laser ablation (193 nm), purification and determination of very low concentrations of solar wind...

The Sun, which represents 99.6% of the mass ofthe Solar System, has a composition which is thoughtto have been constant since its formation about 4.5Ga ago. Comparison of solar isotopic and elementalcompositions, which presumably represent the presolarnebula, with those of the planets provides insightsinto the processes that operated during planetary

formation. The chemical composition of the Sun is rea-sonably well known from photospheric abundancemeasurements combined with analyses of primitivemeteorites (Grevesse et al. 2007). However, the isoto-pic composition of solar matter is still poorly known.The GENESIS mission was designed to recover ionsemitted by the corpuscular radiation of the Sun, known

Laser Ablation (193 nm), Purification and Determination ofVery Low Concentrations of Solar Wind Nitrogen Implantedin Targets from the GENESIS Spacecraft

Vol. 33 — N° 2 p . 1 8 3 - 1 9 4

The GENESIS space mission recovered ions emittedby the Sun during a 27 month period. In order toextract, purify and determine the very low quantitiesof solar nitrogen implanted in the GENESIS targets,a new installation was developed and constructedat the CRPG (Nancy, France). It permitted the simultaneous determination of nitrogen and noblegases extracted from the target by laser ablation.The extraction procedure used a 193 nm excimerlaser that allowed for surface contamination in theouter 5 nm to be removed, followed by a step thatremoved 50 nm of the target material, extractingthe solar nitrogen and noble gases implanted in thetarget. Following purification using Ti and Zr gettersfor noble gases and a Cu-CuO oxidation cycle forN2, the extracted gases were analysed by staticmode (pumps closed) mass spectrometry using electron multiplier and Faraday cup detectors. Thenitrogen blanks from the purification section andthe static line (30 minutes) were only 0.46 picomoleand 0.47 picomole, respectively.

Keywords: nitrogen, laser ablation, purification, mass spectrometry, GENESIS mission.

La mission GENESIS a récupéré des ions émis par le soleil pendant une période de 27 mois. Afind’extraire, purifier et analyser de très faibles quantités d’azote solaire implantés dans des cibles GENESIS, une nouvelle installation a été développée et construite au CRPG. Elle a permisl’analyse simultanée de l’azote et des gaz noblesextraits de la couche d’or par ablation. La procédured’extraction a utilisé un laser Excimer 193 nm qui apermis une étape d’extraction à 5 nm pour éliminerla pollution à la surface, suivie d’une étape qui aextrait jusqu’à une profondeur de 50 nm l’azote etles gaz rares solaires implantés dans la cible. Aprèsune purification à l’aide de getters Ti et Zr pour lesgaz rares et un cycle d’oxydation Cu-CuO pour N2,les gaz extraits ont été analysés en mode statique(pompage fermé) par spectrométrie de masse àl’aide d’un multiplicateur d’électrons et d’une cagede Faraday. Les blancs d’azote provenant de la partie purification et de la ligne en statique (30minutes) étaient de seulement 0.46 et 0.47 picomole, respectivement.

Mots-clés : azote, ablation laser, purification, spectrométrie de masse, mission GENESIS.

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0609

Laurent Zimmermann (1)*, Pete Burnard (1), Bernard Marty (1, 2) and Fabien Gaboriaud (3)

(1) Centre de Recherches Pétrographiques et Géochimiques, Nancy-Université, CNRS, 15 rue Notre-Dame des Pauvres,B.P. 20, F-54501 Vandoeuvre-lès-Nancy

(2) Ecole Nationale Supérieure de Géologie, Avenue du doyen Roubault, F-54501 Vandoeuvre-lès-Nancy(3) Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, Nancy-Université, CNRS, 405 rue de Vandoeuvre,

F-54600 Villers-lès-Nancy* Corresponding author. e-mail: [email protected]

© 2009 The Authors. Journal compilation © 2009 International Association of Geoanalysts

Received 04 Nov 08 — Accepted 09 Apr 09

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

as the solar wind (SW). The GENESIS spacecraft expo-sed different targets to the corpuscular solar irradiationfor 27 months and returned its pay load to Earth onSeptember 8, 2004 (Burnett et al. 2003). Nevertheless,two major problems occurred during this mission: (1) asi l icon oi l present in the spacecraft unfortunatelydegassed and was redeposited and polymerised byirradiation on the surface of the targets, forming the so-called “brown stain”, a film rich in Si, N and C; (2) thereturn capsule crashed at the landing site, which des-troyed the majority of the targets and introduced quan-tities of terrestrial dust into the spacecraft and onto thetargets. Despite these problems, the mission succeededin returning to Earth materials containing solar windions, and, although the atoms were distributed oversquare metres of area, a total of 1.6 x 10-4 mole ofoxygen, nitrogen and other elements were collectedduring the mission. To extract, purify and analyse thesevery small quantities of matter represented a real tech-nological challenge, requiring major analytical deve-lopments. A new analytical instrument was constructedand developed at the CRPG, Nancy, France, specifical-ly for the determination of nitrogen and the noblegases in the GENESIS targets. Solar wind ions from thelow energy regime (quantitatively the most abundant)have an average energy of ≈ 1 keV nucleon-1. Thisenergy resulted in an implantation profi le with a

Poisson-like depth distribution peaking e.g. at ≈ 17 nmfor neon and nitrogen in gold (Figure 1). Consequently,measurement of these SW ions required an extractionprocess capable of excavating the target material atsuch depths, for example using either energetic par-ticles (e.g., a directed ion beam) or photons (laserablation). Because of the low concentrations of implan-ted N predicted in these targets (≈ 2 x 10-14 mole N2

mm-2), the analytical background needed to be of theorder 10-13 mole N. We therefore opted for laser abla-tion under ultra-high vacuum, combined with static gassource mass spectrometry for determining nitrogen asN2 molecu les in para l le l wi th the noble gases .Extraction using an excimer laser (λ = 193 nm) permit-ted sequential ablation with a typical depth of 5 nm,which eliminated, or at least reduced, two sources oferror in our analytical procedure: (1) The nitrogenatmospheric contamination adsorbed on the surface ofthe target and (2) Solid particles adhering on thesample by s tat ic elec t r ic i ty. Fur ther ablat ion wasapplied in order to extract solar nitrogen and noblegases implanted in the target at depths of a few tensof nanometres. In this contribution, we report the tech-niques and procedures aimed at analysing these verysmall samples.

Types of GENESIS targets

GENESIS targets are made of several pure mate-rials (Al, Au, Si, vapour-deposited diamond). Two typesof material were selected for N isotope determinations.The first consisted of plates of sapphire (Al2O3) ontowhich a 300 nm gold layer had been vapour deposi-ted (flown target samples 60132 and 60134). Thesecond material was part of a gold-coated stainlesssteel frame (sample 60009.01) used for holding tar-gets in the centre of an electrostatic lens known as “theconcentrator” (Figure 2). The concentrator increasedthe solar wind fluence in the centre of the lens by afactor of 20 on average (Wiens et al. 2003). The pieceanalysed here was known as the “gold cross arm”, orGCA . The samples were c leaned a t t he OpenUniversity in an ozone atmosphere under UV irradia-tion for 2.5 hours in order to eliminate organic conta-mination present on the surface (Sestak et al. 2006).Once cleaned, samples were handled in a class 100clean cabinet at the CRPG during loading in the laserchamber. The samples were transported out of theclass 100 environment once sealed within the laserchamber. For each type of target, analogue targets(“flight spares”), made under exactly the same condi-tions as the flown targets, were also analysed in order

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0

10

20

30

0 10 20 30 40 50 60 70

Neon

Nitrogen

Figure 1. Simulation of the implantation depth of solar wind

Ne and N2 in a gold layer with an average energy of ≈ 1

keV nucleon-1 using the SRIM-2003 code (Ziegler 2004).

An average of 28% of ions were backscattered. Most of the

ions were implanted between 5 and 30 nm. Therefore,

complete extraction is predicted for an ablation of everything

up to a depth of 50 nm. This profile applies only to the

passively collected SW (sample 60132 and 60134). The

concentrator target depth profiles are similar but may have

a distribution tail that is deeper.

Target depth (nm)

(Ato

ms

cm-3

)/

(Ato

ms

cm-2

) x 1

04

to identify the nitrogen and noble gas concentrationstrapped in the targets prior to bombardment by thesolar wind ions.

Laser extraction

In an initial attempt, a CO2 laser was used to vola-tilise the gold layer of a gold sapphire flight spare tar-get under vacuum using the procedure of Marty et al.(2000). However, it was not possible to accurately cal-culate the surface area heated by the laser, and runa-way coupling between the laser and the test targetsprevented an initial gentle heating step from beingused to eliminate potential surface pollution or toavoid the release of contamination N trapped at thegold-sapphire interface.

To avoid these problems, we initially developed a157 nm excimer laser ( Lambda Phys ik® ; Optex-Propulse energy: 0.1 mJ pulse-1; 400 Hz max.) in orderto ablate the target surfaces (Télouk et al . 2003,

Burnard et al. 2006) and thereby obtain clean ablationwithout fusion of the gold layer. Despite promisingablation profiles, the extraction yield from the goldlayer was poor (< 30%) as estimated from analysis oftest targets artificially implanted with 15N. The reasonfor the low extraction yield with 157 nm photons ispoorly understood, but we note that it was extremelydifficult to maintain constant beam conditions duringablation due to absorption of the laser light in thebeam delivery tube. Photons of 157 nm are readilyadsorbed by O2 molecules in air, consequently thebeam delivery was in a nitrogen-flushed environment.Nevertheless, significant variations in the beam poweroccurred due to traces of O2 in the beam path.

In order to increase our extraction efficiency, thelaser wavelength was changed from 157 nm to 193nm by changing the cavity mirrors and gas composi-tion, which also increased the laser power by a factorten. Furthermore, with 1 to 1.5 mJ pulse-1, it was pos-sible to increase the laser spot size, which reduced the

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11 mm

Flown target 60134 Flown target 60132

7 mm

Gold cross arm 60009.01

30 mm

Flown target 60134 Flown target 60132


Figure 2. Images of GENESIS targets. The first photo shows several flown targets, a gold cross arm and its

flight spare analogue. Following the crash, all targets were broken and the gold surfaces became scratched.

time required for ablation. The different settings of thelaser (193 nm) used for GENESIS targets are describedin Table 1.

Description of the laser system

The beam delivery system imaged a 2 mm x 4 mmslit onto the sample via two spherical lenses (f: 100mm, demagnif icat ion: x50) . The laser beam wasdeflected 90° onto the surface of the target. The laserbeam itself was not homogenised at any point, resul-ting in a reduction in energy at the beam edges.However, because there was considerable overlapduring ablation of the samples, this did not result inuneven removal of material from the sample surface,as can be seen in the depth profiles of the laser pits(see below).

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Table 1.Laser parameters

Initial Secondablation ablation

Ablation depth (nm) 5 50

Wavelengh (nm) 193 193

Pulse energy (mJ) 1 1

Max. repetition rate (Hz) 1 30

Pulse duration (ns) 10 10

Focus (mm) 103 103

Pulse(s) spot-1 1–3 40–50

Size of pulse (mm2) 0.2–0.3 0.2–0.3

These settings allowed an initial ablation of 5 nm depth and asecond of 50 nm simply by increasing the number of pulses perspot. Increasing the repetition rate kept the ablation time constantbetween the different extraction depths. Ablation time for eachextraction was 5-15 minutes.

Laserbeam

45° Mirror

Symmetric-convexspherical lens

f: 100 mm

CCD Camera Mirror

Target to purification

line

Stainless steel

Plexiglass

CaF2 Viewport

Lamp

YXStage

Support Z

Stainless steelblock

(sample support&

volume reduction)

Figure 3. Line drawing of the sample chamber and the laser system (not to scale). The laser beam diverged from a 2

mm x 4 mm slit approximately 2 m from the focussing lenses. It was diverted through 90° before being focussed onto

the sample. The laser beam remained stationary during rastering; instead, the sample chamber was translated using

stepper motors. Focus was achieved by raising or lowering the chamber relative to the lenses.

The laser chamber consisted of a stainless steeltube with two CF63 flanges. The upper flange was fittedwith a CaF2 viewport which transmitted 85% of thelaser beam onto the sample, the lower flange wassolid steel. A CF16 flange welded to the chamber wallconnected the laser chamber to a purification line. Astainless steel block was used to reduce the internalvolume of the chamber and served as a sample sup-port. The laser chamber was placed on a stage, whichpermitted X-Y movement of the sample. A supportadjusted (in Z) the height between the sample and thelens to focus the beam. Finally, a lamp was placedabove the lens to illuminate the target, which wasmonitored with a CCD camera (Figure 3).

After construction, the laser chamber was cleanedby three successive ul t rasonic baths in detergent(Decon90) and then rinsed in demineralised water.Each bath took 2-3 hours and 99.95% of hydrocar-bons were eliminated (Benvenuti et al. 1999). An ace-tone bath with ultrasonic agitation removed traces ofwater. Finally, the chamber was dried at 100 °C.

Quantification of laser ablation rates

Prior to measurement of the flown targets, the para-meters affecting the depth of ablation were investiga-ted. These parameters were: the laser power (0.5-1.5mJ pulse-1), the laser focus (103 ± 5 mm), the pulserate (1-400 Hz) and the rate of movement of thesample relative to the laser beam (0.1-0.8 mm s-1).Glass slides, coated with 30, 60, 120 or 240 nm ofgold (using an Edwards® Scancoat 6 gold-coatingdevice) were used to optimise the laser parameters.

The four parameters were systematically varied,allowing the parameter combinations required toremove 30, 60, 120 and 240 nm of gold to be deter-mined. SEM images were used to explore semi-quanti-tatively the optimum laser parameters (Figure 4). Inaddition, a few ablation pits were selected for accura-te depth profiling by atomic force microscopy (AFM)(Thermomicroscopes Veeco instrument® , Ecu plusExplorer) (Figure 5). These measurements were perfor-med on a 50 μm x 50 μm surface in non-contactmode with a vertical and lateral resolution of 0.1 to0.5 nm and 150 nm, respectively.

We were able to ablate to depths of 5 and 50 nmin a reproducible way using the conditions given inTable 1. The first ablation, at 5 nm, was intended to eli-minate three sources of contamination, namely: (1)

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Plate glass/Au

Thickness :120 nm

(+) Pow

er (-)

Ablation

pits

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Si

Plate glass/Au

Thickness :120 nm

(+) Pow

er (-)

Ablation

pits

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Si

Figure 4. SEM images of ablation pits of a glass plate

covered by a 120 nm gold layer. The large amount of

silica detected on the clear stains (ablation areas 1 and

2) demonstrates that the gold layer was practically

removed by the laser beam, showing that the depth of

ablation was greater than 120 nm. For area 3, the

depth of ablation was about 120 nm. For areas 4 to 7,

the ablation depths were less the 120 nm. Similar tests

were performed for plates with 30, 60 and 240 nm of

gold. Horizontal lines result from inhomogeneities of the

laser beam, originating from the electrode distribution

within the excimer cavity.

-50

-40

-30

-20

-10

0

10

0 10 20 30 40 50

30 Hz

50 pulses spot-1

1 Hz1–3 pulses spot-1

Figure 5. Measurement of the ablation depth by AFM.

1-3 pulse spot-1 resulted in < 5 nm excavation and a

depth of ≈ 40 nm was obtained with 50 pulses spot-1.

X-profile (μm)

Z-p

rofil

e (n

m)

atmospheric nitrogen adsorbed on the surface of thetargets, (2) microscopic terrestrial particles fixed bystatic electricity on the surface after the crash of thecapsule and (3) hydrocarbons that make up the“brown stain”. Then, the second ablation at 50 nmallowed an extraction efficiency greater than 99% ofthe remaining solar gases (Figure 1).

Purification and analysis

Design of the ultra-highvacuum (UHV) line

A UHV extraction line with low blanks designed tohandle small quantities of gas was constructed specifi-cally for measuring the GENESIS samples (Figure 6).The line consisted of twenty-six individual sections (14to 220 cm3 in volume) separated by manual (VAT®

series 57-GE01 and Swagelok® SS4BGV51) and auto-mated valves (VAT® series 57-GE02 and Swagelok®

SS4BGV51VD1M). Each volume was determined to anaccuracy of better than ± 0.5% using an absolute pres-sure gauge (Pfeiffer® CMR 272). The line was dividedinto three principal sections, namely: (1) laser extraction

(460 cm3), (2) noble gas purification (585 cm3) and (3)nitrogen purification (140 cm3). Each of these sectionswas pumped independent ly wi th a tu rbo pump(Pfeiffer® TMU-071P) coupled to a scroll-type primarypump (Varian® SH100 or Boc Edwards® XDS5). This“dry” pumping system was chosen in order to bothreduce residual quantities of noble gases and nitrogendue to their high compression ratios (defined as Tx, theratio of the pressure between the primary vacuum flan-ge and the high vacuum flange of the pump) of TH2

> 105, THe : 6 x 106 and TN2-Ar > 1011, and to facilitateoil-free operation, which reduced residual hydrocarbonspecies in the extraction line. Pressure measurementswere made with an ion gauge (MKS® mini BA) using alow current (2 A) to minimise isotopic fractionationduring measurement. The gases were only exposed tothe ion gauge for < 10 seconds. The low internal volu-me of the ion gauge sensor reduced desorption ofgases from the ion gauge walls. The calibration systemconsisted of five separate bottles containing dilutedair and one with an artificial He reference sample(Matsuda et al. 2002). Each calibration bottle was iso-lated from the line by a 0.200 ± 0.004 cm3 pipette(calibrated by manometer) enclosed between two

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GEOSTANDARDS and



laser

Laser chamber

Mass

spectrometer

VG5400

Reference samples

G1

CT(-183 °C)

CuO

Pi

CT(-196 °C

-155 °C)

He Ne Ar Xe

N2

Reference

sample

TP

Gx: GetterPi: Pirani

TP: TurbopumpIG: Ion gauge

CT: Cryogenic trap

TP

TP

G2

IG

line

bottle of

reference gas

double-valve

system

Figure 6. Diagram of the extraction and purification line for the GENESIS targets with inset showing operation of

aliquots for sampling reference gases. See text for descriptions of pumps, getters and gauges.

pneumatic valves (Swagelok® SS4BGV51VD1M-3C),which permitted aliquots of calibration gas to be introdu-ced into the line to determine the sensitivity (A torr-1) ofthe mass spectrometer. The calibration bottles were filledfrom an aliquot of dry air sampled outside the CRPG,Nancy. The Ne and Ar reference samples were purifiedwith a titanium sponge (Johnson Matthey® mesh m3N8t2N8, lot: F11C11) for 30 minutes (15 minutes at 750 °Cand 15 minutes at room temperature). The purified gaswas expanded into a calibration bottle made of stainlesssteel (Ar reference sample) or cryogenically purified usingcharcoal held at 77 K (Ne reference sample). The nitro-gen reference sample was prepared with an aliquot ofdry air and diluted into a glass bottle to minimise nitro-gen adsorption onto the internal surfaces. The amount ofreference gases were computed from capacitancemanometer measurements and dilutions. Table 2 lists theelemental composition of these reference gases.

In order to check our calibrations, we analysed theK-Ar reference material MMhb-1, (Hornblende fromMcClure Mountain complex for which K-Ar data hasbeen compiled from over eighteen laboratories withan interlaboratory Ar concentration reproducibility of1.89% (1s), Samson and Alexander 1987), which wasmelted using a CO2 laser (Marty et al . 2000). Themean computed radiogenic argon content of MMhb-1was 1.690 ± 0.026 x 10-9 mol g-1 (mean of four runs,calculated using the Ar calibration bottle and the Arcontained within the N2 reference bott le) , whichagrees well with the recommended 40Ar* value of1.635 ± 0.024 x 10-9 mol g-1 (Samson and Alexander1987). We compared the sensitivity of Ne between theAr and Ne reference gases to calibrate the abundancein the Ne reference sample.

After ensuring that the line was free from any leaks,the reference bottles and the line were baked at 350°C and 220 °C, respectively for several days, cyclingthe temperature to 100 °C during the day. The pipework of the primary vacuum system (between turbopumps and scroll pumps) was also heated at 100-150 °C. Finally, we obtained pressures < 2 x 10-9 mbarthroughout the extraction line.

Once the gas had been extracted from the GENE-SIS targets by laser ablation, the sample was split intonoble gas (17.5%) and N2 (82.5%) fractions. The twogas fractions were purified independently as describedbelow.

Purification of noble gases

Noble gases were purified with three getters, theTi-sponge (Johnson Matthey® mesh m3N8 t2N8, lot:F11C11) and two SAES getters (Saes® ST172/HI/20-10/650C). The Ti-sponge and the Saes getters wereinitially activated at 800 °C for several days and wereperiodically re-activated approximately once a year.

The noble gas fraction was purified with the Ti-sponge for 10 minutes at 600 °C (chemisorption ofreactive species) followed by 10 minutes at room tem-perature (in order to reduce the hydrogen pressure inthe line). The gas was then purified again over the twoSaes getters for 10 minutes at 20 °C. Then, argon wasseparated from helium and neon by absorbing argonon a stainless steel frit (0.5 μm) at 77 K. During deter-mination of He and Ne by mass spectrometry, Ar wasdesorbed for analysis.

Purification of nitrogen

To minimise nitrogen adsorption on internal sur-faces, the minimum possible volume was used (140cm3) and the entire N2 purification volume was madeof Pyrex and quartz glass, which have cleaner surfacesthan metal. The N2 purification line consisted of aquartz finger containing copper oxide (CuO), whichproduced an adjustable oxygen partial pressure uponcontrolled heating, a glass tube containing severalplatinum foil sheets (previously degassed at 900 °Cunder vacuum) in order to increase the surface avai-lable for nitrogen adsorption, a U-shaped glass coldtrap and a Pirani gauge. The CuO powder was wrap-ped in platinum foil to prevent reaction between theCuO and the quartz finger when heated at 800 °C.

After ablation, the extracted gas was expandedinto the glass line and nitrogen was trapped by physi-sorption at 77 K onto the cold finger containing plati-num foil. After 3 minutes, the glass section was isolatedfrom the laser chamber, the temperature of the coldfinger was increased to 20 °C to desorb nitrogen andthe CuO was heated to 800 °C in order to generatean oxygen atmosphere. Impurities (CO, CxHy) dissocia-ted and oxidised in contact with the hot CuO and Pt

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GEOSTANDARDS and



Table 2.Mole per pipette of each reference gas

Reference Ne Reference Ar Reference N2

20Ne 20Ne 40Ar N240Ar

2.7 x 10-14 5.3 x 10-15 3.0 x 10-12 8.6 x 10-11 1.0 x 10-12

foil at high PO2 to produce CO2, hydrogen was conver-ted into H2O and the nitrogen and the sulfur into NOx

and SO2. All the oxides produced, with the exception ofNOx, were adsorbed on the cold trap at -183 °C for20 minutes. To re-adsorb the remaining oxygen, theCuO furnace temperature was gradually reduced to300 °C over 40 minutes. At 300 °C, the PO2 was suffi-ciently low so that all the NOx was transformed to N2

(we determined nitrogen as a N2 dimer in the massspectrometer, Boyd et al. 1988). It was essential to re-adsorb all oxygen completely, which would otherwiseresult in CO production and interfere with the nitrogenmeasurement in the mass spectrometer. To improvepurification, the first cold finger, originally designed totrap nitrogen at -196 °C, was cooled to -155 ± 5 °C toincrease the hydrocarbon trapping efficiency.

Mass spectrometer

A VG5400® spectrometer in static mode, where thepumps were isolated and all the available sample gaswas equilibrated with the mass spectrometer volume,was used for these analyses. The instrument had a Nier-type ion source that efficiently dissipated the heat gene-rated by the filament, a flight tube of 27 cm radius withextended geometry and two collectors, a high-massFaraday cup and an electron multiplier. The Faradayand multiplier collector mass resolutions (M/ΔM) were250 and 650 at 2% of peak height, respectively. Anelectro-magnet regulated by a Hall probe was used todeliver a magnetic field across the flight tube.

The source settings were 100 μA for the currenttrap and 60 eV for the ionisation energy. The repellerand the focus currents of the source were adjusted toget the best possible stability and sensitivities for Ne,Ar and N2. With these analytical conditions, the half-lives of these gases in the ion source were greaterthan 40 minutes (Table 3), allowing additional time foranalysis and improving counting statistics.

40Ar+ was measured with a Faraday collector(FC) and an electron multiplier (EM) was used for

20-21-22Ne+ and 36-38Ar+. Nitrogen isotopes were mea-sured successively, as N2, at masses 28, 29 and 30.Ions at masses 28 and 29 were collected on the FC,and those at masses 29 and 30 were measured withthe EM. The results were corrected for CO+ and HC+

contr ibut ions us ing a c lass ical mixing approach(Hashizume and Marty 2004). The N2H contributionwas neglected because the nitrogen pressure in themass spectrometer was above 10-7 torr. The quantity ofreference calibration gas in the mass spectrometer wasadjusted to mimic the range of pressures encounteredfor the samples, in order to minimise sensitivity andmass discrimination variations with source pressure(Burnard and Farley 2000). Finally, the concentrationsand the isotopic ratios were calculated by subtractingthe blank and correcting for instrumental mass discri-mination. Errors were propagated following Hashizumeand Marty (2004) using either derivatives or a MonteCarlo method, depending on the complexity of thecorrection algorithms.

Determination of the laser extraction yield

15N-implanted test targets

The GENESIS allocation committee provided testtargets that had been implanted with pure 15N ation energies (15 keV) and fluences (6.6 x 10-12 molmm-2) comparable to those expected for solar windsampled by GENESIS concentrator targets. For thesetes ts , smal l sur faces (0 .5 mm2) were ablated toavoid 15N memory in the analy t ical sys tem. Thenumber of pulses was increased sequent ial ly toexplore the optimum repetition rate to quantitativelyrecover ni trogen ions. A t 50 pulses per spot , weobserved an extraction yield of 81% (± 4%) with adepth of ablation into the gold layer of 200 nm.Increasing the depth ablation to > 200 nm to improvethe extraction yield was not advisable because thesapphire substrate could be ablated and therebyliberate significant quantities of non-solar nitrogen(Figure 7).

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GEOSTANDARDS and



Table 3.Source settings

Sensitivity Total time of analysis Half-life Gas consumption(A torr-1) (minutes) (minutes) (%)

Ne 2.9 x 10-5 13 >> 60 8Ar 2.6 x 10-4 11 >> 60 2N2 1.5 x 10-4 34 40 39

Extraction yield of SW neon in flown sample60132 and gold cross arm 60009.01

Two types of GENESIS targets were analysed for Nand Ne isotopes and abundances: sample #60132, agold-over-sapphire (AuoS) target and gold cross armsample #60009.01 (see above) . Smal l areas ofsamples 60132 and 60009.01 (S60132 : 0.24-0.48mm2 and S60009.01 : 4.8 mm2) were ablated using thesame settings as for the laser ablation tests on the 15Ndoped target. The results were corrected for 27%backscattering of ions by Au surfaces (#60132).

Several areas of sample #60132 were ablatedwith a variable number of pulses. Each extraction yieldwas calculated based on the neon concentration esti-mated separately at the ETH Zürich (Grimberg et al.2007) (Figure 8). The extraction yield increased from9% SW neon at 2 pulses spot-1 up to 80% SW neonat 50 pulses spot-1 with a typically solar isotopic signa-ture (20Ne/22Ne : 13.5 ± 0.8).

For the GCA, we increased the number of pulsesfor a given area until the neon content matched theNe content as determined by the Zürich group (Heberet al. 2007) (but on a much smaller surface area); thisrequired 63 pulses spot-1 (Figure 9).

There was a strong gradient in SW concentrationalong the GCA. Due to the design of the concentrator,

the Ne flux at the centre of the GCA was enriched bya factor of twenty relative to that of the solar wind, butonly by a factor of three at the edge of the concentra-tor (Heber et al. 2007). We analysed several surfacesalong the GCA and compared our Ne concentrationdata with those of Heber et al. (2007). The results(Figure 10) show good agreement between the twosets of determinations, despite large differences in the

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0

20

40

60

80

100

0 20 40 60 80

Ablation

> 200 nm

Figure 7. 15N extraction yield from implanted gold targets.

The yield is computed by dividing the amount of 15N

extracted by the theoretical implanted 15N, computed from

the integrated 15N+ ion beam current during the implantation

experiment and a backscattering loss of 15N during

implantation predicted using the SRIM code (Ziegler 2004).

λ = 193 nm; P = 1 mJ; focus = 103 mm.

Number of pulses spot-1Number of pulses spot-1

Number of pulses spot-1 (cumulative)

Extr

act

ion

yiel

d o

f 15

N (%

)

SW N

e ex

tra

ct y

ield

(%)

SW N

e ex

tra

ct y

ield

(%)

0

20

40

60

80

100

0 10 20 30 40 50 60

Flown target 60132

Figure 8. Extraction yield of neon in flown sample

#60132, the yield was computed from mean fluence

estimated by Grimberg et al. (2007) and corrected for

Ne backscattering in Au.

Figure 9. Extraction yield of neon implanted in gold cross

arm N° 60009.01. At 1 pulse spot-1, only 2% extraction of

SW Ne was obtained. The second ablation (25 pulses

spot-1) and third (12 pulses spot-1) increased the extraction

yield to 64% and 10%, respectively. The final ablation (12

pulses spot-1), which extracted 3% of solar neon, showed

that the maximun amount of SW Ne was extracted after 63

pulses spot-1.

0

20

40

60

80

100

0 10 20 30 40 50 60 70


ablated surface areas (typically 0.008 mm2 for Zürichand 4-6 mm2 for Nancy). Nitrogen isotope data arepresented elsewhere (Marty et al. 2009).

GENESIS blanks

The predicted amount of 14N for the AuoS targetwas 2.3 x 10-14 mol mm-2, computed from a 20Ne fluence

of 1.4 x 1010 atom mm-2 (Reisenfeld et al. 2007), a solarwind 20Ne/14N ratio of ~ 1.1 (Reames 1995) and a cor-rection for backscattering of Ne of 70% (Ziegler 2004).With such a low nitrogen concentration, the analysis ofthe GENESIS target required an extremely low residualN2 pressure to minimise blank corrections.

The extraction blanks were obtained using exactlythe same analytical conditions as the samples as des-cribed above with the exception that the laser was notused for the blanks. Significant development of the pro-cedure used in the nitrogen purification section of theextraction line (described in the following section) allo-wed N blank levels to be reduced to below 10-12 mole.

Analytical protocol to reduce the nitrogen blanks

The main source of blank nitrogen proved to bethe CuO used as a source of O2. In order to minimisethis source of N2, we used the highest purity CuOavailable (99.9999%). Nevertheless, the N2 blankfrom the CuO (1.5 x 10-11 mole N2) was unacceptable.We then completely degassed oxygen from the CuOby heating it to 850-900 °C while pumping. After 20days of degassing, the blank of the O2-free systemwas 4.4 x 10-13 mole N2 in this section. We then reple-nished the CuO with ultra pure oxygen from a cylinder(noble gases < 8 ppm; N2 < 6 ppm; CH4, CO2 and CO< 0.2 ppm). This was achieved by heating the CuO to900 °C for 330 minutes under an oxygen partial pres-

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Figure 10. Comparison between the extractions for GCA

60009.01 made in the Nancy and Zürich laboratories.

The surface areas ablated were very different between

the two experiments (because the Zürich team did not

determine N isotopes, they were able to analyse smaller

areas). The ablations made at the Zürich laboratory

consisted of round spots of 0.008 mm2 as opposed to

the rectangular areas of several mm2 used for this study.

20Ne picomole mm-2 (Zürich)

20N

e p

icom

ole

mm

-2(N

anc

y)

0 5 10 15 20 25

Time (days)

N2

mole

8x10-13

Evolution of blanks (part purification)10-13

10-12

10-11

10-10

0 4 8 12 16 20 24 28

6x10-13

4x10-13

2x10-13

Figure 11. Evolution of the purification line blank over time after replenishing the CuO trap with ultra

pure O2. The CuO furnace was continually cycled between 450 and 800 °C during the entire period.

Time (month)

N2

(mol

e)

R2 = 0.93

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6

y = 0.67x

sure of 205 mbar, then cooling it slowly to 450 °C. Atotal of 10-3 mole O2 was fixed by the CuO. Finally,the CuO furnace was automatically cycled, thousandsof times between 450 °C (20 minutes pumping) and800 °C (static) in order to degas residual nitrogenfrom the CuO.

The evolution of the blank over 28 months (Figure11) showed a progressive trend towards lower levels.Once stabilised (the last eight runs), the average N2

blank contribution from the purification section was 4.6± 0.5 x 10-13 mole N2, with δ15N = -12 ± 62‰. Thewhole extraction process (including the laser chamber)resulted in blanks of 9.3 ± 1.9 x 10-13, mole N2, withδ15N = -7 ± 30‰. The difference between the com-plete extraction blank and the purification line blank(4.7 x 10-13 mole N2) comes from degassing of thelaser chamber internal surfaces during the 30 minutesextraction, this despite a baking at 150 °C for severaldays, followed by pumping over several weeks atroom temperature.

Nitrogen contents of the non-flown targets

Despite a rigorous preparation procedure (Jurewiczet al . 2003) designed to minimise the amount ofatmospheric nitrogen and noble gases trapped in thegold targets, it was essential to verify the backgroundconcentration of these gases using exactly the sametype of materials as the flown targets. For this, we used“flight spares” (fs). Two types of flight spares were ana-lysed: a standard AuoS target fs and a GCA fs. Forthese tests, we chose laser parameters to ablate to 5nm (1-3 pulse spot-1) and with a second extraction at≈ 50 nm (40-50 pulses spot-1) as used for the ana-lyses themselves. The first ablation simulated the surfa-ce particle and adsorbed N2 clean-up step and thesecond ablation simulated solar wind extraction. Eachresult was corrected for the “purification blank” (4.6 x10-13 mole N2).

The results for the fs AuoS target showed a N2 levelof 3.8 x 10-14 mole mm-2 at the surface (ablationdepth of 5 nm), despite cleaning the target to elimina-te terrestrial particles and baking the laser chamberafter loading the samples for several days to reduceadsorbed nitrogen. We also observed a significantamount of nitrogen trapped within the gold layer itself,despite its high purity (99.9999%), equivalent to acontamination of ≈ 3 x 10-13 mole N2 mm-2 for adepth of ablation of 50 nm, compared to the predictedsolar N2 concentration of only 10-14 mole N2 mm-2.

The amounts of nitrogen extracted from the GCAflight spare (1-50 pulses spot-1) were constant withdepth and were equivalent to 4.7 x 10-13 mole N2

mm-2 (corrected for the blank), with δ15N = 21 ± 12‰,ablating surfaces of about 5 mm2 each along the GCAflight spare in a manner similar to that applied to theflown GCA. The concentrator increased the solar windimplanted in the targets by a factor of twenty, therefore5 mm2 ablation per analysis was sufficient to obtain≈ 10-12 mole N2 (solar). Although this concentration iscomparable to the amount of SW nitrogen expected inthe GCA, the expectation was that SW nitrogen wouldhave an isotopic composition different from that ofterrestrial nitrogen, allowing the solar wind nitrogen tobe distinguished from terrestrial contamination.

Analytical protocol for low blank levels of noble gases

The entire stainless steel extraction line was bakedseveral times at 200 °C to desorb gases from the inter-nal surface. The getters, which are the principal sourceof noble gas blanks, were repeatedly activated untilacceptably low amounts of these gases were presentin the system (about two weeks), notably 1.3 x 10-16

mole 20Ne and 1.7 x 10-16 mole 36Ar.

During analysis, the titanium sponge getters cycledbetween 600 °C and 20 °C and two SAES getterswere used at room temperature. There was negligibleNe or Ar in the flight spares. For all noble gases, theline blank represented < 5% of the amount extractedfrom the actual flown samples.

Conclusions

We have developed a new analytical system formeasuring very low concentrations of nitrogen in theGENESIS targets. For this, we have used a laser (193nm) to ablate the surface of the target. With this newequipment, it is possible to ablate 5 nm of gold withone to three pulses (to eliminate pollution on the surfa-ce) and then increase the number of pulses to 40-50to ablate 50 nm of material (in order to liberate solargases). We have designed and constructed a low volu-me nitrogen purification line. In order to avoid nitrogenloss by dilution in the line, a cryogenic trap was usedto concentrate sample nitrogen in the purification line.After several months of pumping and baking, weobtained a purification blank of 4.6 x 10-13 mole N2.The laser chamber was equally clean (4.7 x 10-13

mole N2 after 30 minutes in static mode). These blanks

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are an order of magnitude lower than those achievedelsewhere (Marty et al. 2000). For the noble gases, theblank was 1.3 x 10-16 mole 20Ne and 1.7 x 10-16 mole36Ar.

Acknowledgements

We are grateful to Bouchaib Tibari and PierreBaillot for help during the construction of this new line.This work was supported by the Centre Nationald’Etudes Spatiales (CNES), the Centre National de laRecherche Sc ien t i f ique (CNRS) and the Rég ionLorraine. We are indebted to the members of theGENESIS science team, in particular J.H. Allton, D.S.Burnett, I. Franchi, V. Heber, A. Jurewicz, R. Sestack, R.Wieler and R. Wiens. CRPG contribution No. 1946.

References

Benvenuti C., Canil G., Chiggiato P., Collin P., Cosso R., Guérin J., Ilie S., Latorre D. and Neil K.S. (1999)Surface cleaning efficiency measurements for UHV applications. Vacuum, 53, 317-320.

Boyd S.R., Wright I.P., Franchi I.A. and Pillinger C.T. (1988)Preparation of sub-nanomole quantities of nitrogen gasfor stable isotopic analysis. Journal of Physics E: ScientificInstruments, 21, 876-885.

Burnard P.G. and Farley K.A. (2000)Calibration of pressure-dependent sensitivity and discrimination in Nier-type noble gas ion sources.Geochemistry Geophysics Geosystems, 1,2000GC000038.

Burnard P.G., Zimmermann L. and Marty B. (2006)Vacuum UV laser ablation of genesis target materials:Results from gold-on-sapphire analogues. Lunar andPlanetary Science Conference XXXVII.

Burnett D.S, Barraclough B.L., Bennet R.,Neugebauer M., Oldham L.P., Sasaki A.C.N.,Sevilla D., Smith N., Stansbery T.E., Sweetnam D.and Wiens R.C. (2003)The GENESIS discovery mission: Return of solar matter toEarth. Space Science Reviews, 105, 509-534.

Grevesse N., Asplund M. and Sauval A.J. (2007)The solar chemical composition. Space Science Reviews,130, 105-114.

Grimberg A., Baur H., Buhler F., Bochsler P. andWieler R. (2007)Solar wind helium, neon and argon isotopic and elemental composition: Data from the metallic glassflown on NASA’s Genesis mission. Geochimica etCosmochimica Acta, 72, 626-645.

Hashizume K. and Marty B. (2004)Nitrogen isotopic analyses at the sub-picomole levelusing an ultra-low blank laser extraction technique. In: deGroot P.A. (ed.), Handbook of stable isotope analyticaltechniques, Vol. 1. Elsevier (Amsterdam), 361-374.

Heber V.S., Wiens R.C., Reisenfeld D.B., Allton J.H.,Baur H., Burnett D.S., Olinger C.T., Wiechert U. andWieler R. (2007)The genesis solar wind concentrator target: Mass fractionation characterised by Ne isotopes. SpaceScience Reviews, 130, 309-316.

Jurewicz A.J.G., Burnett D.S., Wiens R.C., and twenty others (2003)The Genesis solar-wind collector materials. SpaceScience Reviews, 105, 535-560.

Marty B., Humbert F., Libourel G., France-Lanord C.and Zimmermann L. (2000)CO2-laser extraction-static mass spectrometry analysis ofultra-low concentrations of nitrogen in silicates.Geostandards Newsletter: The Journal of Geostandardsand Geoanalysis, 24, 255-260.

Marty B., Zimmermann L., Burnard P.G., Burnett D.S., Heber V.S., Wieler R., Bochsler P,.Wiens R.C., Sestak S. and Franchi I.A. (2009)In search of solar wind nitrogen in GENESIS material:Further analysis of a gold cross arm of the concentrator.40th Lunar and Planetary Science Conference.

Matsuda J., Matsumoto T., Sumino H., Nagao K.,Yamamoto J., Miura Y., Kaneoka I., Takahata N.and Sano Y. (2002)The 3He/4He ratio of the new internal He standard ofJapan (HESJ). Geochemical Journal, 36, 191-195.

Reames D.V. (1995)Coronal abundances determined from energetic particles. Advances in Space Research, 15, 41(7)-51(7).

Reisenfeld D.B., Burnett D.S, Becker R.H., Grimberg A.G., Heber V.S., Hohenberg C.M.,Jurewicz A.J.G., Meshik A., Pepin R.O., Raines J.M.,Schlutter D.J., Wieler R., Wiens R.C. and Zurbuchen T.H. (2007)Elemental abundances of the bulk solar wind: Analysesfrom GENESIS and ACE. Space Science Reviews, 130,79-86.

Samson S.D. and Alexander E.C. (1987)Calibration of the interlaboratory 40Ar-39Ar dating standard, MMhb-1. Chemical Geology (IsotopeGeoscience section), 66, 27-34.

Sestak S., Franchi I.A., Verchovsky A.B., Al-Kuzee J.,Braithwaite N. St J. and Burnett D.S. (2006)Application of semiconductor industry cleaning technologies for Genesis sample collectors. Lunar andPlanetary Science Conference XXXVII.

Télouk P., Rose-Koga E. and Albarède F. (2003)Preliminary results from a new 157 nm laser ablation ICP-MS instrument: New opportunities in the analysis ofsolid samples. Geostandards Newsletter: The Journal ofGeostandards and Geoanalysis, 27, 5-11.

Wiens R.C., Neugebauer M., Reisenfeld D.B., Moses R., Nordholt J.E. and Burnett D.S. (2003)Genesis solar wind concentrator: Computer simulations ofperformance under solar wind conditions. Space ScienceReviews, 105, 601-625.

Ziegler J.F. (2004)SRIM-2003. Nuclear Instruments and Methods inPhysics Research Section B, 219-220, 1027-1036.

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laser ablation (193 nm), purification and determination of very low concentrations of solar wind...

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