Neutrons in Our Future

NEUTRONS in our future

a proposed high-flux spallation neutron source

Roger Pynn

Experimental hall at LANSCE

here is a paradigm in scientific neutrons as probes of the structures 1950s to obtain data for nuclear- research that repeats itself of materials. That area of research, power programs. To this day the Tcontinually—the discovery referred to simply as neutron scat- most productive neutron-scattering that earned yesterday’s Nobel Prize tering, is an important part of program is to be found at a reactor— becomes the tool for today’s re- today’s scientific agenda, which the Institut Laue Langevin (ILL) in search. Take x rays, lasers, and stresses industrial competitiveness Grenoble, France. However, the situ- transistors, for example. Each was and quality of life. To design new ation is changing. A newer tech- worth a Nobel in its day, and each is and improved materials for industri- nique for producing neutrons at pro- now found not only in almost every al applications, scientists build on ton accelerators rather than nuclear research laboratory but also in hos- their understanding of existing ma- reactors is fast becoming competi- pitals, supermarkets, and homes. terials, a large part of which comes tive. The technique, proton-induced The same paradigm applies to neu- from information about their struc- spallation of heavy-metal nuclei, is trons. Discovered by James Chad- tures. Neutron scattering provides currently the basis of the neutron wick in 1932, these neutral particles that information, often in situations source at the Laboratory’s Manuel were the stuff of esoteric research where other techniques fail. Lujan, Jr. Neutron Scattering Center until fast fission and politics com- Successful neutron-scattering ex- and will remain the basis of a more bined to make them central players periments require large number of intense neutron source that the Labo- in the Los Alamos story. Nuclear neutrons to be directed at a sample ratory hopes to build. An upgrade of reactions in which neutrons partici- because only a small fraction of the the LAMPF proton accelerator will pate are at the heart of all of the nu- neutrons are scattered. The first neu- make the more intense neutron clear weapons designed here and tron sources that were sufficiently in- source possible—which brings us elsewhere. Other neutron reac- tense for such experiments were nu- back once more to our paradigm. tions—those in which neutrons are clear reactors, and neutron scattering LAMPF was built more than twenty scattered rather than absorbed by began as a parasitic activity at re- years ago to study nuclear reactions nuclei—are the basis for the use of search reactors that were built in the that involve energetic protons or

1993 Number 21 Los Alamos Science 107 Neutrons in Our Future

pions. Now, one of those reactions, other means. Even a partial list of drogen could be added to other lig- proton-induced spallation, may be contributions from the past decade is ands such as ethylene (catalytic hy- the basis for a new neutron-scattering impressive. During that period neu- drogenation) at a much lower cost in facility. tron scattering revealed the structure energy than the 104 kilocalories per of the first high-temperature super- mole required to break the hydro- he success of neutron scattering conductors; the structure and excita- gen-hydrogen bond of uncoordinated Tand its continuing importance tions of buckminsterfullerenes, or molecular hydrogen. are a result of several properties of bucky balls; the conformation of Polymers and other macromole- the neutron. Because of its neutrali- molecules in a polymer melt; the in- cules absorbed at solid or fluid sur- ty and the weakness of its interac- terfacial structure of artificially pro- faces have many applications to a tions with matter, the neutron—un- duced polymeric and magnetic lay- wide variety of technologies. They like x rays or light— can penetrate ers; the structure and dynamics of are a means for achieving colloidal deeply into solids and liquids and new catalysts; the spin dynamics of stabilization in water-treatment provide information about bulk, as highly correlated electron systems; schemes, ceramic processing, inks, opposed to surface, structure. In ad- and the condensate fraction in super- and fuels; they are used for mechan- dition, because neutrons are scat- fluid helium. It is safe to say that a ical protection of solids against fric- tered by both the nuclei and the un- large part of the conceptual and the- tion and wear in motors and comput- paired electrons in matter, they pro- oretical underpinning of the modern er disks; and surface-active mole- vide information about both atomic theory of solids would be unverified cules at liquid-liquid interfaces are and magnetic structure. The thermal and incomplete without neutron used to clean up oil spills and to en- neutrons generated by nuclear reac- scattering. And without that knowl- hance emulsification and blending. tors or spallation sources have ener- edge our current technology could The variation of polymer density gies that are comparable to those of not exist. close to an absorbing surface had vibrating or diffusing atoms in LANSCE has made its share of been studied theoretically but was solids. Therefore neutrons can contributions during the five years it difficult to study experimentally probe not only the equilibrium posi- has been operating. The discovery until neutron reflection provided the tions of atoms in solids but also by Gregory J. Kubas of the Labora- answer. Work at LANSCE verified temporal structural changes. Be- tory’s Inorganic and Structural theoretical predictions for the pro- cause the neutron-scattering power Chemistry Group that certain metal file of the “polymer brush” formed of atomic nuclei varies erratically complexes can coordinate molecular by the stretching of polymer mole- and often considerably with atomic hydrogen is widely regarded as one cules away from a solid surface into number, neutrons can often distin- of the most significant developments a surrounding fluid and provided a guish between neighboring elements of the 1980s in inorganic chemistry. characterization of the “polymer and can easily distinguish the light- Studies at LANSCE of the vibra- mushrooms” that occur as the graft- est element, hydrogen, even in the tional and rotational dynamics of ing density of the absorbed polymers presence of much heavier elements. those dihydrogen ligands have pro- (the number of attached polymers The latter property makes neutrons a vided insight into the nature of this per unit area) is decreased. particularly powerful probe of bio- unique chemical bond—the first As the transportation industry logical molecules and man-made known example of stable intermole- struggles to improve fuel efficiency, polymers, both of which contain cular coordination of a sigma bond it is turning increasingly to new substantial amounts of hydrogen. to a metal. The system mimics a composite materials—such as alu- For more than forty years neutron catalytic reaction “frozen” in an in- minum reinforced with silicon-car- scattering has played an indispens- termediate state of a type that is bide particles—that provide the dual able role in studies of condensed usually too ephemeral to study and advantages of strength and lightness. matter, providing essential informa- understand. The dihydrogen ligand To understand the mechanisms of tion about materials as different as is important in catalysis because it failure of such materials and to as- antiferromagnets, ribosomes, and can easily exchange hydrogen with sess their lifetimes in real compo- shape-memory alloys. Often the in- other ligands in a complex. It is nents, it is important to understand formation has been unobtainable by conceivable, for example, that hy- the residual stresses induced in the

108 Los Alamos Science Number 21 1993 Neutrons in Our Future

materials during fabrication. De- and short annual operating periods. dressed was chaired by Walter Kohn pending on their distribution, such The problems of LANSCE have been of the University of California, stresses can be devastating—aircraft exacerbated by constant erosion of Santa Barbara, and had been charged fuselages have disintegrated in flight the operating budget of LAMPF over by Will Happer, head of the DOE’s and railroad tracks have cracked and the past five years. LANSCE is now Office of Energy Research, to exam- caused train crashes—or beneficial— threatened with closure because ine the relative merits of reactor and wine barrels have been held together LAMPF is no longer the highest pri- spallation sources for the country’s by metal hoops for centuries. Unfor- ority of the nuclear-physics commu- future neutron-scattering program. tunately no conventional technique nity. Without a new neutron source, After several contentious weeks, the for measuring residual stress, such as U.S. researchers will not remain committee finally concluded that the strain-gauge sectioning or hole competitive. Fundamental research country would be best served if two drilling, is truly nondestructive. as well as technology will suffer. new sources—one of each type— Over the last five years, neutron dif- Nor are our competitors standing could be built. fraction has proved to be a unique, still. A consortium of European lab- Since its beginning in 1987, nondestructive alternative and has oratories has proposed a design LANSCE has been operated as a Na- been systematically exploited at study for an advanced spallation tional User Facility, open to scien- LANSCE. Our work on composite source (the European Spallation tists from industry, academia, and materials has allowed sophisticated Source, or ESS) that would provide other national laboratories. Experi- computer models for stresses—resid- capabilities well beyond those avail- mental proposals submitted by po- ual stresses as well as stress induced able at the ILL. The proposed tential users are peer-reviewed to by applied load—to be verified and, ESS—consisting of a high-energy ensure that the best use is made of in some cases, improved. linac and an accumulator ring— the facility. Most of the national looks very much like an upgraded laboratories host a user facility of n spite of the successes and ac- version of LAMPF and the Proton some sort in fulfillment of one of Iknowledged importance of neutron Storage Ring. It is ironic that just the DOE’s most important missions scattering, the technique is on the as the Laboratory’s spallation source in the area of basic research. The verge of extinction in this country, faces shutdown, the Europeans are new pulsed source proposed by the leadership having passed to our Eu- realizing that spallation sources are Laboratory will remain a user facili- ropean colleagues over a decade the way of the future. ty, and the community of users will ago. With one exception (the neu- Recognizing the national need for define the facility specifications that tron source at the National Institute new neutron-scattering capabilities, best suit its needs. Of course, we of Standards and Technology), all as well as the value of existing in- have an idea of the facility we the neutron sources in the U.S. that frastructure at LAMPF and the would like to build—it is described support neutron-scattering programs strength of its expertise in neutron below—but it is important to recog- are run by the Department of Ener- scattering, Los Alamos National nize that the final design parameters gy. Those sources are old and, by Laboratory has proposed the con- will come from the users rather than modern standards, poorly instru- struction of a new pulsed spallation from the Laboratory. mented. The high-flux reactors at source with an initial power of 1 Brookhaven and Oak Ridge national megawatt (the power of the present new spallation source at the laboratories may reach the end of LANSCE source is 60 kilowatts) and ALaboratory will make use of a their useful lives before the end of a possible future power of 5 mega- number of existing LAMPF assets this decade. A pulsed spallation watts. On August 19, 1992, Labora- that would be expensive to reproduce source at Argonne National Labora- tory Director Sig Hecker announced elsewhere and are very appropriate tory provides only one-tenth of the the proposal to a visiting review as part of a modern accelerator com- intensity of the ISIS facility at the committee, noting that the Laborato- plex. The 700-meter-long shielded University of Oxford. The LANSCE ry wants to change the emphasis of tunnel that contains the present linac source has a peak neutron flux that is research at its 800-MeV linac from will remain, as will buildings, cool- slightly higher than the ISIS source nuclear physics to neutron scatter- ing towers, 30 megawatts of site but has suffered from poor reliability ing. The committee Hecker ad- electrical power, and a 600-meter

1993 Number 21 Los Alamos Science 109 Neutrons in Our Future

part of the existing accelerator called ments requiring the intense high-en- size is even more critical in structur- a coupled-cavity linac. The latter is ergy neutron beams produced by al biology, where the structures of basically a copper pipe—albeit of spallation. In other words, a 1- only about one in every two hundred somewhat exotic design—that is not megawatt spallation source would interesting proteins are now accessi- expected to wear out. give the U.S. the same capability as ble to neutron scattering—sufficient- All of the high-tech parts of the the ILL plus the obvious advantages ly large crystals of the others just proposed accelerator complex will over ISIS. Such a source would also cannot be produced. Although some be new and will take full advantage be complementary to the reactor— improvement in sample size is likely of accelerator technology developed the so-called Advanced Neutron in some instances, there are other here and elsewhere as part of the Source—that the DOE proposes to areas—the study of interfacial struc- Strategic Defense Initiative. Our build at Oak Ridge National Labora- ture by neutron reflection, for exam- present reference design calls for in- tory. Although there is a large area ple—where the scattering volume is jection of 800-MeV protons from the of overlap in the capabilities of inherently small and will always re- upgraded linac into an accumulator these two sources, each has unique main so. For such systems the only ring that is similar in concept to the strengths. way forward is through the use of the existing Proton Storage Ring. neutron sources with higher flux. However, we are studying an option ny prediction of the scientific Higher-flux sources will also that would increase the proton ener- Aimpact of a 1-megawatt spalla- offer scientists the possibility to gy and, perhaps, permit an easier tion source is bound to be incom- study structures as they evolve over upgrade to 5 megawatts of beam plete, at best. Nevertheless, the im- time. Examples include changes in power in the future. pact is fairly obvious in those areas the structure of interfaces during The new accelerator complex will that are extrapolations of current re- corrosion and of electrolytes during produce 60 proton pulses per sec- search. For example, many experi- battery discharge; phase transforma- ond, each of about 0.5 microsecond ments are beyond our current capa- tions induced by propagating shock in duration, and distribute them be- bilities because samples of sufficient waves; and conformational changes tween two neutron-production tar- size are not available. Neutron scat- of polymers during extrusion mold- gets. One target will receive 40 tering is inherently a signal-limited ing. Presently such experiments are pulses per second and the other 20 technique because, as mentioned restricted to model systems that pulses per second. We expect the above, only a small fraction of the change relatively slowly with time 40-hertz target to provide about five neutrons incident on a sample are or to systems that can be arrested or times the average neutron flux gen- scattered. If the sample is not large cycled repeatedly. Examining the erated by the ISIS source. Coupled enough, the informative scattered change in structure of a catalyst dur- cold moderators at the 20-hertz tar- neutrons—the signal—cannot be dis- ing its active phase, for example, is get will give twenty-five times the criminated from background neu- beyond current capabilities because peak flux of either the ISIS or the trons that have suffered spurious the entire reaction is completed in a present LANSCE source. There will scattering processes. One way of time that is much shorter than that be room for between twelve and six- overcoming this limitation is to in- needed for a neutron measurement. teen beam lines around each target. crease the flux of incident neutrons The techniques used for neutron What does all this buy us? How as our new source is designed to do. scattering at high-flux pulsed does it compare with the ILL, for Such a source would make many im- sources are well adapted to neutron- example? This question was an- portant experiments possible. For scattering experiments in which swered by a group of European and example, we would be able to probe samples are subjected to high pres- American neutron-instrumentation the collective excitations of high- sures or high magnetic fields. The experts who advised the Kohn panel. temperature superconductors and equipment required to achieve high That eminent group concluded that a fullerenes and perhaps understand pressures would fail if it had to have 1-megawatt pulsed spallation source the bases for their bizarre properties. large windows to let neutrons in and could duplicate the capabilities of Experiments of this sort now require out, and high magnetic fields can be the ILL and provide facilities that single crystals of a size that cannot maintained only for short periods. exceed those at the ILL for experi- be grown. The problem of sample Powder-diffraction measurements at

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Neutrons in Our Future

LANSCE have already been made at ern computers and the wizardry of visage that those other types of re- pressures above 100 kilobars, a modern techniques for data manipu- search could be carried out without pressure that is three or four times lation and display. The payoff could jeopardizing the primary mission of higher than has been achieved by be immense, however. One can eas- neutron scattering. The prospects of similar experiments elsewhere in the ily imagine a neutron spectrometer such a multidisciplinary research fa- country. Achieving still higher pres- at a new pulsed source with 1 mil- cility are indeed exciting and a fit- sures, such as those needed to mimic lion or 10 million parallel informa- ting continuation of Los Alamos ex- some geological conditions, will re- tion channels instead of just one. pertise in the science of neutrons. quire the use of smaller samples and, concomitantly, more powerful lthough this article has focused neutron sources. Aon neutron scattering, we ex- One of the most exciting new ca- pect that a new spallation source at pabilities offered by a 1-megawatt the Laboratory would support other pulsed source couples the character- types of research as well. Indeed, it istics of the source with the great would be indefensible from the tax- progress that has been made in com- payers’ point of view not to exploit puter science over the past two synergistic uses of the facility, some decades. Twenty-five years ago, of which might help to resolve im- neutron spectrometers were deliber- portant issues in areas such as the ately designed to avoid collecting management of radioactive waste. too much data—it would have been And there are exciting experiments just too confusing to the poor scien- in basic physics and nuclear-physics tists! Spectrometers were designed research to be done with neutrons, to focus on phenomena that occurred as well as complementary investiga- over a small range of length and tions of materials by muon spin res- time scales and to ignore the rest. onance. Experiments with ultracold Although that approach delayed neutrons—those with velocities of Roger Pynn was born and educated in England. some discoveries a decade or two, it only a few meters per second—can He received his M.A. from the University of Cambridge in 1966 and his Ph.D. in neutron scat- worked reasonably well for simple accurately measure the lifetime of tering, also from the University of Cambridge, in samples, especially those that could the neutron and determine whether it 1969. He was a Royal Society European Fellow be grown in the form of single crys- has an electric dipole moment. Both to Sweden in 1970; he did two years of postdoc- toral research in Norway; and then he was an as- tals. Unfortunately, many of the of those properties are important in- sociate physicist for two years at Brookhaven Na- complex materials of interest puts to the standard model used to tional Laboratory. After spending eleven years at today—both in materials science and understand our universe. High- the world’s leading center for neutron scattering, the Institut Laue Langevin in Grenoble, France, structural biology—are interesting power spallation sources also have Pynn was appointed director of the Laboratory’s precisely because they have struc- practical applications. They can Manuel Lujan, Jr. Neutron Scattering Center. ture on a wide variety of length produce neutron-poor radioisotopes, He served as a member of the Kohn panel whose deliberations are described in this article. scales. Examples range from DNA many of which have become indis- molecules packaged as chromatin to pensable to modern nuclear medi- the fractal structures found in cine; they can be used to study radi- porous media. To study such mate- ation damage of materials in regimes rials with neutrons requires spec- that are relevant to fusion-energy trometers with access to a large systems; and they are the basis for range of length scales, a feature pro- many transmutation schemes that vided quite naturally by pulsed have been proposed to solve prob- sources. However, to find some lems ranging from the production of meaning in the vast quantities of in- tritium to the destruction of long- formation obtained by such spec- lived fission products and plutonium trometers requires the speed of mod- from the weapons stockpile. We en-

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