Probing the Structure of Matter a history of accelerators at Los Alamos Adjusting the Van de Graaff Accelerator, 1946. Richard A. Reichelt 1993 Number 21 Los Alamos Science 93 Probing the Structure of Matter he history of Los Alamos is target and interact with the target Lawrence in 1929. He called his in- intimately linked to machines. atoms. If the particles have a high vention the cyclotron. TMachines of all types—from enough energy, they interact with the reactors to computers to lasers— nuclei of the atoms, often yielding have been indispensable in advanc- “secondary” particles whose identi- World War II Accelerators ing scientific knowledge at the Lab- ties depend on the energy of the pri- oratory. Among all of those ma- mary particles and on the target ma- J. Robert Oppenheimer, director chines, accelerators occupy a special terial. Sometimes the accelerated of Project Y during World War II, position. During World War II ac- particles are only a means of produc- was responsible for bringing acceler- celerators provided vital information ing the secondary particles (neutrons ators to Los Alamos. He believed to scientists designing the first nu- and pions are examples of secondary that only by consolidating machines clear weapons, and after the war particles). Although the basic princi- and scientists in one location could a some of those machines were used ple of particle acceleration is simple, nuclear bomb be speedily built. And as tools of basic scientific research. its execution is not. Designing an speed was important—the Germans Then, in the early 1970s, a half- accelerator requires creativity and in- were thought to be well advanced in mile-long accelerator called LAMPF genuity, and operating an accelerator developing a nuclear bomb. Oppen- (formally named the Clinton P. An- can be an art in itself. heimer envisaged concentrating the derson Meson Physics Facility) was completed after a decade of planning and construction. Over the years LAMPF has been the workhorse for many Laboratory programs, includ- ing programs in nuclear physics, weapons research, neutron scatter- ing, radioisotope production, and pion cancer therapy. Accelerators for more specialized purposes have also been developed or improved by Los Alamos scientists. The constant effort to upgrade and redesign accelerators has often led to unexpected results. New technological spin-offs, new vistas of scientific research, have opened up as one generation of machines re- placed another. This photoessay briefly traces the evolution of accel- erators at Los Alamos and examines Technical Building Z. This shack was built for the Cockcroft-Walton accelerator in 1943. their different applications. In the last section the promise of a future Most modern accelerators are work of designing and building the generation of machines is examined. classified as either linear accelera- the first nuclear bombs in one loca- First a few words of explanation. tors (linacs) or circular accelerators. tion so that ideas and findings could Most accelerators work according to The linac design, in which the accel- easily be exchanged. Accelerators, the same basic principle. An electric erated particles move in a straight as suppliers of nuclear data, were ur- field is applied to a stream of charged line, was first developed by Rolph gently needed to provide an experi- particles (typically electrons or pro- Wideröe in 1928 and later refined by mental foundation for the work. tons) and accelerates the particles to Luis Alvarez. The circular machine, Thus, in the spring of 1943, four greater and greater energies. Those in which the particles move in a cir- bulky machines were transported to accelerated particles can then strike a cular path, was conceived by Ernest a remote New Mexico location from 94 Los Alamos Science Number 21 1993 Probing the Structure of Matter predetonation, in the split and release energy and more plutonium bomb—a neutrons—had not been studied at problem arising from all the relevant neutron energies. the spontaneous fis- Several months earlier, in December, sion of fuel impuri- 1942, Enrico Fermi had induced the ties. Of the two prob- first fission chain reaction at the lems predetonation University of Chicago using a “pile” was the more in- consisting of enriched uranium in- tractable and the terleaved with graphite moderators. source of pessimism The moderators were designed to about the feasibility slow down neutrons and make the of a plutonium bomb. fission reaction more efficient. But The accelerators fission induced by fast neutrons was made it possible for expected to be less efficient than fis- The Wisconsin Short Tank. One of five accelerators comman- scientists to deter- sion induced by slow neutrons. In deered for use at Los Alamos during World War II, the Short Tank was an improved version of a Van de Graaff accelerator mine the critical an explosive chain reaction there and was designed mainly by Joseph McKibben at the University masses for each pro- could be no moderators; the neu- of Wisconsin. The lead shielding and concrete blocks surround- posed bomb design. trons would emerge from fission at ing the pressure vessel served as radiation protection. The machines sup- high energies—between 0.1 and 3 universities across the country. To plied neutrons for studying the neu- MeV (1 MeV = 1 million electron throw curious observers off the tron interactions involved in an ex- volts)—and travel unimpeded until track, the accelerators followed a plosive fission chain reaction. At they collided with other heavy nu- circuitous route. They were first di- that time neutron-induced fission— clei. How would the fast neutrons verted to a medical officer in St. the process by which a neutron caus- interact with heavy nuclei? What Louis and then shipped in boxcars to es the nucleus of a heavy atom to percentage of those neutrons would Santa Fe. Finally the accelerators were moved on flatbed trucks to Los Alamos, just as some of the world’s most eminent scientists were begin- ning to gather there. Massive steel pressure tanks for some of the ma- chines arrived during technical con- ferences in mid April. The accelerators, it was hoped, would help scientists tackle two major challenges. The first was to determine the critical masses of the proposed nuclear fuels—plutonium (239Pu) and uranium highly enriched in the isotope 235U. (The critical mass is the smallest mass of nuclear fuel of a certain shape necessary to sustain a chain reaction.) Both fuels were so scarce that direct measure- ments of the critical mass were im- possible; production techniques for the fuels were only just being devel- The Illinois Cockcroft-Walton Accelerator. This accelerator was used by John Manley and his group to investigate the efficacy of different metals as a “tamper”—a liner surrounding the nuclear oped. The second challenge was to explosive that acts as a neutron reflector and makes the explosive chain reaction more efficient. find a way of preventing a “fizzle,” or Gold, uranium, platinum, tungsten, and other metals were investigated as possible tampers. 1993 Number 21 Los Alamos Science 95 Probing the Structure of Matter fission of 240Pu were likely to initi- ate a chain reaction in the two chunks before the gun mechanism could bring them together into a su- percritical mass. The result would be a fizzle. Therefore a gun-type plutonium weapon was out of the question. So in August the Labora- tory threw its resources into achiev- ing criticality in a plutonium weapon by implosion, that is, by detonation of a layer of conventional explosives surrounding a subcritical sphere of plutonium. The idea was that the inward force of the conven- tional explosion would compress the plutonium sphere and thereby create a supercritical mass, provided the implosion was sufficiently symmet- ric. The implosion option had been pursued previously—early experi- ments had involved detonating TNT wrapped around iron pipes—but im- plosive forces of the required sym- metry had not been achieved. A Portion of the Harvard Cyclotron. A group led by R. R. Wilson investigated nuclear reac- To help in the design of an implo- tions induced by the low-energy neutrons provided by this 42-inch cyclotron. sion weapon, a betatron—a circular cause fission? How many neutrons MeV. And a cyclotron from Harvard electron accelerator— was procured would be released in a fission reac- University produced neutrons with in December, 1944. Pulses of x rays tion caused by a fast neutron? What even lower energies. Together the produced by the betatron were used percentage would be reflected back accelerators produced neutrons with to obtain a sequence of images of a into the fissile material from a metal energies spanning the pertinent sphere of mock fuel as it was being liner—or “tamper”—surrounding the energy spectrum. imploded by a particular configura- core? As sources of neutrons with The summer of 1944 signaled an tion of high explosive. This diag- various energies, accelerators were abrupt shift in the program at Los nostic technique, along with others, just the tools scientists needed in Alamos. In mid April Emilio Segré helped solve the problem of uneven order to study the nuclear reactions and his band of graduate students collapse in the implosion weapon. relevant to weapons physics. had discovered some bad news—that On July 16, 1945, a plutonium Within a few months of their ar- 240Pu (an isotopic impurity present weapon was tested near Alamogor- rival, all four accelerators were pro- in 239Pu) had a high cross section do, New Mexico. On August 6 a ducing neutrons in hastily erected for spontaneous fission. The origi- uranium weapon, Little Boy, was wooden buildings. A Cockcroft- nal plan for initiating an explosive dropped on Hiroshima, Japan, and Walton accelerator requisitioned fission chain reaction in either a plu- on August 9 a plutonium weapon, from the University of Illinois pro- tonium or a uranium bomb had been Fat Man, was dropped on Nagasaki.