4th quarter 2000 the actinide research los alamos national ... · the actinide researchlos alamos...

1Nuclear Materials Technology/Los Alamos National Laboratory

In This Issue

1239Pu/240Pu Isotope

Ratios Can Be Deter-mined Using LIBS

4Studies of Actinide

PerfluorodiphenylamidoComplexes Support

Science Mission

6 Actinide Research

Quarterly Celebrates aSix-Year Anniversary–A

Challenging Time

8Recent MeasurementsAdd to Understanding

of Pu Metal’s ElectronicStructure

11

Publications andInvited Talks

12Newsmakers

a U.S. Department of Energy Laboratory

Los Alamos National LaboratoryThe Actinide Research4th quarter 2000

N u c l e a r M a t e r i a l s R e s e a r c h a n d T e c h n o l o g y

continued on page 2

239Pu/240Pu Isotope Ratios Can BeDetermined Using LIBS

Laser Induced Breakdown Spectroscopy (LIBS) is also known aslaser ablation/optical emission spectroscopy. This type of high-resolu-tion emission spectroscopy has received considerable attention as aversatile analyticaltechnique. The tech-nique has severaladvantages overother forms of analy-sis for nuclear appli-cations; those ofgreatest importanceat the Los AlamosPlutonium Facilityinclude reduction ofsample size, directanalysis in inhomo-geneous matrices,reduced turnaroundtime betweensample submittaland results, andin-situ analysis capa-bility. The Pluto-nium Facility currently employs standard inductively coupled plasmamass spectrometry/atomic emission spectroscopy for most chemicalanalyses, with dissolution of a fairly large sample (~ 0.5 g) normallyrequired. Isotope ratios are determined by thermal ionization massspectrometry (TIMS) or gamma spectrometry. LIBS has been previ-ously applied to actinide analysis, including recent work involvingU isotope ratio determination.

Observation of isotope shifts through the use of optical emissionspectroscopy is not a common application for LIBS, mainly because ofthe very high resolution needed. However, a French research group atCEA Saclay has recently reported a LIBS isotope ratio determinationfor uranium using the U(II) line at 424.437 nm with a 238U/235U isotopeshift of 1.39 cm-1 (0.025 nm). Their observed linewidth of 0.67 cm-1

(0.012 nm) was approximately twice as broad as the instrument limit

Figure 1.A LIBS plasmaplume producedfrom a lump ofelectrorefined Pumetal. The laserimpinges on thesample verticallyfrom the top, withemission collectedhorizontally by the40 mm diameter, f/3lens at far left. Thesample chamberviewport is approx.2.5” in diameter.

Quarterly

Actinide Research Quarterly

2 Nuclear Materials Technology/Los Alamos National Laboratory

This article wascontributed byColeman A. Smith(NMT-11). Otherresearchers on theproject include MaxA. Martinez and D.Kirk Veirs (NMT-11and David A.Cremers (C-1).

239Pu/240Pu Isotope Ratios Can BeDetermined Using LIBS (continued)

of 0.305 cm-1 (0.0055 nm). This linewidth wasproduced using experimental conditions of308-nm laser wavelength, 0.46-GW/cm2 fo-cused laser energy density, a delay time of<500 ns, and 3 Pa (~20 mtorr) of air as thebuffer gas.

One of the largest plutonium isotope shiftsoccurs in the Pu(I) emission line at 594.52202nm, which has a 239Pu/240Pu isotope shift of0.355 cm-1, approximately one-fourth of theisotope shift of the U(II) transition. Thesmaller value for this isotope shift is partiallydue to the 1 atomic mass unit (amu) differencebetween common Pu isotopes as compared tothe 3-amu difference between the two mostabundant natural isotopes of uranium. Thissmall isotope shift makes an isotope ratiodetermination more challenging. Under theexperimental conditions reported for the ob-servation of the uranium isotope ratio, wherethe linewidth is 0.67 cm-1, the emission fromthe plutonium isotopes would not be resolved,and an isotopic ratio measurement of 239Pu/240Pu using LIBS would not be possible. Thelinewidth reported in the French work is at-tributed to Doppler and Stark broadening inthe very hot plasma of the French experiment.A higher buffer-gas pressure and longer delaytimes should result in a cooler plasma, inwhich smaller isotope shifts may be observed.We show that this approach allows the LIBSmeasurement of the 239Pu/240Pu isotope ratio.

The LIBS system employs the followingmajor components: a pulsed laser, samplechamber, emission spectrometer, detector, andcomputer. Work within the Plutonium Facilityrequires that radioactive samples be containedinside a glove box. Only the sample chamberand a minimum of optics reside within theglove box (see Figure 1); all other optical com-ponents are mounted on external opticaltables. A pulsed and Q-switched Nd:YAGlaser was used, operating at its fundamentalwavelength of 1064 nm and generating pulsewidths of ~5 ns. The laser energy is typicallyattenuated to ~25 mJ and focused into a <100-mm spot on the sample surface, producing an

energy density of ~1011 W/cm2. This high en-ergy density is sufficient to both vaporize thesample surface and to generate a high-tem-perature plasma consisting of both neutralatoms and ions. The plasma species are elec-tronically excited and emit light characteristicof all elements present in the ablation plume.The sample is enclosed in a vacuum chamberthat is backfilled with helium at 100 torr pres-sure. This atmosphere allows some transla-tional and electronic cooling of the plasmawithout excessive quenching of the emission.The emission is collected and collimated beforeexiting the glove box through a secondviewport. The light is then focused onto theentrance slit of a high-resolution spectrometer,where it is wavelength-dispersed and collectedon an intensified charge-coupled device(ICCD) camera. A broad continuum emissionbackground is avoided by electronically delay-ing the ICCD by 1 ms following the laser pulsecollecting the emission over a gate width of5 ms.

A Pu transition was selected for analysisbased on its large isotope shift, high intensity,and minimal spectral interference from neigh-boring lines. Atomic emission dominates theplasma at longer ICCD delay times, and as theplasma cools, expansion slows, thus allowingmore electron-ion recombinations to occur. Asa result, atomic emission is favored because ofa decrease in both Doppler and Stark linebroadening. Thus, the Pu atomic line at594.52202 nm (16815.576 cm-1) was chosen forthis work, with a previously observed 239Pu/240Pu isotope shift of 0.355 cm-1.

Two samples of greatly differing isotopiccomposition were chosen for characterization.The first sample consisted of Pu metal of anominal 239/240 isotopic ratio of 93/6. Thesecond sample was PuO2 with a nominal ratioof 49/51. The oxide sample was analyzed as apressed pellet.

4th quarter 2000

Nuclear Materials Technology/Los Alamos National Laboratory 3

2002.0e+5

4.0e+5

6.0e+5

8.0e+5

1.0e+6

1.2e+6

1.4e+6

1.6e+6

250 300 350 400

Pixel

Pu Oxide, 239/240 isotope ration = 49/51

Inte

nsity

Pu-239pixel 299

Pu-240pixel 306

The 239Pu/240Pu isotope shift of 0.355 cm-1

from the plutonium atomic line at 594.52202nm is clearly resolved in our plasma condi-tions (see Figure 2). Curve-fitting yields inte-grated peak-area ratios that match isotopicratios as measured by TIMS or gamma spec-trometry within ±0.5%. The curve fit typicallyproduces a peak separation of ~6.7 pixels, withfull-width-at-half-maximum at ~3.2 pixels. Theobserved plutonium linewidths are calculatedto be 0.19 cm-1 (0.0067 nm) based on the knowndispersion of the spectrometer. Theselinewidths are within the experimental error ofthe ideal instrument-limited linewidth, whichis calculated to be 0.15 cm-1 (0.0052 nm) basedupon the known modulation transfer functionfor the ICCD system.

In the uranium work by the French group,the uranium plasma is dominated by ioniclines that exhibit a larger degree of broaden-ing. They study the 238U/235U isotope shift us-ing an ionic line at 424.437 nm with an isotopeshift of 0.025 nm, or 1.39 cm-1, and report linebroadening exceeding the instrument limit bya factor of 2 (observed linewidth ~0.012 nm,instrument limit 0.0055 nm). A Doppler widthof 0.0106 nm was calculated for the U(II) line,with Stark width ~0.002 nm at a delay time of350 ns and ~20 mtorr buffer-gas pressure.

Although the experimental apparatusused by the French group closely resemblesthat of our work, there are major differences inthe experimental conditions that greatly affectintrinsic linewidth. In particular, our condi-tions of increased buffer gas pressure andlonger delay time allow the plasma to achievemuch lower translational temperatures, inspite of much higher laser energy densities.The use of a longer wavelength emission linefor analysis provides greater peak separation.These effects result in smaller linewidths andadequate resolution that allow the plutoniumisotopic ratio to be determined. The linewidthsobserved under our conditions for plutonium

Figure 2. LIBS pro-duced a one-dimen-sional spectrum of thePu emission line from239Pu/240Pu isotopes inPuO2 at 594.52202 nm,clearly showing resolu-tion of the 239Pu/240Puisotope shift. The LIBSsetup at Los Alamosproduces conditionsbetter suited to such finediscriminations inactinides than a similarFrench experiment does(recently conducted atCEA Saclay).

should be applicable to all of the light ac-tinides (Th-Cm) under the same experimentalconditions because the first ionization poten-tials are similar, ranging from 5.97 eV for Amto 6.27 eV for Np, and changes in the relativemasses are small. Since the reported largestisotope shifts for Pu and U are in the range of0.3–0.5 cm-1/amu, the isotopic shifts for all ofthe light actinides should be similar, and LIBSshould be applicable for isotopic ratio determi-nations across the light actinide series.

We have shown that LIBS may be appliedto plutonium isotopic analysis. The techniqueis sensitive, essentially nondestructive, andcan produce accurate results with reasonableprecision. The accuracy of the technique criti-cally depends on adequate spectral resolutionof isotopic emission. We find that our condi-tions are well suited to resolution of 1-amu iso-tope shifts, and we expect that analysis of allactinides are possible under our conditions, ofwhich the most important are long delay timesand high buffer-gas pressures.



Studies of Actinide PerfluorodiphenylamidoComplexes Support Science Mission

This article wascontributed bySusan M. Oldhamand Ann R Schake(NMT-5);Arthur N. MorganIII (NMT-11); andBrian L. Scott,Benjamin P.Warner, and JohnG. Watkin (C-SIC).

Fundamental chemistry of the actinideelements provides important support for themission at Los Alamos. In contrast to otheraspects of plutonium research, details of itsmolecular chemistry remain relatively un-known. This knowledge, however, can pro-vide the basis for understanding importantmetallurgical properties relevant to the weap-ons mission and development of process-re-lated chemistry and separation chemistry, aswell as benefiting the Shelf Life Extension, En-hanced Surveillance, and Stockpile Steward-ship Programs.

Our goal is to elucidate the fundamentalchemistry of the actinides by focusing our ef-forts on preparation and characterization ofsimple, well-defined systems and comparingtheir molecular structure and reactivity to wellstudied lanthanide and transition metal ana-logs. The subject of this article is our work inpreparing a set of actinide complexes sup-ported by amido ligands. The first uraniumamido complex was reported in 1956 byGilman and coworkers at Iowa State College,detailing their efforts to prepare volatile ura-nium complexes for the Manhattan Project.Tetrakis(diethylamido) uranium was preparedby the reaction of the lithium salt of the aminewith uranium tetrachloride and then isolatedby distillation under reduced pressure. Otheractinide amido complexes have also been re-ported to be volatile. The tris(silylamido) com-plexes of uranium, neptunium, and plutoniumsublime around 60°C at 10-4–10-5 Torr.

Currently, several researchgroups in North America, GreatBritain, and Europe are activelyinvestigating actinide amido com-plexes, principally as well-definedprecursor materials for syntheticchemistry. Recent interest lies in

their rich reaction chemistry, including activa-tion of molecular nitrogen. Homoleptic tetra-and tri-amido complexes are generally solublein common organic solvents and are thereforevaluable entries in the synthesis of moisture-sensitive organometallic complexes of the +4and +3 oxidation states. With the dual goals ofexploring the electrophilic chemistry of the ac-tinide elements and also developing volatile or-ganometallic actinide complexes for chemicalvapor deposition applications, we have set outto prepare a series of Th, U, Np, and Pu com-plexes stabilized by perfluorodiphenylamidoligands—N(C6F5)2.

Perfluorodiphenylamido ligands have beenused previously to stabilize highly electrophiliclanthanide complexes. X-ray diffraction studieshave revealed that the lanthanide amido unit isquite electrophilic and readily forms weak in-tramolecular interactions to saturate the coordi-nation sphere of the large lanthanide(III) ions.Of note is the (trisamido) neodymium complex(h6-C6H5Me)Nd[N(C6F5)2]3 (Fig. 1) that coordi-nates an h6-bound toluene solvent molecule aswell as three significant Nd-F interactions (2.6to 2.7 Å) and one weak Nd-F interaction (2.9 Å).

Recently we have synthesized several noveluranium amido complexes that have been char-acterized by single-crystal x-ray diffraction. Theelectrophilic nature of these complexes is exem-plified by the reaction of UCl4 with four equiva-lents of NaN(C6F5)2 in tetrahydrofuran (THF).Crystallization from toluene yields lime crystalsof the “ate” complex [Na(thf)5][U[N(C6F5)2]4Cl],in which one chloride ligand remains coordi-nated to the uranium metal center to yield ananionic U(IV) complex (Fig. 2). Close intramo-lecular contacts between three ortho-fluorineatoms and uranium (2.6 to 2.7 Å) further testifyto the electron deficiency of the uranium atom.

This result was unexpected since we hopedto synthesize the homoleptic U(IV) amido com-plex; however, there is precedence of the forma-tion of “ate” complexes. In France in 1998Ephritikhine prepared the [Li][U(NEt2)4Cl]analogue during his attempts to synthesizeU(NEt2)4. The average U-N bond lengthsof thetwo complexes are similar with [Li][U(NEt2)4Cl]measuring 2.37 Å and [Na(thf)5][U[N(C6F5)2]4Cl]measuring 2.386 Å.

Nd

N(C6F5)2

N(C6F5)2(C6F5)2N

Figure 2. Thermal ellipsoid plot(30% probability ellipsoids) for[Na(thf)5][U[N(C6F5)2]4Cl]. For clarity,the [Na(thf)5] counter ion has beenomitted. Characterization of actinideamido complexes helps to elucidatethe fundamental chemistry of theactinide elements.

Figure 1. Trisamidoneodymium complex.

4th quarter 2000


While Ephritikhine reports that several ex-tractions of [Li][U(NEt2)4Cl] with ether allowsfor the complete removal of LiCl and the for-mation of U(NEt2)4, we find that reaction ofUCl4 with four equivalents of NaN(C6F5)2 inTHF conducted under more concentratedconditions does yield the desired homoleptictetrakisamido complex U[N(C6F5)2]4. Interest-ingly, actinide complexes supported by lessbulky amido ligands readily form dimeric(Fig. 3), or trimeric (Fig. 4) structures. Thehomoleptic diethylamido and dimethylamidouranium(IV) complexes are shown below.

Figure 3. Dimeric actinide amido complex

Figure 4. Trimeric actinide amido complex.

The single-crystal x-ray structure of ourred tetrakisamido complex (Fig. 5) is verysimilar to that of the nonfluorinateddiphenylamido complex. Each monomeric ura-nium metal center is coordinated to four nitro-gen atoms in a distorted tetrahedralstructure. The uranium nitrogen bond lengthsof the fluorinated and non-fluorinatedcomplexes are almost identical withaverages of 2.26 and 2.27 Å, respectively.The tetrakis(perfluordiphenylamido) uraniumcomplex also has contacts between the ura-nium metal center and three fluorine atoms,although they are slightly weaker than thosein the “ate” complex (2.8-2.9 Å). This complexis the subject of ongoing reactivity andvolatility studies.

Reaction of [UO2Cl2(thf)2]2 with fourequivalents of NaN(C6F5)2 in THF yieldsthe red uranyldiamido complexUO2(N(C6F5)2)2(thf)2. The single-crystal x-raystructure reveals an octahedral coordinationenvironment around the uranium metal center.The linear uranyl unit is augmented by twotrans amido ligands as well as two tetrahydro-furan solvent molecules (Fig. 6).

Figure 5. Thermalellipsoid plot (30%probability ellipsoids)for U[N(C6F5)2]4.

N

NU U NEt2

NEt2

NEt2

Et2N

Et2N

Et2N

Et2

Et2

N

N

N

U U NMet2

NMet2

NMet2

Me2N

Me2N

Me2NMe2

Me2

Me2N

NMe2

Me2

Me2N U

Figure 6. Thermalellipsoid plot (30%probability ellipsoids)for UO2(N(C6F5)2)2(thf)2.

Examples of structurally charac-terized uranyl amido complexes arerare. A comparison can be made be-tween our uranyl amido complex anduranyl alkoxide complexes. The ura-nyl moiety in the bis(perfluoro-diphenylamido) uranyl complex isshortened, with an average U-O bondlength of 1.75 Å compared to those ofuranyl alkoxide complexes with typi-cal bond lengths between 1.77-1.79 Å.

We are also attempting to preparetriamido complexes of plutonium(III)and neptunium(III). The tris(silylamido) com-plexes of neptunium and plutonium are theonly amido complexes reported for transuranicmetals. Starting from the tris(silylamido) com-plexes we have employed an aminoly-sis reaction. Reaction of threeequivalents of perfluorodiphenyla-mine with the blue-blacktris(silylamido) neptunium(III) com-plex in hexane forms an insoluble yel-low product that is subsequentlyfiltered from the solution and dried invacuo. Fluorine nuclear magneticresonance (NMR) spectroscopy of theneptunium complex shows reso-nances that have shifted from those offree ligand; however, the data are in-conclusive, and therefore completecharacterization is difficult. Similar reaction ofthree equivalents of perfluorodiphenylaminewith the orange tris(silylamido) plutonium(III)complex in hexane forms an insoluble light-purple product that is also filtered off anddried in vacuo. Attempts to grow single crys-tals of these transuranic complexes have beenunsuccessful to date. Further NMR studies andcrystallization attempts of both the neptuniumand plutonium complexes will resume in PF-4during the second quarter of FY01.

Our work has extended the library ofactinide complexes. Novel uranium amidocomplexes have been synthesized and charac-terized by single-crystal x-ray diffraction. Thereaction chemistry of perfluorodiphenylaminewith both neptunium and plutonium showsinteresting initial results. We are hopeful thatcomplete characterization of these transuraniccomplexes will be possible.



This issue of the Actinide Research Quar-terly marks the sixth year anniversary of thispublication, a significant accomplishment intechnical news reporting by a part-time, three-person publication team. We have not misseda single quarter except the summer of thisyear when we combined the 2nd and 3rd quar-ters for the timely reporting of the “PlutoniumFutures - The Science” conference with ex-tended coverage of this international confer-ence, held in Santa Fe in July.

Six years ago we started out with thesimple objective of highlighting recentachievements and ongoing programs of theNuclear Materials Technology (NMT) Divi-sion. In the beginning we did not have a verygood idea about who might read the newsletter and what might constitute readable materials in a technical newsletter. What has worried me addition- ally is, “Can we continue to tap enough source materials to report and write about quarter after quarter?”

While the NMT organization is large, its pro-grammatic activities complex and diverse, andits mission and national goals compelling, it isnot a daily event, or even a monthly event torecord technical successes and accomplish-ments year after year.

Every quarter brings unique challengesand difficulties in selecting reportable articlesand news items, in writing, rewriting, andediting the selected articles, and in presentingtimely editorials, all within the limited re-sources. This past year presented us with anadditional challenge: When the Laboratorygoes through one of the most challenging timesas witnessed during the past year: when LosAlamos is in the national news almost dailywith the disastrous Cerro Grande fire, thesecurity breaches investigation, the Wen HoLee case, etc., it is extremely difficult for eventhe most scientific minded to focus on scienceand to ignore all the happenings around us as only unneccesssary, irritating noises. An organizational newsletter is a living document so it suffers as do its

authors and readers when things are not going well.

Actinide Research Quarterly Celebrates aSix-Year Anniversary—A Challenging Time


Nuclear Materials Technology/Los Alamos National Laboratory6

Editorial

This editorial wascontributed byKyu C. Kim, NMTChief Scientist.

4th quarter 2000


The past two or three years have beennotoriously difficult for the coexistencebetween science and secrecy at the nation’snuclear Laboratories. For this situation myobservations are

Everywhere we lookUniverse-full of wonders to thestudents of science,A world-full of secrets to the ever-suspecting souls.

Everywhere we lookA limitless horizon of discoveryinto the unknowns,A countless number of opportuni-ties for secrets to the jealoussouls.

Everywhere we lookA deep valley runs between theworld of scienceAnd the proprietors of scientificknowledge.

Readers of this newsletter may not havenoticed any change of technical contents, butwe have expanded the sources of our newslet-ter materials to include other technical organi-zations besides the NMT Division. We nowrely upon continuing contributions of ourbroader actinide science community for thenewsletter’s articles. In this way we reaffirmthe inter-relatedness of the actinide sciencecommunity and highlight collaborationsamong various organizations and researchers.

The number of subscribers has increasedmore than three-fold over the six-year periodalthough a “subscription” to this newsletter isonly for the asking. We mail the newsletterworldwide, free of charge to individuals andorganizations that request it. Once in a while,we receive praise and encouragement from anumber of readers: One reader commented,“I have enjoyed The Actinide Research Quarterlyvery much, but the summer issue is special.The Seaborg article is superb, as it reminds usabout the excitement of discovery.” Anothercomments, “This good looking, quality news-letter is very informative.” We received threeawards of “Excellence in Technical Publica-tions” from the New Mexico Kachina Chapterof the Society for Technical Communicationalong the way.

The newsletter also has served anotherimportant avenue of communicating scientificevents worldwide. When the “Plutonium Fu-tures-The Science” conference convened inSanta Fe in 1997 and 2000, The Actinide Re-search Quarterly was the main medium for usto publish the conference programs and alsofor reporting the main events of the confer-ences. There were 14 countries in the firstconference and 15 countries in the secondrepresented; our circulation of The ActinideResearch Quarterly included all the participat-ing countries.

ARQ is now on the Web also. Our readerscan view entire back issues on-line http://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/AQhome/AQhome.htmlIts Sub-ject and author indexes are arranged.

Over time we, the publication team, real-ize how much we have learned just trying tokeep up with the most current events in theactinide community, while we try to informour readers. We find this to be one of the mostrewarding aspects of publishing The ActinideResearch Quarterly. In the end, every humanendeavor is a learning process—publishingthis newsletter is no exception. We draw im-mense satisfaction knowing that our readersand the publication team members have jour-neyed together the past six years on thislearning path.

4th quarter 2000




Recent Measurements Add to Understandingof Plutonium Metal’s Electronic Structure

This article wascontributed byRoland Schulze(NMT-16). Otherresearchers onthe project areJ. Doug Farr,Jeff Terry, andJeffreyArchuleta(NMT-16);John Joyceand Al Arko(MST-10);Jim Tobin(LLN); andDavid Shuh andEli Rotenberg(LBNL).

Figure 2. A pure plutonium sample in the UHV EELSinstrument. The sample is mounted on a 1-inchpuck, which sits in the hot/cold stage of the samplemanipulator. The sample is illuminated by a greenfluorescent glow in the early stages of ion sputtercleaning as a result of the presence of Pu oxide.The snout of the electron gun and the electronenergy analyzer can be seen directly above thesample.

Figure 1. UHV scanningAuger spectroscopy/EELS instrumentlocated in the Chemistryand Metallurgy Re-search building at LosAlamos. This instrumentis used for fundamentalactinide surface scienceresearch, plutoniumphysical propertiesmeasurements, andanalytical surfacecharacterization.

The electronic structure of a solid deter-mines virtually all of its physical properties,excluding those nuclear in nature. It is ex-tremely important to understand the characterof the electrons near the Fermi energy (outer-most) of the material because these electrons inparticular are involved in bonding or conduc-tion and collectively form the valence band ofthe solid. We have an ongoing effort to mea-sure and understand the electronic structure ofplutonium metal including all of its thermalallotropes (α, β, γ, δ, δ’, ε), and impurity-stabi-lized δ-plutonium. The methods we use in-clude electron photoemission spectroscopy(PES), Auger electron spectroscopy, electronenergy loss spectroscopy (EELS), and photonabsorption techniques in the visible and x-rayregimes. This article describes the progress ofour photoemission and EELS studies of Pumetal with the objective of understanding theelectronic structure of this material and theunique changes it undergoes from low tem-perature to the liquid phase.

The methods we have used are in generalsensitive only to the outermost surface (3 to 10atomic layers) of the solid under investigation.Therefore, it is very important to understandthe nature of the surface relative to the bulk ofthe material, in particular to insure that thesurfaces are atomically clean. It is also neces-sary to distinguish the presence of possiblesurface reconfigurations that intrinsically alterthe surface electronic structure from that of thebulk material. To that end, the sample prepara-tion and spectroscopic measurements weremade in an ultra high vacuum (UHV) environ-ment (~10-10 Torr) in order to preserve the in-tegrity of the surface for the duration of the

measurements. Figure 1 showsthe UHV Auger spectrometer/EELS instrument where someof these experimental mea-surements take place.

The electron energy loss measurementshave shed light on two aspects of theplutonium metal allotropes: electronicreconfigurations at the surface of each allot-rope, and changes in electronic environmentinduced through temperature changes of thepure Pu metal. We have made careful mea-surements of the EELS spectra from the sur-faces of ion-sputter-cleaned and annealed Pumetal. An exhaustive number of sputter-clean-ing and annealing cycles were required to at-tain a surface that was free of contamination(Figure 2). This effort resulted in a surfacethat would remain pristine, in the area ofanalysis, to less than 10% of a monolayer ofoxygen impurity for a period of two hours,allowing us sufficient time to make high-quality measurements.

4th quarter 2000


Figure 3. Schematic representation ofan incident primary electron undergoingenergy losses due to excitation ofplasmons. In the general case theelectron may lose energy by exciting asurface plasmon upon entering andexiting the solid (maximum of 2 surfaceplasmon excitations possible), and maylose energy by exciting any integralnumber of bulk plasmon losses.

The EELS measurements involve bom-bardment of the Pu surface with 150 to 1000 eVelectrons, with subsequent energy analysis ofthe scattered electrons. The scattered electronshave lost energy through interaction of the sur-face by a variety of different processes includ-ing excitation of plasmons (Figure 3) in thematerial, and promotion of electrons in atomicplutonium core levels to unoccupied states inthe valence band of the solid. Plasmons are col-lective excitations of the electron gas (conduc-tion electrons) in the material. Analysis of theplasmon response of the material over therange of primary electron energies that wereused shows substantial changes as the depth ofmaterial sampled by the spectroscopy changesfrom approximately 8 to 18 Å. This behavior isconsistent for each of the Pu thermal allotropicphases and the Ga-stabilized δ-Pu phase. Thisresponse indicates that the surface is beingreconfigured in an electronic, and probably astructural nature, to an estimated depth of 10Å (about 3 atomic layers) in each case. We be-lieve that this reconfiguration involves a relax-ation of atomic positions to something that isapproximately a factor of three less denseatomically than the bulk structure. The precisenature of this reconfiguration is not yet under-stood.

In our measurements of plutonium, theintention is to obtain a value for the free elec-tron count per atom in the solid and to exam-ine how this value differs for each allotropicphase. In the case of EELS measurements at700 eV primary electron energy, these mea-surements probe the electronic environment inthe bulk (not surface layers) of the Pu metal.We observe up to three bulk plasmon lossevents spectroscopically, with peaks evident atapproximately 11, 22, and 33 eV loss energy. Acompilation of the 700 eV EELS spectra foreach thermal allotrope exhibits subtle shifts inthe energy of the plasmon resonance peaks asa result of changes in the free electron volumedensity with the allotrope.

The bulk plasmon resonance energy isused to extract the free electron volume den-sity. The free electron count per atom is calcu-lated by correcting the volume density usingthe atomic volume density specific for each al-lotrope. The free electron count per atom ap-pears to be nearly invariant over the range ofallotropes, but the subtle changes, however,track exactly with changes in other measuredphysical properties such as absolute resistivityand magnetic susceptibility. This pattern indi-cates that the changes in the free electron den-sity measured here are identically responsiblefor these changes in electronic properties overthe allotropic series. These changes, however,do not reflect the traditional view of a shift in5f electron nature from itinerant to localizedwith increasing temperature across the allotro-pic series. Alternatively, the measurementsmade with this techniqueprobe free electron behaviornot associated with the Pu 5fvalence electrons, suggestingthat the 5f electrons largely donot participate in charge con-duction or magnetic propertiesof Pu. Future measurementsand ongoing analysis mayclarify these details.

Our photoemission mea-surements probe the occupieddensity of states (DOS) in theplutonium and include corelevel states as well as valenceband states (see also ActinideQuarterly, first quarter 1999).This measurement involvesabsorption of an x–ray or ultra-violet photon by an atom in thesolid with corresponding ejec-tion of a photoelectron fromthe surface of the material.



Recent Measurements Add to Understandingof Pu Metal’s Electronic Structure (continued)

Figure 4. Experimental electronic structure DOSrepresentation for α-plutonium metal. The occupiedvalence band DOS was probed by valence bandphotoemission spectroscopy (VBPES), and theunoccupied DOS was probed by NEXAFS andISEELS measurements. Some of the deeper corelevels below the Pu 6s state are accessible usingsoft x-ray photoemission (experimental spectrumnot shown).

Through an energy balance, the binding en-ergy of the electron in the solid (as part of theelectronic density of states) is related to themeasured kinetic energy of the photoelectronand the original photon energy. This is the firststep in mapping out the electronic structure ofPu metal, with an ultimate goal of measuringthe band structure of each allotrope. In thisway we hope to understand the electronicband structure changes that plutonium under-goes with each thermal phase transformation,as well as the electronic mechanism for impu-rity stabilization of the δ-Pu phase. In addi-tion, the unoccupied density of states abovethe Fermi energy can be examined through theuse of near edge x-ray absorption fine struc-ture (NEXAFS) and inner shell EELS (ISEELS)measurements. These are analogous processesin which a core-level electron is promoted intoan unoccupied state through absorption of aphoton (NEXAFS) or through an inelastic en-ergy loss of a primary electron in the EELSmeasurements. To a first approximation,within the constraints of the orbital selectionrule, the spectroscopic shape of the absorptioncurve or the loss feature is a convolution of thenarrow core level and the shape of the unoccu-pied states. Thus as shown in Figure 4, wehave been able to assemble a complete experi-mental representation of the electronic densityof states from the atomic core levels to the oc-cupied and unoccupied states of the solid va-lence band.

Fundamental studies involving photonand electron interactions with plutonium sur-faces continue in our laboratories in an at-tempt to understand the electronic structure ofthe metal. An understanding is of paramountimportance with regard to the electronic con-tribution to changes in crystal structure andphysical properties with thermal phasechanges in plutonium metal, and the nature ofthe impurity stabilization of the δ-Pu phase.An accurate description will help us to under-stand materials stability, solid-state reactions,and surface chemistry of plutonium metal.

4th quarter 2000


Publications andInvited Talks(December 1999—December 2000)

If you are the author of a published paper (book,book chapter) or an invited talk, please e-mail theparticulars to [email protected].

Blackadar, J. M., A. S. Wong, and N.D. Stalnaker,“Radiochemistry Calculation of Plutonium andAmericium-241 Using Rad Calc III Program,”Los Alamos National Laboratory report LA-13756-MS (September 2000).

Danis, J. A., H. T. Hawkins, B. L. Scott,W. H. Runde, B. E. Scheetz, and B. W. Eichhorn,“X-ray Structure Determination of Two RelatedUranyl Phosphate Crown Ether Compounds”Polyhedron, 19(#13) pp. 1551-1557 (July 2000).

Fife, K.W., J. L. Alwin, C. A. Smith, M. D. Mayne,and D. A. Rockstraw, “Vitrified Magnesia Dissolu-tion and Its Impact on Plutonium Residue Process-ing,” Los Alamos National Laboratory reportLA-13680-MS (March 2000).

Figg, D. J., A. A. Martinez, L. R. Drake, andC. A. Brink, “Inductively Coupled Plasma—MassSpectrometry Analysis of Plutonium Samples,”Los Alamos National Laboratory report LA-13754-MS (October 2000).

Jacobsen, S. D., J. R. Smyth, R. J. Swope, andR. I. Sheldon, “Two Proton Positions in theVery Strong Hydrogen Bond of Serandite,NaMn2[Si3O8(OH)],” American Mineralogist,85 745-752 (2000).

Lawson, A.C., B. Martinez, R.B. Von Dreele,J.A. Roberts, R.I. Sheldon, T.O. Brun, andJ.W. Richardson, Jr., “Vibrational Order inPu0.098Ga0.02‰,” Phil. Mag. B 80(11), 1869-1891(2000).

Park, J. J., “Creep Behavior of W-4Re-0.32HfC andIts Comparison with Some Creep Models,” Interna-tional Journal of Refractory Metals and Hard Materials17, 331-337 (December 1999).

Park, J. J., D. P. Butt, and C. A. Beard, “Review ofLiquid Metal Corrosion Issues for Potential Con-tainment Materials for Liquid Lead and Lead-Bis-muth Eutectic Spallation Targets as a NeutronSource,” Nuclear Engineering and Design 196, 315-325 (April 2000).

Pillay, G., S. R. Billingsley, and J. J. Balkey, “Electro-chemical Treatment and Minimization of Defense-Related Wastes,” Keynote Address, in EnvironmentalAspects of Electrochemical Technology: Proceedings ofthe International Symposium, E. J. Rudd and C. W.Walton, eds., PV 99-39, The Electrochemical Society,Inc., Pennington, NJ (2000).

Reimus, M. A. H., M. G. Stout, “Properties ofHafnium Oxide with Respect to Its Use as aSimulant for 238PuO2 in Safety Studies,” Los AlamosNational Laboratory report LA-13746-MS(August 2000).

Salimi, B, L. A. McNair, T. Baros, R. A. Pereya,W. B. Hutchinson, J. A. Kennison, “Special Designfor Ellipse Alpha,” Los Alamos National Laboratoryreport LA-13691 (February 2000).

Sheldon, R.I., T. Hartmann, K. E. Sickafus, A. Ibarra,B. L. Scott, D. N. Argyriou, A. C. Larson, andR. B. Von Dreele, “Cation Disorder and VacancyDistribution in Non-stoichiometric MagnesiumAluminate Spinel”, J. Am Ceram. Soc. 82(12), 3293-3298 (1999).

Sickafus, K. E., J. M. Wills, S. Chen, J. H. Terry,T. Hartmann, and R. I. Sheldon, “Development of aFundamental Understanding of Chemical Bondingand Electronic Structure in Spinel Compounds,”Los Alamos National Laboratory document LA-UR-99-2731, Los Alamos, NM (1999).

Tandon, L., “Radiolysis of Salts and Long-term Stor-age Issues for both Pure and Impure UO2 Materialsin Plutonium Storage containers,” Los Alamos Na-tional Laboratory report LA-13725-MS (May 2000).


Nuclear Materials Technology/Los Alamos National Laboratory12

LosN A T I O N A L L A B O R A T O R Y

Alamos

The Actinide Research Quarterly is published quarterly to highlight recent achievements and ongoingprograms of the Nuclear Materials Technology Division. We welcome your suggestions and contributions.If you have any comments, suggestions, or contributions, you may contact us by phone, by mail, or on e-mail([email protected]). ARQ is now on the Web also. See this issue as well as back issues on-line (http://www.lanl.gov/Internal/divisions/NMT/nmtdo/AQarchive/AQindex/AQindex.html).

Nuclear Materials Technology DivisionMail Stop E500Los Alamos National LaboratoryLos Alamos, New Mexico 87545505/667-2556 FAX 505/667-7966

LALP-00-48

Director of NMT: Timothy GeorgeChief Scientist: Kyu C. KimWriter/Editor: Ann MauzyDesign and Production: Susan L. CarlsonPrinting Coordination: Lupe Archuleta

Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36.All company names, logos, and products mentioned herein are trademarks of their respective companies. Reference to any specific company or product is not to be construed as an endorsementof said company or product by the Regents of the University of California, the United States Government, the U.S. Department of Energy, nor any of their employees. Los Alamos NationalLaboratory strongly supports academic freedom and a researcher’s right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee itstechnical correctness.

Newsmakers ■ Award: The NMT-9 team responsible for supplying heat sources to the Galileo mission wasrecently awarded the National Space Club’s Nelson P. Jackson Aerospace Award for outstandingcontribution to planetary exploration. Team members are Roy Zocher, Tim George, GaryRinehart, Theresa Cull, and Art Herrera.

■ Pollution Prevention Award: Bob Grundemann and Susan Ramsey of Power Source Technol-ogy (NMT-9) and Scott Ferrara of the Idaho National Engineering and Environmental Laboratorywon a Pollution Prevention Award this year for improvements they made to the plutonium resi-due solidification process.

■ New Division Review Committee member: Dr. William (Bill) F. Weston, (Boeing Company,Rocketdyne Division), has agreed to serve as a member of the Division Review Committee. Thenext review is planned for May 8–11, 2001. The committee will accomplish the work of the reviewthat was scheduled for May 2000 but was postponed as a result of the Cerro Grande fire. Westonjoins Dr. Richard A. Bartsch (Texas Tech University), Dr. Rohinton K. Bhada, Dr. Darleane C.Hoffman (Lawrence Berkeley National Laboratory), Dr. Todd R. LaPorte (University of Califor-nia, Berkeley), Dr. W. Lamar Miler (University of Florida, Consultant), Dr. Anthony (Tony) D.Rollet (Carnegie Mellon University), Dr. Ned A. Wogman (Battelle, Pacific Northwest Laborato-ries, Consultant), and Dr. Susan Wood (Westinghouse Savannah River Company, CommitteeChair).

■ New SLC Members: Eleven NMT scientists and engineers were nominated for this year’sselection of the Science Leadership Council in NMT, and five have been selected. SLC membersare all accomplished senior scientists within NMT who have demonstrated significant scientificand technical achievements and hold peer recognition, external reputation, and visibility in theirrespective scientific and technical fields. All non-managers, the SLC is charged with providingleadership in directing the division’s efforts in scientific and technical research. The newlyselected members are Gary Eller, Laura Worl, Robert Margevicius, Ed Garcia, and John Park.

4th quarter 2000 the actinide research los alamos national ... · the actinide researchlos alamos...

Documents