The Bomb^ the , and the Space-Charge Problem A participating scientist relates the story of a World War II realizing he had to deal with only two fractions, "^Li and '^Li, Smith suggested project dedicated to electromagnetically separating trying a geometry, illustrated in figure 1, uranium-235 from uranium-238. similar to that of the electron mag- netron. An arc source would be along the centerline parallel to the William E. Parkins , and would be ac- celerated radially outward. They ranium! Uranium! Uranium!" A voice sbouted out into would describe circular paths, and the fields would be ad- U tbe night from the second floor of a dormitory in Oak justed so that the heavy fraction would collect near the Ridge, . It was 6 . That day, Presi- 180° focus on a peripheral cylinder while the light fraction dent Harry S Truman bad announced to the world that the returned to be collected near the center. Smith reasoned US had dropped a new weapon, a uranium bomb, on the tbat the symmetry would eliminate space-charge fields in city of Hiroshima, Japan. For years, tbose of us on the the 0 direction (in the usual cylindrical coordinates). bomb project were cautioned not to say the word uranium, Space-charge fields would have r and z components, and but now it was okay. There were code words and code let- Smith had calculated how much ion current one would ex- ters for tbe things we worked with, and eacb of our new pect before the radial fields ruined the resolution in designs received a new name. The teletype messages tbat separation. went back and forth between tbe radiation laboratory in The experimental apparatus was readied in the sum- Berkeley, , and the Y-12 plant in Oak Ridge were mer of 1941. Almost immediately, we observed clean reso- total gihherish. The purpose of our effort was to separate lution with higher currents than should have been possi- "P," or ^^''U from "Q," or ^^^U. Tbose were easy to remem- ble. Shortly thereafter, we realized that we had stumbled ber hecause P stood for precious and Q stood for qrap. onto a process wherein the ion beam automatically neu- Afew days later, another word burst on the scene with tralizes itself by ionizing residual gas in the vacuum cham- the issuance of the , the official government ber. The positive ion beam presents a potential well to elec- account of the history of the bomh project.' That word was trons. They are trapped while the ions they leave are "calutron." Now that the device had achieved its objective, immediately swept out along magnetic-field lines in the z wished to give recognition to the Uni- direction. And tbe process is fast. Even when the acceler- versity of California by using the name calutron for the ap- ating voltage is swept at 60 Hz, the neutralization follows paratus developed to separate P from Q. He had made an the beam location perfectly. arrangement with report author Henry Smyth that the Furthermore, the process is self-limiting. Electrons name he included, but never divulged the deal until the accumulate while oscillating up and down along magnetic- war was won. field lines only until tbe potential well is filled. They move The calutron's separation method was based on elec- laterally with small cycloidal paths because of any resid- tromagnetic . (See box 1 on page 46 for a ual space-charge fields perpendicular to the magnetic tutorial.) All critical material was transported in beams of field. Collisions of the electrons with gas molecules cause positive ions on wbich electric and magnetic fields could act. the electrons to start new cycloids—fortunately always The needed quantity of material demanded very intense closer to the center of the beam. {See box 2 on page 50 for beams witb a bigh density of electric charge. But the posi- more on heam neutralization.) tive beam itself should create so-called space-charge fields We at Cornell didn't know tbat at this very time, the whose repulsive forces would alter ion trajectories and pre- Uranium Committee, an arm of the Office of Scientific Re- vent the desired isotopic separation by mass from occurring. search and Development (OSRD) under , At least in 1940, any thinking knew that. was negotiating with Smyth and Lawrence to start proj- ects at and tbe University of Cali- fornia. These would investigate whether space-charge ef- The calutron story starts around 1940 at Cornell Univer- fects might be overcome sufficiently to permit use of an sity. Fellow student A. Theodore Forrester and I were fin- electromagnetic method for quantity separation of ^^^U. ishing our graduate work under the direction of Lloyd P. Lawrence, who was in the process of building a giant 184- Smith, who had obtained a contract to separate a quantity inch magnet at UC Berkeley, volunteered the use of it and of lithium-6 for use in an experimental study of a new can- the existing 37-inch magnet. He would try the cer therapy. Knowing about the space-charge problem and classic Dempster mass-spectrometer arrangement'^ and see how well he could do. Smyth proposed a time-of-flight method he called the isotron, invented by Robert R. Wil- Bill Parkins retired from Rockwell International, where he was son. It used no magnetic fields, but employed a broad two- director of research and for the energy systems dimensional ion source to increase currents. Both of those group. He now lives in Woodlarid Hills, California. projects were started in late 1941.

© 2005 American Inslitule ot Physics. S-0031-9228-0505-020-9 May 2005 Physics Today 45 Word reached Lawrence of our work Figure 1. The radial magnetic separator at at Cornell. He contacted Smith and in- Cornell University was used to separate vited the three of us to join his proj- lithium isotopes. This cross section ect at Berkeley. Pearl Harhor had shows the relationship of its ion tra- recently been attacked, and the jectories with those of the con- country was at war. We felt a ventional Dempster mass spec- duty to go, although Smith trometer (blue area). Also would have to return within shown are a circular arc (red a few months. In mid- area) struck trom the cathode February we boarded a (A), short alternating sec- train in Ithaca and left tions of metal tubing and the ice and snow for screen at ground potential sunny California. From a (B), and matching sections railroad station on the of metal tubing and open way, we mailed a manu- mesh grid (C) at the accel- script to Physical Review erating potential. The loca- with the request that, be- tion of the collector pocket cause it should now be re- for the lighter isotope is indi- garded as secret, its publi- cated (D), as is the focus cation he postponed until the where the heavier isotope col- war was over. We wished to lected (E). For ease of viewing, the elements A, B, and C have get credit for having discovered been somewhat enlarged. and explained the automatic self- neutralization of intense ion beams where there are no applied electric fields. Our manuscript was received on 18 February 1942 and published on 1 Decem- until the additional width is equal to the ber 1947 after declassification.^ separation distance of the two isotopes being sep- arated, then tbe useful beam current limit has been The Berkeley radiation laboratory reached. The current / of the desired isotope in milliamps per centimeter of height of beam in the magnetic-field di- When we arrived at Berkeley, experiments were already rection is^ under way in a small vacuum-chamber tank placed be- tween the poie pieces of the 37-inch magnet. Also, tbere was arc-ion-source development using a smaller magnet /=8.55x VH, from the cosmic-ray program. Getting sufficient ion cur- rent from the source was the greatest problem. By mid- where rj is tbe fractional abundance of the desired isotope, March the ion currents were up and, for the first time, ex- Aj and A^ are the atomic weights of the two isotopes being ceeded those possible without some space-charge separated. Vis tbe accelerating voltage, and H is the mag- neutralization. netic field in gauss. The current above which resolution in mass separa- For separating '^'^'^U from '^^''U, the maximum / of ^^"^U tion is lost may readily he calculated from tbe divergence is 4 X 10"^ niA/cm for a voltage of 35 kV and a magnetic of the ions at the heam's boundary, which is caused by the field of 3500 gauss. With that current, it would take more space-charge field there. In the Dempster spectrometer, tban 5 years to accumulate 1 kg of'^^•''U with 1000 separa- the beam is accelerated at the ion source from a narrow tor units having beams 60 cm in height and operating at slit that is long in the direction of tbe magnetic field. Tbe full capacity without interruption. That was considered beam takes tbe shape of a douhle-bladed wedge bent into unachievable, so our challenge was to see how much beam a semicircle, and comes close to focusing at tbe 180" point. current might be increased above the space-cbarge limit. If the space-charge field widens the focus even further, A significant change took place in the beginning of

Box 1. Electromagnetic Mass Spectrometry espite their varied geometries and field combinations, field Hare perpendicular and the ion moves in a straight line Dmass spectrometers incorporate two steps, each of orthogonal to both fields. Equating the electric and magnetic which filters ions of similar energy, momentum, or velocity, forces determines that v - E/H. tn each step, the trajectories are completely determined by From any two of the three equations, velocity can be elim- the physical parameters of the apparatus along with the inated as a variable. Thus, using two mass-spectrometer steps mass, charge, and velocity of the ion. For example, an en- allows a discrete solution for mass, which effectively results ergy filter can be a simple acceleration of the ion with in spatial separation of different isotopes. charge e and mass m through a potential difference V. By For some mass spectrometers, the electric field varies in equating the energy gained through that acceleration with time. Such time-of-flight spectrometers typically use a fixed- the ion's kinetic energy, one obtains mv'/2 ^ eV. where v is potential accelerating electric field as an energy filter, fol- the ion's velocity. lowed by a velocity t'llter that uses an RF electric field. No A magnetic field H perpendicular to the path of the ion magnetic field is necessary. provides a momentum filter. The ion subjected to such a field A Dempster-type mass spectrometer' uses the fixed- will describe a circular path of radius r. After equating cen- potential energy f^ilter, followed by a semicircular path in a tripetal and magnetic forces, one obtains the momentum ex- uniform magnetic field as a momentum filter. The calutron pression mv - Her. developed during World War II modified that basic arrange- A velocity filter can use crossed electric and magnetic ment to increase ion currents while still retaining adequate fields. In a simplest case, the electric field E and magnetic resolution in mass separation.

46 May 2005 Physics Today http://www.physicstoday.org Figure 2. The 184-inch magnet at the University of California, Berkeley. The gap between the 184-inch diameter pole pieces is 72 inches. The photo, courtesy of the Lawrence Berkeley National Laboratory Image Library, is from

June 1942: The giant 184-inch magnet on the hill hehind the campus became ready for oper- ation (see figure 2). Smith had returned to Cornell. More and more of Lawrence's former stu- dents were arriving to join the effort. The center of activities moved to the 184-inch magnet huilding, although work contin- ued at the old radiation lah huilding and at others on the campus. Rohert Oppenheimer's theoretical group, which was as- sisting us, operated from the physics huilding, Le Conte Hall. Big-time physics The large circular huilding that housed the giant magnet was ideal for our purposes. Around its wall on the inside were nu- merous shops and hatches of heavy electrical equipment. An upper level included offices face, it was always Ernest. Lawrence, seen in figure 3, was and conference rooms. In the center was the hig magnet a big man with strong hands, a healthy boyish complexion, with a hastily erected platform at the level of the lower a ready smile, and a hig shock of hair. But most impressive pole-piece surface. Two large vacuum-chamher tanks with were those penetrating hluish eyes. Nobody worked harder slightly over 2 feet of inside clearance in the magnetic-field or had more enthusiasm than Lawrence, and his approach direction were stacked in the 72-inch gap. They had access was perfect for the kind of development heing done. He be- faces on opposite sides for mounting ion sources and col- lieved in thinking a little and experimenting a lot. There were lectors. Control rooms for each tank were nearhy at plat- so many variables and we lefl none uncovered. I can't begin form level. to explain all of the interesting avenues our work explored. Crews for each tank worked around the clock in three But I must mention one that had critical consequences. shifts. Much of the work was done inside the magnet gap On the matter of neutralization of the , with the magnet turned on. We all knew not to wear a no means of introducing electrons into the beam worked watch or carry keys. The nails in our shoes were no proh- better than simply depending on the beam to ionize the lem, hut they made walking seem as though we were work- residual gas in the tank. But the pressure had to he ahout ing in a muddy field. We had nonmagnetic tools made from 2 X 10"^ torr. Half of that and the beam became unstable, a beryllium- alloy. Occasionally, a nail or other fer- especially with sparking. Unfortunately, we were trying to rous object would get loose and come flying like a bullet to separate a lighter isotope from a much more abundant the lip of the pole piece. After a couple of accidents, we heavier one, ^-^"U. Scattering of the beam by residual gas learned that the liquid-nitrogen-containing Dewar flasks, caused the heavier ions to reduce energy and radius, and which were in large metal canisters on casters, needed to to enter the collector intended for the -'"^U, therehy reduc- he chained to the railing at the edge of the platform. A sofa ing the enrichment obtained. We knew this, because ex- was placed on top of the magnet where anyone detained periments decelerating the collected ions to near zero en- for an extended run could catch a nap. Also, it was the ergy improved the enrichment. But such a collector warmest location in the huilding on a foggy night. reduced the final currents too much. One immediate need was for experts in high-power- A compromise had to be struck, and reluctantly the circuit design. Lawrence contacted movie studios in Hol- decision was made to go to a two-stage process to achieve lywood. They were looking for work and immediately sent enrichments necessary for the bomh design. The stages a very competent team that stayed through our entire proj- would be called the alpha and beta. In the beta stage, there ect. Marcus Oliphant, who had come to the US from Eng- would be the new problem of chemical recovery of all ura- land, arranged for a group of superb to come nium used because beta-stage feed material would be so from that war-torn nation. Both Oliphant and Harrie valuable. Massey, who also came, were subsequently knighted by the The atomic homb project was now being taken very queen. The General Electric Research IJahs sent a good seriously. The Manhattan District of the US Army Corps group headed hy Kenneth Kingdon. Westinghouse sent a of Engineers, under the command of Major General Leslie team led hy William Shoup, who was joined later hy Ed- R. Groves, had been brought in to take charge. It had al- ward U. Condon. Lots was happening; everybody was co- ready set up the Los Alamos weapons lab in New Mexico. operating and one could feel the excitement. We were like The isotron project at Princeton was shut down for lack a swarm of bees in a huilding that even looked like a hive! of any positive results. Key personnel from there, includ- But there was no question as to who had the role of ing Wilson and , went to Los Alamos. queen bee. It was, as we all called him, E.O.L. But to his The project at Berkeley was transferred from OSRD to

http://www.physicstoday.org May 2005 Physics Today 47 the US Army on 1 May 1943. A short time later came the surprising tiews that a plant for its process would be built in eastern Ten- nessee, where there was access to power from the Tennessee Valley Authority. The calutron The electromagnetic isotope separator design developed at Berkeley was a variation of the Dempster mass spectrometer. Did it really deserve the name calutron, which advertised the university? I believe it did, hecause of four important new features that contributed to increased throughput and resolution. These are features other than the use of extremely high accelerating voltages. The first new feature was the use of mag- netic shims. As designed by Oppenheimer's theoretical group, the shims employed two shaped iron sheets approximately 3 feet wide and extending all the way across the tank, as Figure 3. Ernest Lawrence as he poses on the big viewed from the ion hind the University of California, Berkeley, campus. The source. They were bolted 184-inch magnet, housed In the building visible in the to the top and bottom sur- background, became operational in June 1942, about faces in the tank's central the lime this picture was taken. {Courtesy of the Emilio region. Their purpose Segre Visual Archives, PHYSICS TODAY Collection was to slightly increase the magnetic field. That preferentially, if only very slightly, decreased the radii of the tra- jectories of ions exiting the ion source with small diver- was at ground potential, so the region around the source gence, and brought those ions to a focus at the collector to- had exactly tbe conditions for the classic Phillips dis- gether with the ions of wider divergence. In effect, the charge—magnetic field extending between negative end altered magnetic field produced better resolution with plates and an electrode maintaining a positive region in wider angular divergence from the source, which improved between. both throughput and resolution. A disadvantage was that The region immediately surrounding the source pre- the focus at the collector was an odd-shaped nonplanar sented a positive-potential well to electrons, and trapped curve instead of a straight line. them much as the ion beam did. But in this case, the ap- Lawrence wanted to alleviate the problem with so- plied field was one tbe electrons could not neutralize. And called fish shims that would be anchored at midplane like now the electrons could have many thousands of volts of a flat fish. To Lawrence's great disappointment. Groves energy, depending on where they were created by ioniza- said "No," and that was that. Groves felt the urgency and tion. At the residual pressure around the source, the elec- wisely foresaw new problems. But he had the greatest ad- tron current oscillating up and down the magnetic-field miration for Lawrence, whom be considered a national lines could multiply exponentially into an avalanche that treasure. He even refused to allow Lawrence to fly, and re- would destroy any positive electrode surface that finally quired him only to travel by train. collected it. Holes were melted through quarter-inch-thick tungsten plate. The second calutron innovation was tbe use of multi- ple beams. Several arc ion sources were located a few The solution, while hard to descrihe, was truly ingen- inches apart on a line perpendicular to the initial direction ious. A grounded and fitted shield, with plenty of perfo- of tbe accelerated beam. Of course, multiple collecting rated holes to allow for vacuum pumping, was installed to pockets bad to be provided at the 180" focus. In traversing closely surround the big source block. The shield had ob- f"rom source to collector, the beams had to cross, and at first long bulges called blisters that were each several inches their mutual interference caused great trouble. Hans long and that enclosed fins—short lengths of copper-plate Bethe, who was visiting at the time, quipped, "Lawrence strips brazed to the source block. Two overlapping se- expected multiplication, but he got division." With further quences of fins and blisters just above and below tbe mid- experimentation, we eventually learned the conditions for plane perpendicular to the magnetic field reduced the dis- stable operation with multiple beams. Production designs tance in which the trapped electrons could oscillate from incorporated eitber two or four beams per tank. about 2 feet witbin tbe tank to about 2 inches within the blister. Furthermore, as the electrons traveled laterally Other features within the blister, they executed cycloids in tbe direction Up until tbat time, the ion source had been operated at of E X H. Tben, as tbey went around tbe end of a fin, tbey ground potential and the accelerating slit at high negative would see, along magnetic-field lines, tbe overlapping fin voltage. That required the collectors to be at higb negative in the otber plane and be collected. That arrangement pre- vented wasteful electron drain currents from building up, voltage and to bave a metallic tank liner at the high neg- and it worked! ative. The liner was a constant problem, and it reduced tbe usable beam height. The next major innovation was to The fourth major feature of the calutron was the so- eliminate the high-voltage liner, put the collectors at called accel-decel. The positive source now made it possi- ground potential, and operate the source at high positive ble to interpose a high negative accelerating slit between voltage. But that created a monstrous problem: The tank the ion source slit and the final slit at ground potential. The

48 May 2005 Physics Today http://www.physicstoday.org Figure 4. Tbe Y-12 plant in Oak Ridge, Tennessee, was a part of (be . The site included nine production buildings and was dedicated to electromagnetically separating uranium-235 from uranium-238. (Courtesy of Oak ' National Laboratory.)

Manhattan Project. Ours was the Y-12 plant (see figure 4), which eventu- ally included nine large production buildings. Each building contained one or two racetrack- sbaped assemblies sucb as tbe Alpba-I track sbown in figure 5. The racetracks alternated tanks, set on edge, with magnet excitation wind- ings. Copper was scarce, and the US Treasury De- partment loaned 15 000 tons of for the windings. As many as 96 ions would first be accelerated by, for example, 70 kV, and gaps for tanks were designed into a single racetrack. It was then decelerated by 35 kV to follow trajectories at 35 kV in staggering! tbe grounded tank. The very high initial voltage allowed us Big industry had been called in. The engineer- to extract higher ion currents. But because of tbe deceler- constructor was Stone and Webster of Boston; the race- ation step, one could retain the same radius of trajectory in tracks and their large generators were built by Allis tbe tank without increasing the magnetic field. Chalmers; Westingbouse made the internals, sources, and The large amount of electrical equipment at high volt- collectors; General Electric handled tbe electrical cubicles; age presented our greatest hazard. We took care to use cir- and tbe operating company was set up as a division of cuit interlocks on gates and cages. Those were essentially Eastman Kodak called Tennessee Eastman. switches that, wben opened, prevented tbe bigh voltage Tbe first track to go into operation, in early 1944, was from being turned on. Fortunately no one was electro- the Alpha-I. It had shims, two beams per tank, a grounded cuted, altbough in the frenzy of work, there were a few source, and no accel-decel. Its performance was not nearly close calls. I was one of those, and I owe my life since tbat as good as that of tbe Alpha-II, which followed. Alpba-II day to a scream by a quick-thinking . had sbims, four beams, a high-positive source, and Tbe ion sources, except for tbeir large scale, were more accel-decel. The Beta tracks tbat followed next were like conventional. Each was a full-length arc struck between a Alpha-II, except parameters were halved—two beams, replaceable, large-gauge tantalum wire filament and a 2-foot-radius paths instead of 4-foot, and a beam height of grapbite box enclosing the arc. Tbe heavy source block, 7.5 inches instead of 16. The magnetic field for tbe Beta supported on porcelain bushings, bad beaters that tracks bad to be doubled to between 6000 and 7000 G. And warmed a large reservoir of uranium tetracbloride. Care- tbe Beta tanks required a water-cooled stainless steel liner ful temperature regulation ensured optimum vapor pres- for the cbemical recovery of all uranium tbat did not reacb sure in tbe boxes. Willard Libhy, later of carbon-14 fame, tbe collector pockets. came from the project at Columbia Uni- Tbe design of all of tbis equipment was hased on re- versity to acquaint us with tbe tricks of using uranium sults from experimental apparatus at Berkeley. The rush bexafluoride. That compound had the advantage of being to get into production omitted any pilot-plant phase, and a gas at room temperature and would have been easier to many serious start-up problems resulted.^ Tbe magnet use. In tbe end, UF,. was rejected because of a mucb re- coils in tbe first Alpha-I track bad shorts to ground from duced lifetime for the arc filament and its colhmating slot. rust and sediments in the cooling oil. A four-montb delay In late 1943, the production design of what was called ensued wbile all of tbe silver coils from tbat track were Alpba-I bad to he frozen before development work on tbe sbipped back to the Allis Chalmers plant in Milwaukee, higb-positive source and accel-dece! bad been completed. Wisconsin, for cleaning and otber corrective measures. Tbose features would later be incorporated into tbe Alpba- Alpba-II suffered devastating failures of the large in- II design. Tbe plant in Oak Ridge was coming together. sulating busbings supporting tbe source block. Operations Tbe first team to go tbere bad tbe assignment of mapping were hampered for months until we obtained bushings tbe magnetic fieids. Others for plant start-up would soon made from improved porcelain. Even tbe cbemical recov- be following. ery of enricbed uranium was initially so poor tbat it tbreat- ened the viability of the overall project. These and other The Y-12 plant difficulties were overcome one by one, as Lawrence The US Army Corps of Engineers had created a town in steadily maintained his unflagging optimism and drive. tbe mud of the Tennessee hills west of Knoxville. In adja- One of tbe best beam diagnostic techniques was sim- cent valleys, army contractors were building plants for the ply to look. Through a window with protective shutter built http://www.physicstoday.org May 2005 Physics Today 49 Box 2. Space-Charge Neutralization ince the time of Charles Darwin, discovery of simple, tion. Given their transit time, the beam would be neutralized Seffective, and beneficial biological processes has not In a few milliseconds. There is then continuing production of been a surprise—they all have had the slow guiding hand of trapped electrons; those having a little more momentum in evoiution. But it is extremely rare to discover such a process ihe direclion of the magnetic field are constantly replaced by in the physical world. Instead, society undertakes to bend electrons of lower energy. With any interruption of beam cur- nature to serve its technological needs. Automatic self- rent, the electrons would be swept out in microseconds. neutralization of ion beams is a wonderful solution to the Discovered' in 1941, beam neutralization problem of space-charge repulsion, and it occurs totally • is established in all beam regions at a rate independent of without human intervention. the beam current in the region, With some form of Dempster mass spectrometer, all that • can follow fluctuations in beam location and intensity at is required is a low pressure of residual gas in the vacuum frequencies up to aboul 1 kHz, chamber to permit by the ion beam itself. Scatter- • appears to be 100% effective with steady beams, and ing of ions by that gas reduces the enrichment of separated • places no limit on total beam current. isotopes, but a broad window of operating conditions makes The process is a gift from Nature and, in contrast to those in possible essentially 100% neutralization and reasonably high the iiiological world, is one of a small number of beneficial enrichment factors. natural physical processes. It has made possible the quantity With the Alpha-ll calutron operation, about 10% of the separation of isotopes of elements throughout the periodic beam ions traversing from source to collector cause ioniza- table for use in , technoiogy, and medicine. into the tank wall, one could easily see the beam floating able. Its output, obtained independently of the Y-12 efTort, like a pale blue 3D ghost. Depending on conditions, ion- was introduced at a time when Y-12 was at its full capac- ized chlorine or Cl^, ionized combinations of U and Cl, and ity of 1152 tanks. A big push was mounted around the be- even doubly ionized U might be visible. And in the Beta ginning of 1945 that led, within a few months, to the ac- tank, one could actually see the beam of ^^''U, although it cumulation of the necessary , and to was approximately lC/f of the total U current. Truman's announcement of 6 August. The calutron was a temperamental piece of equip- Just three years earlier, our challenge had been to see ment. Each unit was operated from a control panel at the how much we could exceed the limit set by the space-charge front of a cubicle containing the electrical equipment. The operators were mostly young women scoured from the back effect on ion beam currents. Now we knew. A good run on country of Tennessee. Many were uncomfortable having to an Alpha-II unit would last 7 to 10 days and was usually wear shoes, and their drawl was often hard to understand. terminated when the graphite facing on the ^^^U collector But when it came to patience, no one was their superior. pocket had been sputtered away. During that time, the total And the calutron required patience. With no scientific un- charge measured to the ^•^^'U collector would be in the neigh- derstanding, those women became much better operators borhood of 15 A-h per beam. If averaged over a one-week pe- than our lab personnel from Berkeley. riod, that amounts to a beam current approximately 400 As many as 22 000 nonscientist employees worked at times the calculated space-charge limit! Y-12.'^ What did they think we were doing? The plant had no receiving dock and no loading dock. Nothing went in or out, but everybody was very busy. For a time, they were given a cock-and-bull story about broadcasting radio signals that jammed the communications of our enemies in Europe and the Pacific. But it was wartime, and people accepted the fact that our work, whatever its purpose, must be important. Late in 1944, a fortunate change boosted the throughput and final ^^''U enrichment from Y-12: Low-enrichment feed ma- terial from Oak Ridge's thermal diffusion plant became avail-

Figure 5. This Alpha-I racetrack was part of the Y-12 plant in Oak Ridge, Ten- nessee. In it, process tanks with calutron units alternate with magnetic windings made of silver. Electrical-equipment cubi- cles with control panels are in a separate bay. Vacuum pumps are on a lower level.

50 May 2005 Physics Today http://www. physicstoday.org The legacy The electromagnetic separation of ^^^U could not have taken place without our having overcome the space-charge Step Into the limitation. We at Cornell and, later, Lawrence at the Uni- versity of California independently suggested using a Dempster-type instrument—-the only basic electromag- netic method of that offers a solution. 3D World of AFM Its elements are an accelerating-potential-based energy filter in which space-charge fields are unimportant, fol- lowed hy a momentum filter consisting of a drift space in with the MFP-3D° a magnetic field, where no applied electric fields exist and where space charge can he neutralized by trapped elec- trons. No other combination of steady electric and mag- netic fields has that capability. During World War II, the Germans gave up any at- tempt to separate ^'•'"'U electromagnetically because they did not consider the space-charge problem soluble.^ The Japanese also considered the electromagnetic method, hut gave up on it for the same reason.''" We at Cornell were the first to observe and explain the ion beam neutralization process. Our article, withheld from pubhcation for more than five years, was titled "On the Sep- aration of Isotopes in Quantity hy Electromagnetic Means." That, of course, was the purpose of the calutron project that came later and produced the Hiroshima bomh material. But the bomb has not been the project's most important legacy. Other methods were being used to make bomh material. A bomb was dropped over Nagasaki, Japan, three days afler Hiroshima was hombed, and the gaseous diffu- sion plant was coming on line. It would produce enriched uranium at a much faster rate and lower cost. The most important legacy of the project has been the contribution to science, technology, and medicine made possible through the use of separated isotopes of nearly all the elements of the periodic table. Hundreds of kilograms bave been prepared for research and diagnostics in physics, . Earth , biology, and medicine. This service bas heen provided at cost for almost 60 years through the use of in the pilot units and Beta tracks at Y-12, all operated by Oak Ridge National Labo- ratory. Nationally and internationally, thousands of cus- tomers and millions of medical patients bave benefited.^ AFM 3D rendering (top), anaglyph (below) of The development and use of the calutron to produce quantum dot structure created on a GaAs enriched uranium for the first atomic bomb that was ex- wafer using oxidation lithography, 2.5 pm scan ploded in warfare, and then to produce tbe full spectrum of separated isotopes for uses in peacetime, is tbe greatest Image courtesy D. Graf and R. Shieser, example of heating swords into plowshares in tbe history Ensslin Group, ETH Zurich of humankind. For its contribution in both wartime and peacetime, tbe physics profession can be proud. References Email us for your own pair 1. H. D. Smyth, Atomic Energy for Military Purposes, Princeton of 3D glasses U. Press, Princeton, NJ (1948). 2. A. J. Dempster, Phyn. Rev. 11, 316 (1918). 3. L. P. Smith, W. E. Parkins, A. T. Forrester, Phys. Rev. 72, 989 (1947). 4. R. G. Hewlett, 0. E. Anderson, A History of the Atomic Energy Commission, vol. 1, Pennsylvania State U. Press, University Park {1962). 5. C. W. Johnson, C. O. Jackson, City Behind a Fence: Oak YLUM Ridge, Tennessee, 1942-1946, U. of Tennessee Press, Knoxville(1981). 6. D. Irving, The German Atomic Bomb, Simon and Schuster, www.AsylumResearch.com New York (1967). 805-696-6466 7. F. H. Schmidt, Science 199, 1286 (1978). [email protected] 8. W. E. Parkins, Science 200, 255 (1978). 9. L. O. Love, Science 182, 343 (1973). •

May 2005 Physics Today 51 wfww.pt.ims.ca/5988-25 or Circle #25