magine alchemists struggling in the · magine alchemists struggling in the dark with materials they...

I magine alchemists struggling in thedark with materials they do not un-derstand; see modern industrialiststrying to control the properties of

polymers, colloids, and gels; think of atheorist grasping for the essentials ofcomplicated nonlinear phenomena. Doyou feel you need some solid ground?Neutrons traveling through mutter seemostly empty space, but their few inter-actions with nuclei begin to show us thestructures of materials-concrete infor-mation that can crystallize our questionsand theories into a framework of futurediscovery, Relating the basic propertiesof materials to their structures trans-forms alchemy to technology and bindsindustrial research to basic science.This is the world of neutron scattering.

As director of the Los Alamos Neu-tron Scattering Center, Roger Pynn iscaught up in every aspect of this world.When the user rooms at LANSCE arejammed with biologists, chemists, physi-cists, and industrial researchers, hejumps from question to question andfield to field in only the time it takes himto hurry through the halls. (There weretimes when we thought he might writeall the articles for this issue instead ofonly three.) Even his own ongoing re-search projects range from phase transi -tions and surface phenomena to instru-ment design and data analysis.

Given the present state of neutronscattering in the United States, however,Roger’s most important work may beas LANSCE’s ambassador to the larger,more political world. In the last weeksof 1989, one hundred participants in acondensed- matter physics conferencepetitioned Presidential Science AdviserAllan Bromley for- a new commitmentto funding for neutron-scattering re-search. Citing aging facilities and alack of young scientists entering thefield, these researchers warned that theUnited States neutron-scattering effortmay lose irretrievable ground to thethriving community in Europe. We inter-

Los Alamos Science Summer 1990

An Interview with Roger Pynn

viewed Roger to find out where LAN-SCE fits in this picture, and he gave usa unique perspective on neutron scatter-ing’s history and on the most pressingproblems facing the field today.Science: Last winter, a petition to se-cure funding for neutron scattering’sfuture was sent to Presidential ScienceAdviser Allan Bromley. Can you ex-plain why?Pynn: The petition actually relates tothe present as much as the future. Rightnow, the two most powerful nuclear re-actors used for neutron scattering in thiscountry are closed to address safety con-cerns. Brookhaven has been closed fornearly a year, and Oak Ridge has beenclosed for over three years. In addition,the National Institute of Standards andTechnology reactor was closed during1989 while neutron guides were be-ing added. It is bad enough that thesefacilities haven’t been available for re-search, but a larger and larger share ofthe neutron-scattering budget has alsobeen spent on their safety studies andrepairs—and that drains the budgets ofthe remaining facilities. For example,this year at LANSCE we’ll be able torun our beam less than half the year,which means we won’t accommodatenearly as many experiments as we’dlike.Science: Is the situation likely to im-prove in the near future?Pynn: Well to make matters worse,these reactors are all around twentyyears old, and their neutron-scatteringinstruments are antiquated. If fundswere available, we could improve ex-periments at these facilities by a factorof five to ten simply by modernizingequipment. For a while, we wouldn’teven need to worry about getting higherbeam fluxes. I should say, however, thatthe future of this field depends on build-ing facilities with higher beam fluxes. Ifthe U.S. wants to keep pace with Eu-rope and Japan, it is critical that webuild a next-generation neutron source.

Science: Why is it so critical to keeppace? The petition to Bromley men-tioned economic growth.Pynn: As far as that goes, neutronscattering has all sorts of technologi-cal applications. We can use neutrondiffraction to do nondestructive testingof residual stresses and strains in a widevariety of industrial products. Small-angle scattering can examine the struc-tures of polymers and colloids, which

The future of this fielddepends on building facili-ties with higher beam fluxes.If the U.S. wants to keeppace with Europe andJapan, it is critical that webuild a next-generation neu-tron source.

are the basic ingredients of many mod-em materials. Neutron reflectometry canlook at the structures of protective coat-ings and lubricants. Look what cameacross my desk today: “A Neutron Scat-tering Study of Diffusion and Perme-ation Processes through Pores in Clay.”You can imagine applying that to un-derground waste disposal or oil mining.Really, the industrial uses of neutronscattering are endless.Science: So you argue that we shouldfund neutron scattering because it iscrucial to the industrial future of theUnited States?Pynn: Partly. I am very interested inpromoting industrial uses of neutronscattering, but that’s not the only reasonto promote the technique. We shoulddevelop materials-research techniquesand do science with them whether theirapplications are instantly apparent ornot. For example, when new materialslike high-temperature superconductors

come along, you want to understandthem and you use every resource youhave--electron microscopy, nuclearmagnetic resonance, neutron scatter-ing, x rays, or whatever. In the caseof high-temperature superconductors,neutron scattering gives unique infor-mation about structure because it canlocate light elements and also look atmagnetic properties. Researchers atBrookhaven were doing valuable ex-periments of this kind when the reactorwas closed down. For other problems,other methods might be more valuable,but it is impossible to predict whichones. So we should maintain a capabil-ity in each technique if we want to havean effective materials research program.Furthermore, techniques need to be ex-plored because they open new areas ofbasic interest. As a matter of fact, thedevelopment of neutron scattering il-lustrates this point quite well. Back inthe forties, scientists shot neutrons intosamples because they wanted to find outabout the fundamental properties of thisnew elementary particle. Today, neutronscattering provides useful informationabout the samples themselves to physi-cists, chemists, and biologists in addi-tion to all the industries I mentioned.Science: Go back and tell us a littlemore of the early history.Pynn: Soon after Chadwick’s discoveryof neutrons in 1932, researchers begantrying to understand the properties ofthese particles by sending them throughvarious materials. Theorists knew fromquantum mechanics that neutrons wouldbehave like waves, and that low-energyneutrons would produce interference ordiffraction patterns much like x rays.But there were many theories and dis-agreements about specifics. For exam-ple, people wondered what details ofthe interaction between a neutron anda nucleus could be determined from ascattering experiment, and they won-dered whether the neutron’s expectedmagnetic field would resemble that of

Los Alamos Science Summer 1990 35


a bar magnet, a current loop. or some-thing in between. Fermi resolved thefirst problem in 1936 by proving theo-retically that the neutron-nucleus inter-action is described by only one mean-ingful parameter—what we call todaythe scattering length. The second prob-lem grew into a fertile debate betweenBloch and Schwinger. In 1939, Halpemand Johnson suggested experiments tosettle this argument and to test a numberof other theories. But of course in theearly forties, pressure from the Manhat-tan Project for critical neutron data wasdictating most of the neutron research.Science: Were these early neutron-scattering experiments much the sameas those done today?Pynn: Until the first reactors werebuilt, researchers couldn’t generateenough flux for anything but trans-mission experiments, but after the warFermi and Marshall began experimentscomparable to ours today. They mea-sured neutron diffraction from a crystalwhose structure was known from x-rayexperiments-some simple thing likerock salt—and then they drew conclu-sions about the interaction of neutronswith different nuclei by comparing theneutron data with the x-ray data. At thesame time, Wollan and Shun were us-ing a similar method to study neutrondiffraction from crystalline powders andgetting some very important results. In1947 they demonstrated that neutronscan see hydrogen atoms in a crystal,and that unique ability has become neu-tron scattering’s great contribution tothe study of biological systems. An-other seminal experiment was Hughesand Burgy’s verification of Schwinger’scurrent-loop hypothesis. They pro-duced the first fully polarized neutronbeam by using magnetic mirrors—precursors to the reflection techniquethat we use today in neutron guidetubes. I should say that in all these ex-periments people were still primarilyinterested in understanding the neutron.

The great breakthrough forneutron scattering camein 1952 with the first [mea-surements of] the internaldynamics of condensed-matter samples.

It was the opposite of most researchtoday, where we assume we know aboutthe neutron and draw conclusions aboutthe sample from the scattering.Science: Even so, it sounds like theearly researchers conceived of most ofthe techniques in use today.Pynn: To a large extent that is true.Most of the gains made in neutron scat-tering have been technological. For ex-ample, if you want to make guide tubesthat transport neutrons with minimumlosses, you have to make glass whichis optically flat enough and you haveto learn how to vapor-deposit nickel.We have made many improvements likethat. The neutron spin-echo techniquethat came along in the early seventieswas a genuinely new method of gettinghigh resolution without losing much in-tensity. but for the most part the basicexperimental concepts go way back tothe beginning.

Science: When did researchers startusing neutrons to probe materials?Pynn: Once the properties of the neu-tron were understood, it was natural toturn scattering experiments into a meansof studying static crystalline structures—a change which occurred in the early tomid fifties. This wasn’t a completelynew research field, however; essentiallyit extended the x-ray crystallographywork that had been going on for someforty years. The great breakthrough forneutron scattering came in 1952 withthe first inelastic-scattering experiments,which investigated the internal dynamicsof condensed-matter samples.

In an inelastic-scattering experiment,you measure the energy and momen-tum a neutron transfers to the atoms ofa solid-energy which can then vibratethroughout the sample as a collectiveexcitation called a phonon. Theoristshad already described phonons as vibra-tional waves whose frequencies relate tothe interatomic forces in solids, but neu-tron scattering was the first way to makemeasurements in real samples. Groupsin France and the U.S. began measuringphonons using time-of-flight spectrom-eters, and in Canada Blockhouse beganusing what he called a constant-Q scanon his triple-axis spectrometer. Such amachine measures only at well-definedscattering angles and energy transfers,which makes for very precise, very fo-cussed inelastic-scattering data. Block-house used to say something like younever get more data than you need froma triple-axis spectrometer, and you al-ways get it at a rate that lets you figureout exactly what to measure next. As Isaid, many people were working to de-velop instruments for measuring inelas-tic scattering processes, but the three-axis spectrometer became the prevalenttool. For whatever reason, Blockhousewas able to apply it to a wider rangeof materials and problems than anyoneelse. Given the technology at the time,he made some spectacular measurements

36 Los Alamos Science Summer 1990


of phonons in semiconductors, and met-als, and ionic crystals, and everythingyou can think of. He just knocked themoff one after another.

The study of phonons in all sortsof materials became a major focus ofneutron-scattering research throughoutmuch of the sixties. As the experimen-tal part of my doctoral thesis, for exam-ple, I studied the phonon spectrum ofmagnesium. I had this huge single crys-tal of magnesium—four inches long andan inch and a half across—and I mea-sured phonons in the damn thing. To-day people would say, “So what? Whywould you bother to measure phononsin a single crystal of magnesium?” Theanswer is that we were just beginning tolearn how to calculate phonon frequen-cies in metals from first principles, Peo-ple would propose models of the differ-ent bonding forces in metals, calculatewhat the phonon frequencies should be,and then compare them with neutron-scattering measurements. If they didn’tsee agreement, they would go back andfool around with their models or comeup with better ones. Efforts like thosegave us a much better qualitative andquantitative understanding of what holdsmetals together.Science: What were some other majordiscoveries from the fifties and sixties?Pynn: Perhaps the most striking wasVan Hove’s elegant formulation of theneutron-scattering law in 1954. BeforeVan Hove people used neutrons to studystructure and dynamics in a variety ofways, but they didn’t understand howthese different techniques related to eachother. Van Hove’s analysis unified thewhole field of research. It brought to-gether in one simple equation the staticstructure factor, which we measure indiffraction experiments, and the collec-tive excitations, which we measure ininelastic-scattering experiments. BeforeVan Hove no one had really demon-strated the simplicity and power ofneutron scattering as a research tool.

There were also neutron-scattering re-sults which affected materials physics asa whole. In 1951 Cliff Shun used neu-tron diffraction to verify Neel’s theoryof antiferromagnetic structure, whichsaid that in certain materials the mag-netic moments of electrons line up inalternating sequences. There had beenno way of proving the existence of suchantiferromagnets until neutron scatter-ing came along. Once this structure hadbeen verified, people used inelastic neu-tron scattering to discover the collectiveexcitations in these antiferromagnets andthen developed theories to explain them.In 1961 neutron scattering was also usedto observe rotons, a type of collectiveexcitation in superfluid helium whichLandau had predicted on the basis ofGod knows what genius.Science: It sounds like neutron scatter-ing research was beginning to broadenits scope. How did that happen?Pynn: After a while it becomes tire-some to measure a sample of tediumboride for the seventy-fourth time justto find out what the interatomic forcesare at some other temperature. In themid sixties people gradually started todo experiments that involved some spe-cial physical phenomena; for example,if a material went through a ferroelec-tric transition, they would use neutronscattering to see whether the phononshad played a role in this transition. Themotivation became, “Well, here is thisphenomenon called a phase transition.What can we learn about it with neu-trons?” rather than, “Here we havethis piece of solid garbage on the shelf.Let’s measure it.”Science: What is a phase transition?Pynn: A phase transition is a disap-pearance of order in a sample of matter,brought on by a change in some exter-nal factor like temperature or pressure.Everyone is familiar with ice chang-ing to water or water to steam, transi-tions in which the structural order aswell as the bulk properties of the mat-

[Van Hove] brought togetherin one simple equation thestatic structure factor, whichwe measure in diffractionexperiments, and the col-lective excitations, whichwe measure in inelastic-scattering experiments. Be-fore [that], no one had reallydemonstrated the simplicityand power of neutron scat-tering as a research tool.



When I look at the theory ofcritical phenomena, it seemsclear that the universalityclasses could not have beenidentified without the cluesprovided by [neutron scat-tering] experiments.

ter change suddenly; but there are alsophase transitions in which order changesgradually. The order can be structural,or magnetic, or ferroelectric, or what-ever. Since the early sixties, there hasbeen a huge intellectual effort to de-scribe the gradual changes in order asdifferent systems approach the so-calledcritical point where order disappears al-together. By the early seventies, the-orists succeeded in organizing thesecontinuous phase transitions into uni-versality classes defined by the way theorder changes. They discovered that thesymmetries of the system. and not theparticular details of the forces responsi-ble for creating the order, determine theuniversality class of the phase transition.Some very high-powered people haveworked on this theory of critical phe-nomena, and a Nobel prize was given afew years ago for work in this field.Science: How did neutron scatteringcontribute to this study?Pynn: Let’s take the specific exam-ple of’ a magnetic phase transition. Asyou heat a magnetic sample, its spa-tially averaged magnetism will decreasecontinuously until you reach the Curietemperature [critical point] where themagnetism disappears completely. Thismight seem very simple, but it is not.Although the average magnetism goesthrough a smooth decrease, the mag-netism at any point in the sample fluc-tuates more and more about the averageas the sample nears its critical point.In addition, the fluctuations in magne-tization at one point in the sample arecorrelated with fluctuations at a new-bypoint. As the Curie temperature is ap-proached, the spatial extent of thesecorrelations becomes very large andthe fluctuations slow down. The wayin which the correlation length increasesand the fluctuations slow down-thatis, the dependence on temperature—characterizes the universality class ofthe transition. Neutrons can measureboth of these quantities, as well as the

average magnetic order. Without all theneutron scattering experiments. I don’tthink theorists would have been ableto understand these immensely compli-cated phenomena. Now when I lookat the theory of critical phenomena, itseems clear that the universality classescould not have been identified withoutthe clues provided by the experiments.So neutron scattering had a large role inthat development.Science: Was neutron scattering a well-recognized research field at that time?Pynn: It was beginning to become one.Until the early seventies. the peopledoing neutron scattering were actu-ally condensed-matter physicists whohad become interested in the technique.More important than that. however, wasthe lack of dedicated research facili-ties. In the fifties and much of the six-ties, neutron scattering was just a para-sitic operation at research reactors thathad been built to study things like iso-tope production and radiation damage.Those experiments had first priority atall the facilities because the people do-ing neutron scattering didn’t decide thepolitics of reactor use. Instead, theyhung around the edges, borrowing beamlines and setting up spectrometers whenthey could. The first reactor built ex-clusively for neutron scattering was theBrookhaven High Flux Reactor, whichcame on line in the mid sixties.Science: Did the Brookhaven reactorbegin the field as we know it today?Pynn: I’m not sure I can define a be-ginning. Certainly the advent of theBrookhaven reactor gave neutron scat-

tering a dedicated tool. but it didn’tchange things that drastically. Eventhough this new reactor was dedicatedto neutron scattering, it was also ded-icated. in a sense, to the few peoplewho were employed at Brookhaven. Es-sentially, you still had a small neutron-scattering group working by themselves.The field as we know it today-withscientists from all over doing their re-



search at large central facilities—startedduring the early seventies in Europe.Science: Was there a definable begin-ning to this change’?Pynn: The first real user facility—andincidentally still the pre-eminent neutronscattering facility in the world today—isthe Institut Laue-Langevin in Greno-ble, France. Sometime after the war,a German professor of physics by thename of Maier-Leibnitz proposed build-ing a large research reactor as part ofthe France-German cooperative effort,and this reactor became the ILL. It wasborn of politics, not because peoplesaid. “We have to do neutron scatter-ing.” It was born because the Germansand the French wanted to get togetherfor scientific and cultural exchanges.As the rumor goes, it was born becauseMaier-Leibnitz had a relative who wasclose to Adenauer, but that may not betrue, At any rate, Maier-Leibnitz per-suaded the politicians that a reactor ded-icated to neutron scattering was some-thing they needed as well as a scientificneed. Next, he toured the United Statesand Canada, which were strong in neu-tron scattering at that time, asking foradvice about designing a first-rate re-search program. The advice was, “Well,first you build your three-axis spectrom-eters and get a program established, andthen you think about doing somethingelse.” Triple-axis machines were verypopular at that time, especially in theUnited States, and many of the ques-tions asked in inelastic neutron scatter-ing were dictated by the machine’s char-acteristics. Even today that is true to acertain extent. Anyway, Maier-Leibnitzsaid, “Thank you very much, but I willnot build a single three-axis machineat my institute.” And at first he didn’t.Instead, he hired a bunch of young peo-ple, many of whom knew nothing what-soever about neutron scattering, andset them working on some of his ownbright ideas. They were happy to tryanything because they didn’t know what


was impossible. They invented thingsand incorporated ideas from prototypicalinstruments at smaller reactors in Franceand Germany, and they wound up withall sorts of novel instrumentation-andonly one three-axis machine when theinstitute became operational.Science: What were some of Maier-Leibnitz’s bright ideas’?Pynn: There were several. One of thegreat successes at the ILL has been theuse of long-wavelength neutrons—whatwe call cold neutrons. There were othercold-neutron sources in operation at thetime the ILL was built, but they werecreated by putting moderators on ex-isting beams, Maier-Leibnitz proposedbuilding the moderator in next to thereactor core so cold neutrons would begenerated in copious quantities when-ever the reactor was running, At thetime, many people said the ILL peoplewere crazy to tie the operation of thereactor and the cold source together inthis way, Next, Maier-Leibnitz decidedto use hundreds of meters of opticallyflat, nickel-coated glass tubes to trans-port neutrons away from the reactorand into a huge new guide hall wherethe background radiation would be low.

In the fifties and much of thesixties, neutron scatteringwas . . . a parasitic opera-tion at research reactors....The people doing neutronscattering . . . hung aroundthe edges, borrowing beamlines and setting up spec-trometers when they could.

The Brookhaven reactorgave neutron scattering adedicated tool, but... it wasalso dedicated, in a sense,to the few people who wereemployed at Brookhaven.

39


This was an incredibly courageous deci-sion given that guide tubes of that typehad only been benchtop tested and costseveral thousand dollars a meter. TheILL researchers also improved the angu-lar resolution of small-angle scatteringexperiments by putting their detector 40meters away from the sample—a dis-tance that had never been tried before.There is still not an instrument in theworld that comes close to the resolutionof that machine.

Maier-Leibnitz also had some wildideas that didn’t work. For example.he wanted to replace triple-axis spec-trometers with three separate remotelycontrolled units that could be movedaround on air pads. So the ILL peoplelaid down enormous, smooth marblefloors called tanzboden, or dance floors,and built these air-levitated units—sortof nineteen seventies R2D2 units. Thosethings never worked as planned. In theend someone clamped them togetherinto a traditional three-axis spectrometerthat moved on air pads instead of on thenaval gun mounts used in the sixties.That technology has now spread almosteverywhere.Science: Were these developments ininstrumentation motivated by specificscientific questions’?Pynn: Not really. Maier-Leibnitz hadspecific ideas about improving the mea-surement techniques themselves, and heknew this would lead to new and excit-ing science—an obvious idea that seemsto have been largely misunderstood inthe United States. Remember, neutronscattering is a signal-limited technique.You can’t measure a particular effectunless enough neutrons reach the detec-tor. Let’s take a simple example. Theoriginal triple-axis spectrometers usedbig, flat monochromator and analyzercrystals. usually aluminum or copper orsomething else that grew well in singlecrystal form. That is analogous to doingoptics with rather poor flat mirrors, andit is very inefficient. To maximize the

intensity of a light beam, you usuallyuse good-quality reflecting surfaces andfocus the beam with curved mirrors orlenses. People tried all sorts of thingsto make single crystals more efficientat transmitting neutrons, for instancelaying them on a table and beating themwith a hammer. That helped a little bitbut it wasn’t very controlled.

Maier-Leibnitz and his co-workersthought they knew how to improve theflat crystals, and they approached thisproblem in a systematic way-tailoringnew materials and using multiple crys-tals to achieve a focusing effect. Spec-trometers today deliver much better in-tensity and resolution than the originalinstruments, mostly due to improve-ments in individual components ratherthan to increases in neutron fluxes.Science: How do you increase the scat-tering efficiency of crystals?Pynn: It is a very sophisticated tech-nique that involves well-defined distor-tions of’ the crystal. You start with acrystal that has less than a certain den-sity of’ dislocations, then you cut it to aspecific shape, then you squeeze it

Neutron scattering is asignal-limited technique.You can’t measure a par-ticular effect unless enoughneutrons reach the detector.

along a specific direction usually whileheating it to a certain temperature. Thistechnique has really only been pursuedat the ILL. which fits with their his-tory of developing instrumentation. Letme say right now that one-third of theinitial budget of the ILL was for in-strumemt and spectrometer design andconstruction. By comparison. the planfor a next-generation source in the U.S.allots only one-fifteenth of the budget toinstrumentation, and you know exactly

Maier-Leibnitz had specificideas about improving themeasurement techniquesthemselves, and he knewthis would lead to new andexciting science—an obvi-ous idea that seems to havebeen largely misunderstoodin the United States.

what that will produce—nothing new.Science: How important has instrumentdevelopment been to the field?Pynn: Initially people used diffractome-ters and triple-axis machines, which lim-ited experiments to a small range of’phenomena. Now we use small-angle-scattering instruments, backscattering in-struments, diffuse-scattering instruments.time-of-flight instruments, spin-echo in-struments. reflectomcters, and all sortsof’ other things—and this variety is veryimportant. Basically, you can imaginethat any experiment you want to do ex-ists in a space whose dimensions arcmomentum transfer, energy transfer. andresolution. Because neutron scattering islimited by the flux of’ neutrons you havein your beam, you must develop specialtypes of instruments to get you into dif-ferent corners of that space. A genericinstrument simply won't take you ev-

40 Los Alamos Science Summer 1990


erywhere. So people have built specialnew instruments to study phenomenainvolving high momentum transfer. orvery high resolution, or low energies,or whatever specific problem they wereinterested in. The ILL has done a greatdeal to expand the momentum-energyspace to which we have access. andspallation sources like LANSCE alsoexpand it. I should point out that fu-ture experiments could fall anywhere inthis space at random; so it is difficult tooveremphasize instrument development.Science: When did the ILL people be-

gin inviting outside researchers to comeand do experiments’?Pynn: I suppose they must have re-alized the enormous potential of’ theplace once they began building all thespectrometers and opening all the beamlines. Mossbauer, who succeeded Maier-Leibnitz as director of’ the ILL, reallyinitiated the user system by encourag-ing proposals from universities and bysetting up a committee to evaluate theseproposals and decide who should getbeam time. By attracting a variety ofusers, this system made a great contri-bution to the expansion of the field. TheILL uses it today, and we are copying ithere at LANSCE.Science: Say more about how neu-tron scattering came to be used for thewidely varying research we see today.Pynn: For the most part, the ideas camefrom outside the field, not from the pro-fessional neutron scatterers. I know thata very strong group from Oxford drovemuch of’ the expansion of chemistry re-search at ILL, and the push toward usesin polymer science and biology alsocame from the outside.Science: How did these outsiders findout that neutron scattering was such auseful tool?Pynn: I can at least answer that ques-tion for the Oxford chemists. Sometimeback in the late fifties, Cockcroft, whohad been the director of the Atomic En-ergy Research Establishment at the Har-

welt reactor, was made chairman of a In the United States, smallcommittee to get university researchersinvolved in work at government labs. groups working around theirSo be gave the people at the Harwellreactor the equivalent of $3 million intoday’s money to develop neutron scat-tering, and they drove up the road toOxford and started trying to interestpeople. Among others, they talked toa chemist named John White, a real dy-namo, and he started to figure out theexperiments you could do with neutronscattering in chemistry. He later be-came one of the directors of the ILL.establishing strong connections with thechemistry group at Oxford.

Some physicists who began with neu-tron scattering also helped widen thefield. for example Bernard Jacrot. Hehad been doing scattering experimentssince the fifties, studying magnetismand magnetic properties-he did someof the early time-of-flight experiments.Anyway, the story goes that sometimeduring his stay at the ILL, Jacrot put adump truck outside his office window,threw everything into it, and started do-ing biology instead. He was one of thepeople who showed the power of thecontrast-matching technique. which isnow almost second nature to biologists.

All these people came together at theILL, So neutron scattering got a largefacility where it could grow with influ-ences from other fields, and Europe gota centralized facility where other fieldscould use neutron scattering. Today,for example, the U.K. sees the ILL asa training ground for Ph.D.’s in meth-ods of research. No such movementhappened in the United States. In theUnited States. small groups workingaround their own reactors never reallygot together, and that is as true today asit was twenty years ago.Science: Why did that happen’?Pynn: Perhaps it has something to dowith the way things are funded here.I’m not a great fan of the peer-reviewsystem as it works in the U.S. because

own reactors never reallygot together, and that is astrue today as it was twentyyears ago.

41


it doesn’t encourage cooperation or syn-thesis of ideas. Anyway, the sociol-ogy of neutron scattering in the UnitedStates has centered the work in small,parochial groups that are very defen-sive of what they have acquired overthe years. In contrast, the ILL becamea user facility-people from universi-ties and laboratories came to do sciencethere. Until quite recently the idea of afacility catering to outside users didn’texist in the DOE. Isolated groups ofprofessional neutron scatterers ran everyone of the U.S. facilities.Science: Did outside researchers comeand ask to do experiments?Pynn: The doors to the U.S. facilitiescertainly weren’t wide open. In the sci-entific sense, the neutron professionalsdetermined everything that happened inneutron scattering. I’m not saying theydidn’t do good science—they did—butthe field was inbred and cut off fromnew ideas and influences.

Also, because researchers from differ-ent facilities did not cooperate and be-cause there was no user group to ask formore facilities, the field got no moneybeyond what was necessary to keep thesmall groups going. In fact, the U.S.is missing two generations of scientistsin neutron scattering. Almost all thepeople who were trained as postdocs atBrookhaven or Oak Ridge in the sixtiesand seventies couldn’t get a permanentjob in neutron scattering and went onto something else. Despite my air ofvenerability, I‘m young in the Americanneutron-scattering scene. The number oftwenty-five- to thirty-year-olds is essen-tially zero.Science: Did your coming here coincidewith the DOE’s idea of encouraginguser groups?Pynn: I think the idea of a designateduser facility came earlier—near the be-ginning of this decade. A number ofDOE panels have looked into neutronscattering. The first one asked whatwould happen to neutron scattering in

42

the United States if its budget remainedconstant. They concluded that neutronscattering in the United States was veryhealthy, thank you. It was doing betterthan everybody thought because Amer-icans were smarter and didn‘t need ex-pensive, modern facilities. This mistakewas eventually recognized when suffi-cient people managed to get out of’ theUnited States and see for themselves.After that, various committees were setup to look into the field, and they said,“We have to do better. The Europeansare way ahead of us, and [he Japaneseare coming.” I remember sitting in frontof one of these committees during avisit to Brookhaven during 1980. whenI was still at the ILL. Various other labshad testified during that hearing, andthe Oak Ridge people, for example, hadsaid, “We had one hundred sixty visitorslast year.” That was a nice introduc-tion to my pie chart showing how allthe ILL users broke out into differentsubjects. There were sixteen hundredof them—an order-of-magnitude differ-ence!Science: Has neutron scattering in theUnited States changed in response tothese findings?Pynn: It is not obvious to me that thesociology of the field has changed muchhere. There is just no coordinated lead-ership anywhere in this country. Asa small beginning, LANSCE and thepulsed source at Argonne now havethe same advisory board passing judg-ment on experimental proposals, butany talk of a national committee seemsto fall on deaf ears. It is very hard toget people from the various facilities towork together at this, but we have to try

and collaborate as we can. 1 don’t thinkthe field will move ahead in the UnitedStates unless we do.Science: Is that why you left the ILLand came here?Pynn: Not really. You have to realizethat the ILL has set the standard in neu-tron scattering for the last decade andwill probably do so for the next, but Ihad basically done everything I wantedto do there. I had built a polarized-

Until quite recently the ideaof a facility catering to out-side users didn’t exist in theDOE. Isolated groups ofprofessional neutron scat-terers ran every one of theU.S. facilities.

neutron spectrometer; I had participatedin many exciting experiments; and I hadworked throughout the ILL organiza-tion in various capacities. 1 could havestayed and done my own research there.but LANSCE was a challenge to me.After coming here as a consultant, I be-gan to wonder if it could be made betterthan its European competition. I thinkwe have succeeded in some ways, butwe could do a lot better with only 20percent more money.Science: Is LANSCE now a state-of-the-art neutron source’?Pynn: Before answering that questionwe should ask if LANSCE is a state-of-the art spallation source. Most neutron-scattering facilities, and most of the



ones I’ve mentioned today, use beamsof neutrons produced by reactors. Spal-lation sources, on the other hand, directa beam of accelerated particles onto atarget, which then emits bursts of highenergy neutrons. With that said, oneway to answer my question is in termsof the reliability of the beam-deliverysystem.

In 1988, we had an awful time; onaverage, we had neutrons on the sam-ples in our spectrometers during only50 percent of the time we scheduled.Since we run a user program with peo-ple coming in from out of town, thatis just a complete disaster. It really is.Suppose you have some guy come inwith samples that last a few hours ordays, and the beam is down. There goes$10,000 worth of samples to the waste-basket because they couldn’t be run onthe machine. So we made reliability ourhighest priority this year and finishedwith the beam operating 74 percent ofthe time. That is an acceptable levelfor a user program, because a lot of the26-percent loss is an hour here and anhour there. In fact, that is almost ashigh a reliability as you can expect froma state-of-the-art accelerator source be-cause they are incredibly complicatedbeasts. The accelerator itself has allsorts of power supplies and magnets.You need to tune beams to get them intoclosed orbits-all sorts of complicatedthings like that. If an accelerator hasvery high reliability—and some do-itsdesign and performance are probablynot at the forefront of technology.

Another way to ask if we’re state-of-the-art involves the intensity of theneutron beams on our spectrometers.By the end of the 1989 run cycles, wehad a higher peak neutron flux than anyother spallation source in the world—and when the proton-storage ring isoperating at full capacity, our neutronfluxes will be even higher.Science: How does LANSCE comparewith the best reactor sources?

IPNS/LANSCE External Program Advisory Committee

Pynn: I like to use our small-angle- I have always taken thescattering machine, the Low-Q Diffrac-tometer, as an example, because many view that reactor and spal-people thought spallation sources would Iation sources are comple-not be suited to small-angle-scatteringexperiments. As always, the standard

mentary—that you needfor any comparison in neutron scattering both types if you want ais the similar machine at the ILL. Wehave optimized the LQD at LANSCE

complete neutron-scatteringso that our results are as good or better program.than the ILL’s when we probe lengthscales up to 500 angstroms or so. Atlarger length scales the ILL instrumentwins, but most experiments fall in therange I just mentioned. In that sensewe are competitive. However, I havealways taken the view that reactor andspallation sources are complementary—that you need both types if you want acomplete neutron-scattering program.You can do a lot of things with eachone that you can’t do with the other.For example, spallation sources are bet-ter for powder diffraction, but a sim-ple three-axis spectrometer at a reactorsource still produces inelastic-scatteringdata that we cannot duplicate.Science: Are you also competitive withthe ILL in the number of users?Pynn: In 1989, one hundred and eighty-six scientists were involved in exper-iments at LANSCE, and one hundredand twenty of them actually came andworked. In addition, about three thou-sand people are now on our mailing list.The IPNS at Argonne, which was thefirst neutron-scattering facility in thiscountry to have a user program. hasdone a tremendous job of bringing inusers and expanding the user commu-nity. We are gradually attracting moreand more people from universities and



industry, but it is a long uphill battle.Science: In what sense is it uphill? Dopeople have to be very brave to try aneutron-scattering experiment?Pynn: If you think of going to a facil-ity to do something that you have neverdone before, just wanting to get the an-swer to your scientific question and notknowing exactly what is involved, yourealize it must be quite daunting. Evengoing to the lab next door and borrow-ing a simple piece of equipment can beextremely difficult, and might even keepyou from an important discovery.Science: That’s human nature isn’t it?Pynn: I guess, but the adventurous peo-ple who overcome these barriers canbe extremely successful. Lots of peo-ple in Europe now use neutron scatter-ing and no other technique. They maybe at a university and have no lab oftheir own, but they can rely on doingneutron-scattering experiments at userfacilities.Science: They get hooked on the field?Pynn: Perhaps they get hooked on go-ing to Grenoble and skiing, either thator on French wine and cuisine! But Ido think it is very important to over-come the barriers that prevent scientistsfrom coming to user facilities, and wework very hard at it. In some sense theILL succeeded because they requiredonly a one-page experiment proposaland would pay travel fare and living ex-penses if they accepted it. So peoplehad nothing to lose. The ILL is veryunusual in paying those expenses; wedon’t have that kind of budget at LAN-SCE. Including the proton-storage ring,our budget in 1988 was between $14million and $15 million dollars per year,whereas the ILL’s was over $50 million.Science: How do the committees deter-mine who gets to do experiments?Pynn: My answer to that depends onwhich hat I wear, my user’s hat or myLANSCE director’s hat. Recently Igot the results from four proposals Isubmitted to the ILL. In that case I’m

the user, so I sometimes think the ILLcommittees toss a coin and don’t con-sider scientific merit at all. I had oneproposal out of four turned down, yousee, so I argue that the committee justdidn’t understand that proposal. I’msure all users have that attitude, but wetry, in principle, to get together a groupof people who can judge. That is ex-tremely hard to do. It comes back tothe question whether theory should leadexperiment or not. If I propose to you

Lots of people in Europenow use neutron scatteringand no other technique...They can rely on doing neu-tron-scattering experimentsat user facilities.

an experiment to look for Landau’s ro-ton, you will probably give me beamtime, provided you understand the the-ory of the roton and understand thatneutrons can find it. It is an essentialexperiment; I could end up verifyingLandau’s theory or demolishing it. Butsuppose I said to you, “There is an ex-tremely good theory of magnetic ex-citations in a material called TMMC.Among other things it predicts fourmodes, and I want to see whether therereally are four.” You would probablytell me to do something rude, right? Infact, I took part in that experiment, andwe happened to identify five modes in-stead of four. We got beam time onlybecause we had proposed somethingelse.Science: Do you think the committeesare too conservative?Pynn: Quite often, yes. That is oneof the great disadvantages of the usersystem, and I assume it must be thesame for a grant system unless you getsomebody who says, “Let’s risk it. Thislooks like wild stuff but it just may pay

off.” Even so, I’m sure an open systemof proposals and reviews is better than aprivate party where a few people controland use the beam time.Science: Who at your facility choosesthe experiments?Pynn: I mentioned that we have a jointprogram advisory committee with thefacility at Argonne National Lab. Peo-ple submit proposals to both facili-ties, and the committee breaks up intothree subcommittees: one that looks atdiffraction, one that looks at small-anglescattering and reflectometry, and a thirdthat looks at inelastic scattering. Un-fortunately, these are technique-orientedgroups, so there may be only one per-son in each group who is an expert ona particular type of science. It is hardto be sure that you always get the bestdecisions out of a committee like that.But to have a wider scientific debatein each subcommittee, you need moremembers, and that costs money. Theway to solve the problem, in my view,is to involve other scattering centers inthe same committee and share the costs.Science: Is there a collaborative ar-rangement for the people who comehere to do science?Pynn: Users aren’t required to haveany experience with neutron scatteringto come do an experiment at LANSCE.Instead, we generally set it up so eachuser has a local contact, some card-carrying member of the neutron Mafiawho doesn’t mind what he shoots hisneutrons at.Science: Let’s talk about your connec-tion with the rest of the Laboratory.Pynn: Well, we have a number of peo-ple from different divisions who workmore or less permanently at LANSCE.We have people from the Life SciencesDivision, someone from Materials Sci-ence and Technology, and someonefrom Chemical and Laser Sciences aswell. That’s about the extent of it rightnow. Even here inside the Lab there arestill a lot of people who don’t realize

44 LOS Alamos Science Summer 1990


the useful information they can get withneutron scattering. I know this becauseI sat on a committee which reviewedproposals for internally supported re-search [ISRD], and last year a numberof them could have used neutrons butdidn’t propose to do so. They were notintentionally ignoring neutron scattering;they just didn’t think of it because theydidn’t know it was available or what itcould do. To a large extent that is ourfault—we have to get the informationout there. With that in mind, we havebeen trying to set up a committee withrepresentatives from lots of different di-visions to try and acquaint people withthe neutron-scattering facility, and toteach them which experiments in theirwork can use neutrons. We hope theywill eventually become advocates ofneutron scattering in the Lab. In partic-ular, I think we haven’t done a good jobof selling to the weapons community.There is a communications problem be-cause we are in an open area, and thereare a number of us, including me, whocan ‘t hear classified information.Science: That’s right. You’re an alien.Pynn: I’m an alien and if I’m going todiscuss weapons-related neutron scatter-ing with people, it has to be in a verygeneric sense—but that is often goodenough. It is only when you are plan-ning the details of the experiments thatyou need to know details of materialsand composition or of shapes and formsand sizes, So I can usually tell someonewhether their experiment makes sensewithout knowing classified details.

The problems I have as an alien arenot technical; they're bureaucratic. Onething which seems to have changedsince I came to work in Los Alamos isthe thicket of rules about foreign nation-als at DOE facilities. I can understandthe need for security and I respect it,but some of the new rules aren’t wellconsidered or in the national interest.For example, there was a draft regula-tion which would have prevented for-


eign nationals from using almost anycomputer at a DOE facility-includingPCs—for any kind of work, classifiedor not. That’s not very smart. The U.S.needs its scientific contacts with the restof the world-perhaps now more thanin the past. So a sensible compromisehas to be found which encourages inter-national science and preserves nationalsecurity. I have to say, though, I havenever experienced any discriminationtowards me here at Los Alamos. Thepeople here are always very thoughtfuland hospitable, even though the bureau-cracy can sometimes give real meaningto the word “alien”-a green thing withhorns that spies or worse.Science: What do you see as the futureof neutron scattering?Pynn: I talked before about the needfor a next-generation source. Ideally,we should build both a reactor sourceand a spallation source—and also keepthe older facilities for more patient de-velopment of the field. That way wecould do the science which needs high-intensity sources, and people wouldalso have places to think, work, trynew ideas, and train students withouttoo much pressure. 1 don’t think youcan have a very healthy program with-out such places. However, on a morerealistic level, we need to expand ouruser base in order to generate supportfor at least one new source. If basic re-searchers and people from industry com-bined their support, their voice would bedifficult to ignore. So we have to con-tribute as we can to technological andindustrial problems, as well as to ba-sic research. Finally, neutron scatterersneed to speak with a unified voice andwork together to produce a coherent na-tional policy. At the moment, some ofus are trying to create a national steer-ing committee. If we can accomplish

The U.S. needs its scientificcontacts with the rest of theworld—perhaps now morethan in the past. So a sen-sible compromise has to befound which encourages in-ternational science and pre-serves national security.

that and maintain the present fragileunity, I think the field will be poisedmove ahead, ■

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magine alchemists struggling in the · magine alchemists struggling in the dark with materials they...

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