many photons. The short lifetime of the 91 © 1977 SCIENTIFIC AMERICAN, INC MIRROR ,�------VACUUM C",M"RY MIRROR � �---�------I ------_t /' . ------. / - ::::fj _ ------, +---/ : URANIUM-238 ATOMS _ • •/ � e..:;:� . ....r_ - -- ' . . . · o:.-- · , URANIUM-235 IONS --/. . i . ---� -- ; J" f -...... -., XENON-ION LASER CAVITY [./" .. r ��.. . ---.-� : .. B ! �) COLLECTOR PLATE (-1 ,500 VOLTS) URANIUM OVEN

SELECTIVE IONIZATION of atoms was employed in the laser en­ cavity of the krypton-ion laser. By means of an etalon, a device that richment of uranium in a technique demonstrated at the Lawrence narrows the bandwidth of a laser, the xenon laser was adjusted so that Livermore Laboratory. Two lasers, one operating with a medium of its wavelength corresponded to one of the absorption lines of the iso­ xenon ions, the other employing krypton ions, were arranged on a tope uranium 235. In this way uranium-235 atoms were promoted common axisand l\beam of uranium atoms was introduced into the to an excited state, whereas atoms of the commoner isotope uranium excited atoms is essential so that they by the loss of excited atoms that decay Al and A2• If a laser can be tuned so that can quickly return to the ground state to not to the ground state but to metastable it excites only those molecules that in­ absorb another laser photon. It should states from which another photon can­ corporate Al atoms. then only those be noted that each time an excited atom not be absorbed. Even under the best molecules will be dissociated. The ener­ returns to the ground state by emitting a conditions the angular deflection is gy of the chemical bond is dissipated in photon the atom recoils with a momen­ small. and a highly collimated beam of both the internal excitation of the prod­ tum exactly equal to that gained when atoms is necessary. ucts and in their motion; the recoil mo­ light is absorbed. The recoil momentum Barium has a number of stable iso­ tion. which is much larger than the re­ is randomly oriented, however. with the topes. of which the most abundant one. coil associated with photon absorption. result that after many recoils the aver­ making up about 72 percent of the natu­ de fle cts the atoms from the beam. There age momentum from the reemissions is ral element. is the isotope with an atom­ is the potential for a substantial deflec­ zero. The momentum of the absorbed ic mass number of 138. The atomic tion triggered by a single photon. but so laser photons always pushes the atoms mass number is the sum of the number far the technique has not been success­ in the direction of the laser beam. of protons (in this case 56) and the num­ fully demonstrated. The first successful application of this ber of neutrons (82) in the nucleus. By technique was reported in 1974 by An­ selective excitation the Lawrence Liver­ nstead of deflecting atoms that are ex­ thony F. Bernhardt. Donald E. Duerre. more group was able to produce barium I cited but electrically neutral, another Joe R. Simpson and Lowell L. Wood of enriched in this isotope by a factor of approach to isotope separation converts the Lawrence Livermore Laboratory. two or three. them into ions. which are much more They prepared an atomic beam of bari­ One disadvantage of the technique is easily manipulated. A strong electric um and tuned a laser to a line in the that it requires many laser photons. each field can efficiently sweep the charged barium spectrum at a wavelength of of relatively high energy. for each atom particles from a beam of neutral atoms 5.535 angstroms. which falls in the separated. A third proposed method of in the same way that an electron beam is green part of the visible spectrum. The separation by de fle ction overcomes this swept across the face of a cathode-ray absorption of a single photon of this difficulty by obtaining part of the energy tube. The ionization can be accom­ wavelength changes the transverse ve­ required from another source: the laser­ plished in two steps: first one isotopic locity of a barium atom by about .8 cen­ induced rupture of a chemical bond in a species is selectively excited with a fine­ timeter per second. That speed is about molecule. ly tuned laser. then the excited atoms are 50.000 times smaller than the longitudi­ The technique relies on choosing a ionized with another. less precise laser nal velocity of the atoms in the beam. so molecule that is stable in the ground or even with some other source of ener­ that repeated absorption and reemission state but decomposes spontaneously gy. The method was demonstrated in is clearly essential. The number of pho­ when it is promoted to an excited state. 1974. when the isotopes of calcium were tons that can be captured is limited by For the sake of simplicity it can be as­ separated by investigators at the Uni­ the transit time of the barium atoms in sumed that the molecule is diatomic and versity of Cologne. and when the iso­ the laser beam. by the finite time re­ can be represented by the arbitrary sym­ topes of uranium were separated by a quired for the excited state to decay and bols AB. where A again has two isotopes group at the Lawrence Livermore Labo- 92

© 1977 SCIENTIFIC AMERICAN, INC grou nd state into sublevels. glvmg a IONIZATION CONTINUUM "fi ne structure " to the atomic spectrum.) It is the atoms in the metastable state that are excited and ultimately io nized and isolated. The excitatio n is provided by a laser whose operating medium is a gas of xe­ no n io ns; its radiation at a wavelength of 3.78 1 angstroms (in the ultraviolet re­ gion of the spectrum) excites only the lighter and easily fissionable ura ni um isotope of atomic mass number 235 and only those atoms of uranium 235 that happen to be in the low-lying metastable state. Atoms of the heavier isotope ura­ nium 238 are not affected. whether they are in the ground state or in the metasta­ KRYPTON-ION LASER PHOTON ble state. (3,507 OR To ionize an atom a specific wave­ 3,564 ANGSTROMS) length is not needed; any photon with enough energy to overcome the ioniza­ tion potential barrier will suffice. In this case the uranium atoms excited by the xenon laser have only to absorb another photon with a wavelength less than 4,400 angstroms in order to be ionized. The photons are supplied by a second laser. employing krypton ions rather KRYPTON-ION LASER CAVITY than xenon. with two strong emission lines at 3.507 and 3.5 64 angstroms. The 238 were not affected. Each excited atom second laser is not isotopically selective; could then be ionized by a photon from the it would ionize excited states of either krypton-ion laser; atoms that had not been uranium 235 or uranium 23 8. excited could not be ionized. Ions were swept The excited state of uranium 235 has out of beam by an electricaUy charged plate. a brief lifetime (235 nanoseconds). and t EXCITED STATE so it is important that the second photon >­I I------:�---i be supplied immediately after the atom a:C) 'I' ratory. The commercial application of is excited. That is ensured by arranging zw the technique is now being considered. the two lasers on a common optical axis. w In the experiments in Cologne and at with the beam of uranium atoms inside Livermore only microscopic quantities the cavity of the krypton laser. The ion­ of material were separated. but there is ized atoms. which are stripped of one reason to believe the devices and tech­ electron and hence have a positive niques can be scaled up. In 1975 Sam A. charge. are attracted to and deposited Tuccio. Richard J. Foley. James W. on a collector plate with a negative Dubrin and Oscar H. Krikorian at Liv­ charge of 1.500 volts. ermore reported the enrichment of mil­ The system was run continuously for ligram quantities of uranium. and it sub­ about two hours. with the xenon-ion la­ sequently became apparent that similar ser operating at a power of 70 milliwatts work had been started earlier by Rich­ and the krypton-ion laser at 30 watts. XENON-ION ard H. Levy and G. Sargent Janes at the The much higher power of the second LASER PHOTON (3,761 ANGSTROMS) Avco Everett Research Laboratory. laser was required to compensate for the Avco Everett and the Exxon Corpora­ small probability that a given excited tion are now building a pilot plant for atom would absorb a photon and be ion­ the laser enrichment of uranium at ized. The yield is somewhat reduced by Richland. Wash. Because of security considerations and proprietary secrecy many of the details of this process (as well as those of other uranium-isotope­ IONIZATION of uranium atoms in the Liv­ separation methods) remain obscure. ermore experiment was accomplished in two but a ma jor portion of the work at Liver­ steps, only one of which was isotopically se­ more has been made public. lective. The atoms susceptible to laser excita­ In the Livermore experiments a beam tion were those in a metastable state Iyiug only of uranium atoms emerges from an slightly above the lowest possible energy level, oven at a temperature of 2. 600 degrees the ground state. Atoms of uranium 235 in the Kelvin. At this temperature 45 percent metastable state were stimulated by the xeuon of the atoms are in the lowest possible laser to make a transition to an excited state, energy state and 27 percent are in a but urauium-238 atoms were not excited. Any atom in the excited state could then be ionized metastable state of only slightly higher by absorbing a photon with a wavelength of energy. (This metastable state is not an less than 4,400 angstroms. The kryptou-ion METASTABLE STATE independent. excitational energy level. laser, emitting photons with wavelengths of and it is not reached by radiative transi­ 3,507 and 3,564 angstroms, was capable of ion­ tions; it is formed by the splitting of the izing excited atoms of eitber uranium isotope. GROUND STATE 93 © 1977 SCIENTIFIC AMERICAN, INC the fact that some uranium atoms are cumstances the atoms might be stimu­ For the purposes of isotope separa­ thermally ionized in the oven. a process lated to emit radiation as they drop to a tion the reaction that has probably been that is not isotopically selective. Never­ lower energy level. In other words. the studied most extensively is the laser de­ theless. four milligrams of uranium uranium vapor could itself become the composition of gaseous formaldehyde. were obtained with a uranium-235 con­ medium of a laser. Nevertheless. these an organic molecule with the structure tent of about 3 percent. That is four experiments represent the first steps times the abundance of uranium 235 in toward the commercial development of natural uranium and is approximately a new method of uranium isotope sepa­ the level of enrichment required for nu­ ration. It remains to be seen whether or clear power reactors. not this particular method will turn out Several variations on the method of to be the most economical. selective ionization have been proposed. For example. instead of a two-stage class of isotope separation tech­ When formaldehyde is decomposed process several or many photons could A niques that has the attraction of by laser irradiation. it can yield two dis­ be employed. Another appealing idea is great simplicity is the decomposition of tinct sets of products. molecular hydro­ the creation of very highly excited molecules into stable products that can gen (H2) and carbon monoxide (CO). or states. lying just below the threshold of then simply be removed from the mix­ the free radicals Hand HCO. The free ionization. so that the final ionization ture. A prerequisite. however. is a mole­ radicals are unstable and react promptly step would require only a gentle electric cule that satisfies a rigorous list of speci­ with one another or with other sub­ field or collisions with charged particles. fications: it must be possible to excite stances present. The molecular prod­ On the other hand. certain parasitic the molecule with laser photons. the ucts. on the other hand. are stable and processes must be suppressed. Colli­ molecule must then dissociate spontane­ could easily be separated by chemical sions between positively charged urani­ ously and the products of the dissocia­ means. with the result that any isotopic um-235 ions and neutral uranium-238 tion must be stable. so that there are no composition they might be given by se­ atoms can transfer the charge to the further chemical reactions. The last re­ lective excitation would be preserved. heavier isotope. thereby scrambling the quirement is a particularly difficult one The photochemistry of formaldehyde isotopic content of the product. A.more to satisfy; many molecules that can be decomposition is quite complex. but the dramatic failure would be the creation decomposed by electromagnetic radia­ relative yield of molecules and free radi­ of a population inversion among the ex­ tion give rise to free radicals. fragmen­ cals apparently depends on the wave­ cited uranium atoms; under some cir- tary molecules that are highly reactive. length of the light absorbed. so that the

ONE GIGAHERTZ I I

��------� EMISSION OF UNTUNED XENON· ION LASER ")., ...... ,,, ...... ,/ '... ,, URANIUM·235 " ' ...... " ABSORPTION " " ,,'" t " ,,'" " " " " " " " ," " ," "

EMISSION OF XENON·ION LASER TUNED WITH AN ETALON

FREQUENCY�

ABSORPTION SPECTRUM of uranium isotopes can be closely that it would not be suitable for discriminating between them. The matched by the emission spectrum of the xenon-ion laser. The ab­ emission of the laser is narrowed by inserting an etalon into the laser sorption spectrum includes two adjacent peaks, the one at higher fre­ cavity; the etalon confines the laser to only a few modes of reso­ quency (or shorter wavelength) representing the absorption of ura­ nance, giving rise to a spectrum of distinct spikes. The sharpened nium 235. The naturalemission spectrum of the laser is comparative­ emission can be tuned so that it coincides with the maximum uranium- ly broad and encompasses the absorption peaks of both isotopes, so 235 absorption and lies outside the range of uranium-238 absorption.

94 © 1977 SCIENTIFIC AMERICAN, INC process may be amenable to experimen­ _____ REPULSIVE POTENTIAL CURVE tal manipulation. Three groups of inves­ tigators have already reported the suc­ cessful application of this procedure to the separation of hydrogen isotopes. EXCITED·STATE POTENTIAL CURVE Hydrogen has two stable isotopes. the I common one with a mass number of DISSOCIATION LIMIT (l H or merely H). which has a solitary proton as its nucleus. and the much rar­ er deuterium (2H or D). with a mass number of 2 and a nucleus that consists of one proton and one neutron. Formal­ dehyde can be prepared with pure hy­ drogen (H2CO) or with pure deuterium (D2CO). Employing a mixture of equal quantities of these gases. Edward S. >­ Yeung and C. Bradley Moore of the (!)a: University of California at Berkeley zw achieved a deuterium-enrichment ratio w ___LASER·INDUCED TRANSITION of six to one in 1972. and with the same DISSOCIATION LIMIT starting materials other workers later reached a ratio of nine to one. In 1975 John B. Marling of Livermore succeed­ ed in selectively exciting HDCO. the form of formaldehyde in which both hy­ GROUND·STATE POTENTIAL CURVE drogen isotopes are present; the deuteri­ um content of the products. in the form oi HD. was 14 times that of the starting materials. Dissociation of the formaldehyde �====:;;;6- ISOTOPICALL Y DISTINCT VIBRATIONAL ENERGY LEVELS molecules does not come about through excitation all the way to the dissociation continuum. If photons with sufficient INTERATOMIC DISTANCE------7 energy to achieve that result were sup­ plied. they would be absorbed by all PREDISSOCIATION OF MOLECULES into chemically stable fragments offers another ap­ formaldehyde molecules. without iso­ proach to laser isotope separation. The energy of the chemical bond in a molecule is a function topic selectivity. Instead the molecule is of the distance between the atoms, and the curve describing this relation is called a potential curve; in a stable molecule the curve has an energy minimum where the forces of attraction disassembled at a lower energy through and repulsion are in balance. Actually a molecule can have a number of potential curves, each a phenomenon called predissociation. It curve associated with an electronic energy state, and some of them may not have a stable mini­ is raised to a precise energy level where mnm; on such a curve the force between the atoms is always repulsive. The absorption of a the force between the constituent atoms laser photon can induce a transition from one potential curve to another. Ifthis excited-state can change from attractive to repulsive; curve is intersected by a repulsive potential, the molecule may then dissociate. The process is even though the energy supplied is less called predissociation because it takes place at an energy helow the usual threshold for dissoci­ than that ordinarily required to break ation on that potential curve. It can be made isotopically selective by tuning the laser so that the chemical bond of the molecule in only one isotopic species is promoted to the excited-state potential(horizontal curve. Withinlines), a bound po­ that electronic energy state. the mole­ tential there are a finitenumber of molecular vibrational states and the posi­ cule comes apart. tions of these energy levels are slightly different for each isotopically distinguished molecule. The separation of deuterium from hy­ drogen is facilitated by the comparative­ ly large differences between the spectra of these isotopes. which can be attribut­ ling has shown that wavelengths greater formaldehyde. symmetric tetrazine. an ed to the large ratio of the atomic mas­ than 3.300 angstroms suppress the cre­ unstable organic molecule with the ses. The isotopes of carbon and oxygen ation of free radicals; 80 percent or structure display much smaller spectral shifts. but more of the yield is in the form of mo­ recently they too have been separated lecular products. He has been able to by the photodecomposition of formal­ enrich oxygen 17 and oxygen 18. both of dehyde. John H. Clark. Yehuda B. which are rare. as well as carbon 13 and Haas. Paul L. Houston and Moore of deuterium. Of particular importance. he the University of California at Berkeley. was able to achieve enrichments by a working with the carbon isotopes of at­ factor of three or four with formalde­ omic mass 12 and 13. have increased the hyde made up of elements in their natu­ abundance of carbon 12 incarbon mo­ ral isotopic abundances. When rare iso­ noxide to 81 times its normal value. topes are being separated. most of the thereby enriching the un decomposed effort required in conventional methods formaldehyde in carbon 13. Their ex­ is often exerted in reaching the enrich­ periment employed laser light with a ment level of a few percent. Marling's wavelength of 3.032 angstroms. where results illustrate one of the potential When tetrazine is irradiated, it de­ the creation of free radicals predomi­ benefits of laser technology: large sin­ composes into two molecules of molec­ nates. Nitric oxide. which combines gle-step separation factors for the very ular nitrogen (N 2) and two molecules of with free radicals, was added to the re­ rare isotopes. hydrogen cyanide (HCN). Robert R. action mixture in order to achieve these Studies of separation by photodecom­ Karl and K. Keith Innes of the State enrichment factors. More recently Mar- position have centered on. in addition to University of New York at Binghamton

95 © 1977 SCIENTIFIC AMERICAN, INC have shown that each photon absorbed zine . but their method diffe red in an im ­ sitio n . the selected molecules comp le te­ by tetraz ine leads to the decompos it io n portant respect from that of most other ly decomposed . Irrad ia tion for a few of one molecule . The process has been exper im ents . Instead of exc it in g mole­ minutes enr ic hed the crystal in ni trog en employed to separate the is otopes of hy­ cules in the gas phase they ir rad ia ted a 15and carbon 13 by a factor of 1.000 or drogen . carbon and nitrogen . crystal of symmetr ic tetraz in e cooled to more . Rob in M. Hochstrasser and David S. 1.6 degrees K. (1.6 degrees Cels iu s King of the Un iv ersity of Pe nnsylvan ia above absolute zero) . When the laser ot all molecules can be made to pre­ have also demonstrated is otop ic enr ic h­ was tuned to spectral features of mole­ N dissoc iate in to stable fragments : it ment through the photolys is of tetra- cules with a part ic ular isotop ic compo - is often necessary to work with sub­ stances whose propert ie s are not so co n­ ven ie nt for the chem ist . In many ca ses DISSOCIATION LIMIT DISSOCIATION LIMIT dissoc iati on requ ir es a two-p hoto n process . in wh ich a molecule is first ex ­ cited. then in duced to make a second trans it io n either to the dissoc ia tion con­ tinuum or to another exc it ed or metasta ­ ble state that decomposes . Such a tw o­ step process fo r the separat io n of ura ni­ um is otopes is be ing in vest ig ated at th e J Lo s Al amos Sc ie nt ifi c La boratory . An in frared photon select ively exc it es mol­ ecules of uran iu m hexa flu or ide gas th at , conta in an atom of uran iu m 235 but not .jjj those with the heav ie r is otope . An ult ra­ violet photon then decomposes the ex­ � � cited 235UF6but not the unexc it ed feed ­ stock . The products of the decompos i­ 2 tion are 35U F 5 and atom ic fl uor in e; th e 2 35UF 5 subsequently prec ip it ates from the gas . To aid in the select ive exc it at ion of 235U F6. the absorpt io n spectrum of the molecule is simpl ifi ed by expand in g VISIBLE OR ULTRAVIOLET the gas through a nozzle . caus in g it to LASER PHOTON cool but not to condense . In stil l another techn iq ue dissoc ia tion is in duced by the absorpt io n of many low -e nergy in frared photons . rather than just one or two of rather high ener­ gy . Indeed . the discovery of th is process is one of the most excit in g to emerge from research in to laser separat io n . An example of it s appl icat io n is the separa ­ tion of the is otopes of sulfur . sulfur 32 and sulfur 34. by the decompos itio n of sulfur hexa flu or id e (SF6)' In an exper im ent conducted by R. V. Am bartzum ian and V. S. Letokhov of the Inst it ute of Spectroscopy in Moscow sulfur hexa flu or id e was ir rad ia ted with a pulsed carbon diox ide laser supply ing a power of between one and two bill ion watts per square cent im eter . The laser was tuned to an in frared wavelength co­ in ciding with a vibrat io nal exc it at io n of ISOTOPICALLY 32SF6. Virtually all the 32SF 6 was de ­ DISTINCT VIBRATIONA�\ composed . and the res id ual gas was en­ ENERGY LEVELS riched in 34SF6almost 3.000-fold. Whe n AlB AlB the laser was ad justed to another wave ­ \ length . wh ic h exc it ed 34S F6. that mole ­ A2B cule was decomposed . Essentia lly the INFRARED LASER PHOTON INFRARED LASER PHOTON same results were obta in ed indepen ­ dently by John L. Ly man. Reed J. Jen­ sen . John P. Ri nk . C. Paul Ro binson and GROUND STATE GROUND STATE Stephen D. Rockwood at Los Al amos . Subsequent molec ular -b eam stud ie s MULTIPLE PHOTON ABSORPTION is another(left ) method for separating isotopes by the dis­ have confirmed an early conclus ion that sociation of molecules.arti In a two-stage process one photon selectively excites molecules the sulfur hexa flu or id e molecule de ­ containing a p cular isotope; a second photon then dissociates the excited molecules. The composes without coll isions after it ha s first photon is usually an infrared one, causing a transition between vibrational states, whereas cl imbed a "l adder " of success iv e vibra ­ the second photon is in the visible or ultraviolet region of the spectrum. In an alternative pro­ cedure (right) a molecule absorbs many infrared photons in rapid succession, increasing its vi­ tiona lly exc it ed states up to the dissoc ia­ brational energy until a chemical bond breaks.This method has been applied successfully only tion cont in uum . The process is not yet with comparatively large molecules made up of several atoms; such molecules have many close­ well understood . but it has already bee n ly spaced vibrational states. Only the first few infrared transitions are isotopically selective. employed in the enr ic hment of is oto pe s 96 © 1977 SCIENTIFIC AMERICAN, INC LASER CAVITY >1

I IODINE MONOCHLORIDE I AND BROMOBENZENE L� ______�\ /� ______��� I I LASER MEDIUM INTRACAVITY FILTER OF IJS CI REACTION VESSEL BEAM STOP MIRROR MIRROR

J7 Br CI

IODINE MONOCHLORIDE BROMOBENZENE I CHLOROBENZENE 1 #C� c� �c PHOTON + +Br+ ABSORBED 1 Cr�� /IIC c� /cII C �c/

Br Br

IODINE MONOCHLORIDE BROMOBENZENE 1 BROMOBENZENE 1 c� �c c��c JS I CI + IJSCI + cI � / IIc c�I / IIc c C

CHEMICAL SCAVENGER was employed in the separation of iso­ an atom of chlorine 37. (The compound in its excited state is written topes of chlorine at Colnmbia University. Natural chlorine, which 137CI*.) Bromobenzene has the property that it reacts with excited has stable isotopes with atomic weights of 3S and 37, was combined states of iodine monochloride but not with the ground state. As a re­ with iodine to form the compound iodine monochloride (lCI). The sult chlorine 37 was extracted from the 137CI* molecules (ultimately compound was then mixed with bromobenzene, the scavenger mole­ forming chlorobenzene) but not from 13sCI. The laser was tuned with cule, and the mixture of gases was irradiated. The laser was tuned to an intracavity filter of 13sCI gas, which quenched the laser's out­ excite only those molecules of iodine monochloride that contained put at precisely those wavelengths that would excite that molecule.

of hydrogen. boron. carbon. si li con. (CsHsBr) was irradiated with a dye la ser The Colu mbia work has also shown ch lo rine, titanium, mo lybdenum. tung­ operating at a wave length of 6,050 ang­ that an understanding of the scavenger sten and osmium as well as su lf ur. So far stro ms (in the orange-r ed). At this wave­ mechanism may make possib le the di ­ it has been observed on ly in mo le cu les le ngth the la ser excited mo lecu le s o f io­ rect preparation of isotopica lly la be led made up of more than three atoms. sug­ dine monoch lo ride containing ch lorine compounds. Since many isotopes are gesting that the high density of excited 37 but not those containing the other emp lo yed not as pure elements but as states characteristic of these mo le cu les stab le isotope. ch lo rine 35. In subse­ compounds , the technique could in ­ may be needed for the chain of photon quent co llisions of the excited I37C I* crease the uti li ty of la ser isotope separa­ absorptions to continue unbroken all the mo lecu les with bromobenzene the bro­ tion. Indeed. research on isotopica lly se­ way to dissociation. mobenzene abstracted the chlo rine-37 le ctive photochemistry not on ly promis­ atom to form an unsta ble radica l inter­ es to yiel d practica l methods for isotope As a ru le excited states of atoms are mediate , 37C ICsHsBr. that rapi dly ex­ separation and isotope la be li ng but also .f\.chemica lly more reactive than the pe lled bromine to yie ld the sta ble prod­ may provide a means for fo llowing and ground state, and this distinguishing uct ch lo robenzene (CsHs37CI). Af ter understanding the chemistry of excited­ trait suggests sti ll another method for irradiation for about two hours several state reactions. isotope separation. The method consists mi lligrams of CsHsCI was produced; it in mixing with the feedstock a scavenger was en riched sixfo ld in ch lo rine 37. idespread interest in the la ser sep ­ mo le cu le , one that does not react with A specia l feature of the Co lu mbia Waration of isotopes derives la rge ly the atom or mo lecu le to be separated work is the use of an isotope to tune the from its possib le appli cation to the nu­ when that atom or mo le cu le is in the la ser for the se le ctive excitation of an­ clear power industry. At present the ground state but combines irreversi bly other isotope. A samp le of }JsCI was dominant techno lo gy for the enrich­ with it when it is in an excited state . placed in the cavity of the dy e laser. ment of uranium is gaseous diffu sion. A Finding such an idea l scavenger is not a where by its absorption spectrum it brief review of this process suggests in­ trivia l undertaking, but when one is quenched the output of the la ser at pre­ centives for deve lo ping alternatives. avai la ble, it off ers a significant benefit. cise ly those frequencies that would ex­ In a gaseous -di ffusion plant vapors No additional photons. and thus no au­ cite }J5C l. · By the use of this intracavity of uranium hexa flu oride (UFs) are ditiona l energy, are req uired to ionize or filter I37C I mo le cu les were se le ctive ly pum ped through a porous barrier, the dissociate the excited states. excited without taking extraordinary li ghter 2 35U F s di ffu sing more rapid ly Chemica l scavenging has been em­ measures for narrowing the emission than the heavier 238U F s, so that the ployed by Doug las M. Brenner and Sa ­ li ne of the la ser and lo cking it onto a transmitted vapor becomes slight ly en­ swati Datta at Co lu mbia University in particu lar I37C I transition. The method riched in uranium 235. The enrichment separating the isotopes of ch lo rine. In offers the se le ctive excitation of an iso­ in passing through a sing le stage is their experiment a mixture of iodine topic species without detai led know l­ sma ll, and so hundreds of stages must be monoch lo ride (I CI) and bromobenzene edge of its spectrum. li nked together in a cascade, where the 97

© 1977 SCIENTIFIC AMERICAN, INC fee d to each stage co nsists of the en­ tween 30 an d 40 pe rcent of all the uran i­ riche d stream from th e stage below um 235 that has ever been mine d! Al ­ comb in e d with the dep leted stream th ough a mo re soph is tic ate d econom ic from the stage abov e. In ord er to sepa­ analy sis might be less opt im isti c, it rate a quant it y of UF6 co nta ining one wo uld seem that a sav in gs of several kilogram of uranium enric hed to 3 per­ hu ndred bill io n dollars coul d be real ­ cent uran iu m 235, with waste UF6 at .2 ized by th e end of the century if the laser Queen Anne percent uran ium 235 disca rded, almo st te ch no logy wo rks . Lowboy 1,500 kilograms of natural ura nium The se calculat io ns assume the cont in­ r 17g ���IEC���=nd must enter the casca de at the firs t st age ued use of light -water reactors like those A HenrMuseumy Ford an d more than two mill ion kilog ra ms of now op erating in the U.S. An other type Reproduction uran iu m must be pumpe d and rec ircu ­ of reactor el im in ates the nee d for uran i­ late d through the casca de . An energy of um is otope separatio n by "b urn in g " a Ready to about five mill io n electron volts is ex­ fu el of na tural uran iu m . These heavy­ pen ded for each atom of uran iu m 235 wa ter (D20) reactors , however , requir e Assemble and Finish that is separate d. large qua ntities of de uter iu m : about one 18'h Century Furniture The present cost of obta in ing urani­ ton of D20 per megawatt of electr ic al One of 24 Bartley classics, in hand crafted um 235 of 90 percent pur ity by gaseous capac ity. The widesprea d adopt ion of solid mahogany, oak or cherry. To tally diffus io n is about 2.3 cents per mill i­ heavy-wat er reactors would therefore authentic in design and beautifully gram . Th is compares favorably with the mo tivat e another kind of laser is otope constructed. Each kit is easily assembled cost of from $1 to $10 per mill ig ram of separat io n program . and finished in your own home without tools. All pieces also offered completely most other is otopes at the 90 percent Us es of is otopes in fields other than assembled and hand finished. A $5.00 enr ic hment level , but a more fam il ia r nu clear power generat io n are diverse , coupon included with catalogue. stan dar d of econom ic value shows im ­ bu t much smaller amounts are con­ r;::------, me diately that all pur ifi e d is otopes are sume d. Is otopes are im portant as tracers " qu ite prec io us . The present pr ic e of gol d in resea rch , me dicine and agr ic ulture, I �i.l::;':1furniture reprod,�,: uctions 1::;";: available:::f�:� I is about .3 cent per mill igram . but because th ey are so costly there is I � in kit form or hand finished. I am I . enclosing Sl.00 to cover postage The U.S. has three gaseous- diffusio n litt le incenti ve fo r finding ne w applic a ­ I I. ...-.�- I and handling. I I- plants capable of separat in g 3,960 met­ tions. Th is may change dramat ic ally if I Name I ric tons of 3 perce nt uran ium 235. The lase r is otope separatio n techn iq ues can I Address I cap it al cost of a new plant with an annu­ reduce the present cost by a factor of I I ity/State/Zip al capac it y of about 2,050 metr ic to ns of 10 0 or 1,000 . For example , in chemis try I C I 3 percent uran iu m 235 is estimat ed to be an abun da nt supp ly of lo w-cost is otopes I TheBartleyGol 1ection,ltd . I about $3 bill io n with an annu al operat­ migh t lea d to the widesp rea d adop tion 747 Oakwood Ave., Dept. SA3 Lake Forest,lI 60045 in g cost of about $500 mill ion. Esti­ of is otop ic ally labele d compoun ds fo r mates of the deman d fo r enr iched uran i­ st ructu ral analy sis,fo r analyt ic al de tec­ um are uncerta in , but do mestic con ­ tion and for kine tic studie s . sumpt io n coul d requ ir e a capac it y equ iv alent to between four and ei ght he separati on of is otopes is an im ­ new plan ts by the en d of the century , Tpo rtant process , wh ich th e intense , an d fore ign dema nd coul d do ub le ortr i­ mo noch ro ma tic light of the laser may ple that number. The adopt ion ofot her soo n make cheape r and there fore com ­ phys ic al separat io n techn iq ues , such as mo ner. It is po ssible that the applic at ion gaseous centr ifugat io n , might re duce of the las er to chemi stry may ult im ately the cost of uran iu m- is otope separa tion have ev en greater significa nce , a poss i­ by from 10 to 30 pe rc ent . If large-scale bility mad e pla in whe n the lase r it self is laser separat io n proves feasib le , it off ers viewed in th e light of hi story. the potent ial of much greater savin gs . Th e origins ofchem is try are custom­ Th e energy requ ir ed to ge ne rate visi­ ar ily traced to th e alchem ist s, who se ble phot ons or in frare d photo ns for laser wo rk was fu nded by royal patr ons eag er sepa rat ionmi ght be from one to 10 elec­ to tra ns fo rm lead into gold . The alche ­ tron volts per uran iu m-235 atom sepa ­ mists of cou rse fa il ed to ach ie ve th ilt rated . Even if the effic ie nc y ofthe laser tra ns fo rmati on, but in th eir efforts th ey in convert ing electr ic al ener gy into ligh t ass em bl ed a body of knowledge th a t is no gr eater than .00 2, the energy cost prov id ed a fa ctual bas is fo r much of ofthe process is bet wee n 500 and 5,000 mod ern chem is try. Today the ro yal pa ­ elec tro n volts per uraniu m- 235 atom. tro ns of th e sc iences are gov ernme nt year of service to the World's finest That is at least 1,000 times less tha n th e agencies, wh ich urge th e developme nt craftsmen and technicians. energ y cost of ga seous diffusi on. Be­ of new me th ods fo r sepa rating is otopes , Naf;ona'�2000w"I'II";." A.·a".o" De op t.JAA cause the las er is al so expect ed to mak e again with th e prosp ect of ec onom ic re ­ Camera �!���:-;�:�; 7�-:�;::· poss ible a high separa tionfac tor in ea ch wards. Th is time th e goa l will proba bly stage, the po tential ex ists fo r removin g be ach iev ed . As before, howe ver , th e ul­ p------� Send a FREE copy of the He Flasher almost all th e uran iu m 235 in th e or e. tima te reward may prov e to be of far I Ga seous diffus io n can eco nom ical ly ex ­ greater value th an th e immed iate object I name tract only 60 to 70 perc ent ofth e urani­ being sough t. By understa nding th e I �------um 235 in th e feedstock , leav ing urani­ mech anism of la ser isoto pe sepa rat ion address um co ntaining .2 to .3 pe rcent uranium we may be lay ing th e fo und atio n for a I city 235 as "tai ls ," or waste. Higher ex trac­ new ch em is try , one in which com­ I tioneffic iency wo uld effectively increase pou nds are made not by supplyin g th e I state zip us eful uranium reserves. Indeed, a la ser random energy of heat bu t by exc iting Union .u_ Dept.JAA separ ationpla nt migh t be ableto reproc­ only th ose internal moti ons of mol e­ Na fiona' ��p:��e,,��::;:,��,�::. 2000 W"I' '.O"O ess the stored wastes from gaseous dif· cu les th at facilitate th e transfo rmat ion Camera � fu sionpl ants; th ese wastes represent be- ofre ac tants into prod uc ts. 98 © 1977 SCIENTIFIC AMERICAN, INC © 1977 SCIENTIFIC AMERICAN, INC