Particle accelerators have playedan important role at LosAlamos ever since the

Manhattan Project, when beams fromtwo Van de Graaff accelerators and acyclotron were used to answer experi-mentally important nuclear-physicsquestions. The data obtained fromthose machines helped ensure successof the “gadget” and its descendents.

Today, an 800-million-electron-volt(MeV) proton linear accelerator, orlinac, forms the backbone of theLaboratory’s national user facility, theLos Alamos Neutron Science Center(LANSCE), shown in Figure 1.Protons from the linac, travelling at84 percent the speed of light, smashinto a heavy-metal target and, througha process known as spallation, pro-duce copious numbers of neutrons.The neutrons are used in experimentsthat support the weapons program andadvance basic research. The linac’sproton beam can simultaneously beused in a capability known as protonradiography, or pRad, to make high-speed “movies” of dynamic systems.(See the article “The Development ofFlash Radiography” on page 76.)

A Brief History of the LANSCE Complex

The proton linac used by LANSCEwas originally built in the late 1960s toproduce pi-mesons, also known aspions, for the Los Alamos MesonPhysics Facility (LAMPF). The pionsand their decay products (muons, elec-tron, and muon neutrinos) were used toconduct fundamental studies innuclear, atomic, and particle physics.

The idea for a pion factory was

first mentioned in a memo by LouisRosen, dated May 16, 1962. As Rosendescribed the situation, this was a crit-ical time in the history of what wasthen known as Los Alamos ScientificLaboratory. The Cold War was at itsheight, and the Laboratory had suc-cessfully developed our nuclear deter-rent—the fission and thermonuclearweapons now in the stockpile. Newchallenges and new facilities in thefield of nuclear science would beneeded to maintain and advance theLaboratory’s world-class capabilities.The meson factory would benefit theLaboratory and the nation by provid-ing the experimental capabilities thatwould support the Laboratory’s pro-grammatic needs for the weapons pro-gram. In addition, the facility wouldfoster opportunities in basic research,and be the catalyst for a scientific userprogram that would bring leading sci-entists and students to Los Alamosfrom all over the world.

However, it was a tremendoustechnical challenge to produce protonswith enough energy to support pionproduction. The protons had to havemore than 290 MeV of kinetic energyin the laboratory just to create thepion rest mass, but at that time, pro-ton linac energies had not exceeded100 MeV. After two years of designstudies, a bold proposal was made inSeptember 1964 for an 800-MeVhigh-current (1-milliampere averagecurrent) proton linac for medium-energy physics research. The innova-tive development that made thisenergy scale feasible was the side-coupled linac accelerating structure(see Figure 2).

The Medium-Energy PhysicsDivision was formed in July 1965

(with Rosen as division leader) to con-tinue with the design and developmentof the meson facility. Constructionfunds were authorized by Congress inthe fiscal year 1968 budget, and physi-cal construction began with a ground-breaking ceremony on February 15,1968. Only four years later, on June 8,1972, this new linac delivered its firstbeam. The machine builders can takegreat pride that LAMPF was com-pleted on schedule and within budget($57 million). LAMPF also achievedall its major goals. In September 1972,the facility was dedicated to U.S.Senator Clinton P. Anderson of NewMexico, who was instrumental in turn-ing the project into reality.

An interesting footnote to theLAMPF story is that the side-coupledlinac was later widely adopted byindustry. As a result, thousands of side-coupled linacs were eventually pro-duced around the world for radiationtherapy. Thus, Los Alamos researchand development resulted in one of themost beneficial uses of nuclear-physicstechnology for mankind.

It was clear even before LAMPFwas built that, with a neutron-rich,heavy-element target, rather than thelight-element target that was suitablefor pion production, neutrons wouldbe produced by spallation and thatsuch a source would compete veryfavorably with nuclear reactor-basedneutron sources. Based on that prem-ise, the Weapons Neutron Research(WNR) facility, consisting of aproton-beam transport system and aspallation-neutron target, was built inthe LAMPF complex to support theLaboratory’s materials-science andnuclear-physics programmatic needs.The WNR facility produced neutrons

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for the first time in May 1977 andgave the Laboratory an intense neu-tron source that could be used tostudy the behavior of matter atextreme temperatures and densities, aswell as to study radiation effects andobtain nuclear data needed forweapons design.

The LAMPF complex expandedagain in the early 1980s with the addi-tion of a unique 30-meter-diameterProton Storage Ring (PSR), whichaccumulates the proton beam injectedfrom the linac and compresses the rel-atively long 625-microsecond pulsesinto short 125-nanosecond pulses.These short bursts of protons are thendirected to the heavy-element target toproduce very short bursts of spallationneutrons. The short pulses allow time-of-flight measurements for a moreprecise determination of the energyand wavelength of the neutrons.Construction of the PSR began inMay 1982 and was completed in April1985. The first beam was circulated inthe ring on April 26, 1985.

In 1986, construction was startedon a new experimental area (adjacentto the existing WNR facility) that wassummarily filled with a suite of world-class instruments. The new area wasnamed the Manuel Lujan Jr. NeutronScattering Center (the Lujan Center) inhonor of a popular congressman fromNew Mexico. Scientists use the low-energy neutrons at the Lujan Center toperform novel studies of materials.The old WNR facility was also rebuiltduring this construction project to pro-vide another spallation source thatconcentrated on producing beams ofhigher-energy neutrons for nuclear sci-ence. Both new sources came intooperation in the early 1990s, justbefore the medium-energy program ofpion and nuclear physics at LAMPFcame to an end.

With the closeout of the nuclearphysics user program and anincreased national need for neutrons,the mission of the LAMPF accelerator

complex was changed from nuclearphysics to neutron research. InOctober 1995, the complex wasrenamed as LANSCE.

The LANSCE ComplexToday

The combination of the LujanCenter and the WNR facility givesresearchers access to high-intensityneutron beams covering 16 ordersof magnitude in energy, and theLANSCE complex has evolved intoa major international resource; arecord 750 user visits by scientistsfrom around the world occurredduring the last operating period.Because so many students and pro-fessors visit each year, LANSCE isone of the Laboratory’s most impor-tant “windows” into the academiccommunity and a source of many ofour brightest early-career scientists.It is estimated that LANSCE and itspredecessor, LAMPF, have servedas a gateway to 10 percent of theLaboratory staff.

Within the past year, users haveachieved many significant accom-plishments in materials science,nuclear science, and technology. Afew highlights are described below.

Research at the WNR Facility.The WNR facility is the only remain-ing broad-spectrum neutron sourceavailable to conduct the kinds of pre-cise nuclear science measurementsneeded to develop predictive capabil-ity in the weapons program. As anexample of this need, a problem thathas been around since the early daysof nuclear weapons was recentlysolved through accurate measure-ments of the plutonium (n,2n) crosssection, which is needed to quantifythe production of plutonium-238 fromplutonium-239. Knowledge of the plu-tonium-238 production rate over arange of incident neutron energies isessential for understanding pastNevada Test Site (NTS) data. This

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Figure 1. The LANSCE ComplexThe half-mile-long 800-MeV proton linac is the backbone of the complex.

Figure 2. From 100 to 800 MeVThe side-coupled linac acceleratingstructure (100 to 800 MeV) wasinvented by Los Alamos acceleratorscientists during the design of LAMPF.

definitive measurement has allowed usto significantly improve our predictivecapabilities and resolve complicatedphysics issues in the performance ofour weapons that were previously notunderstood.

Another use of WNR neutronsarises because electronic assemblies,particularly those used in high-altitudeaircraft, are subject to neutron-induced “upsets” at the altitudes atwhich they operate. The neutron-beamenergy spectrum from WNR mimicsneutrons seen by aircraft electronicsin flight, but with an intensity onemillion times stronger. WNR is nowthe standard facility for testing neu-tron-induced upsets in electronics.Thirteen major companies used thefacility in 2002 to validate the opera-tion of selected electronics. Similarly,Los Alamos researchers must addresswhether neutron-induced upsets couldaffect the ASCI Q machine, one of themost powerful supercomputers in theworld and a pillar of the StockpileStewardship Program. The intense

neutron source at LANSCE was usedto conduct a study of the impact ofcosmic-ray-produced neutrons on theQ machine’s reliability.

Research at the Lujan Center.Neutron-scattering experiments atLANSCE have provided a newmethod for characterizing the basicmaterial properties of plutonium, andwe now have the ability to compareplutonium parts that were produced bydifferent manufacturing processes. Theoriginal parts were made at the RockyFlats facility, which is now closed, andnewly manufactured parts are made bya different process. The new character-ization method allows us to addressthe question of whether the changewill affect weapons performance orsafety, two critical issues for theenduring nuclear-weapons stockpile.

Stockpile stewardship also requiresthat weapons modelers have an accu-rate, well-understood equation of state(EOS) of plutonium. Otherwise, it isimpossible to construct a dependablemodel of weapons performance.Inelastic neutron scattering from aplutonium-gallium alloy allowed thefirst-ever determination of phonondensity of states, which is an integralcomponent in understanding the EOS(see Figure 3).

The high-pressure preferred orienta-tion (referred to as HIPPO) diffractome-ter at the Lujan Center can make in situneutron-diffraction measurements ofsamples that are at high temperature.Recent experiments revealed texturechanges in quartzite that establish thisrock as a shape-memory system. Insuch a system, the orientation of grainsis controlled by internal stresses. Shapememory is a desirable attribute formany applications. For example, if aneyeglass frame made from ashape-memory alloy were to becomebent, only modest heating would berequired to return the frame to its origi-nal condition. The revelation that Earthmaterials have a similar attribute could

have profound implications about theplasticity of Earth’s crust.

Another research avenue involvedhelium, which is the decay product ofthe tritium used in nuclear weapons.The partial pressure of helium in aweapon’s neutron generator is a life-time-limiting factor for components ofthe stockpile. Working with SandiaNational Laboratories, scientists at theLujan Center investigated heliumretention, providing information thathas significantly influenced the design.

The Lujan Center has completedconstruction of a major new instru-ment called Asterix, which provides apolarized neutron beam for studies ofmagnetic materials and spin polariza-tion. Asterix made it possible to ana-lyze the structure of crystalline andpolycrystalline materials through theuse of neutron scattering while thematerials are located in a magneticfield that is 100,000 times strongerthan Earth’s magnetic field.

The Lujan Center is also home tothe protein crystallography station,the only neutron-scattering instru-ment in the world designed specifi-cally to investigate the properties ofproteins. It performed the first stud-ies of electric-field-induced structuralchanges in an organic single crystal(see Figure 4).

Finally, LANSCE has completedconstruction of a major new detectorsystem that will enable, for the firsttime, measurements of the nuclearproperties of radioactive targets usingas little as 1 microgram of material.These measurements will affect suchdiverse areas as the analysis of radio-chemical information from pastunderground nuclear tests, models ofastrophysical processes, and technicalissues in homeland defense.

pRad. Protons that are used to pro-duce neutrons for the Lujan Centerand the WNR facility can instead beused directly. Similar to the way anordinary photon source is used to take

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0.45

0.40

0.35

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0.000 5

Pho

non

DO

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15 K

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Figure 3. EOS Measurements Shown here is the phonon density ofstates of a δ-phase plutonium-aluminumalloy at different temperatures. The peaksin the data give the distribution ofphonon-mode frequencies in the alloy.

a picture, in pRad the properties of theprotons are exploited to create a radi-ographic “motion picture” with aframe speed of 50 billionths of a sec-ond. The pRad capability has allowedus to understand better the aging andperformance of weapons systemscomponents and to observe the prop-erties of materials shocked by highexplosives. During the 2002 run cycle,42 dynamic pRad experiments wereperformed at LANSCE in support ofweapons-physics research efforts atSandia and, Lawrence LivermoreNational Laboratories and at theAldermaston Weapons Establishment.A unique permanent-magnet proton“microscope” system, designed byLANSCE and fielded by the Physics

Division, has produced radiographswith up to 15-micrometer resolution,opening an entirely new realm forstudying material features underextremes of pressure and speed.

New Facilities. There are two newareas under construction to furtherenhance the versatility of LANSCE. In2003, a medical radioisotope facility,called the Isotope Production Facility(IPF), will begin operations. The IPFwill allow LANSCE to provide theresearch community with medicalradioisotopes, many of which are nototherwise available in the UnitedStates. In a separate area that reusesone of the old experimental areas fromLAMPF, researchers have demonstratedthe ability to produce the most intensesource of ultracold neutrons (UCNs) inthe world. UCNs move at speeds notmuch higher than those at whichhumans can run, and they have uniqueproperties that allow them to be “bot-tled” and used for research. Workfunded by the Department of Energy isunder way to complete a facility forprecision studies of forefront problemsin physics and cosmology using UCNs.

The Future

The LANSCE spallation neutronsource of 2003 owes its existence to theaccelerator work done more than 30years ago to design and build the origi-nal LAMPF complex. Today, majorprogress in accelerator technology con-tinues. Los Alamos is doing workimportant for the Spallation NeutronSource (SNS) under construction at OakRidge National Laboratory inTennessee. The $1.4 billion project—expected to deliver first beam in 2006—uses a particle accelerator that operatesat an average power 10 times that of theLujan Center. Mercury containers wereirradiated at LANSCE to help scientistsdevelop techniques to increase the life-time of the SNS target (see Figure 5).

Although the future of neutron sci-ence is shifting to the SNS, LANSCEwill still play an important role. Withits low-repetition rate, high peak-intensity pulse from the PSR, andinnovative neutron-production source,the Lujan Center will be a powerfulforce in neutron scattering and funda-mental physics research for manyyears. The WNR facility will remainthe only neutron source in the worldthat can perform basic and appliedneutron science over the range neededfor defense and advanced nuclearapplications. In addition, the pRadfacility will continue to be invaluablefor defense and, possibly, for otherresearch.�

Paul W. Lisowski became the director ofLANSCE in 2001. In his many years at LosAlamos, he has served in a broad range of tech-nical and management positions. These includegroup leader for neutron and nuclear science,project leader and projectdirector for the NationalAccelerator Production ofTritium (APT) Project, anddirector of theLaboratory’s AdvancedHydrotest Facility (AHF)project. His technicalinterests include basic andapplied science with neu-trons and protons, applications of spallationsource technology, and particle acceleratordesign and operation. Paul received his doctor-ate in nuclear physics from Duke University.

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Figure 4. Protein CrystallographyPaul Langan of the BioscienceDivision points out the single-crystalα-glycine sample used in the proteincrystallography station.

Figure 5. Target Investigation A scanning-electron-microscope image ofa large pit produced in a steel container.The pit resulted when a bubble of mercurycollapsed.The bubble had been generatedby an intense proton pulse during a test ofSNS target technology at LANSCE.

Thomas P. Wangler received his Ph.D. inphysics from the University of Wisconsin in1964. He worked in experi-mental high-energy physicsat Brookhaven andArgonne NationalLaboratories and hasworked in the particleaccelerator field atArgonne and Los Alamosfor 28 years. Thomas is atechnical staff member inLANSCE Division, as well as a Los Alamos fel-low. His main areas of interest in the acceleratorfield are the physics of high-intensity beams,proton linear-accelerators, and superconductinglinacs. Thomas is author of the book Principlesof RF Linear Accelerators published in 1998.