Neutrons in Our Future

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.

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