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Nuclear Instruments and Methods in Physics Research B 244 (2006) 427–435

NIMBBeam Interactions

withMaterials &Atoms

Carbon buildup monitoring using RBS: Correlationwith secondary electrons

E.F. Aguilera a,b,*, P. Rosales a, E. Martinez-Quiroz a, G. Murillo a,b, M.C. Fernandez a

a Departamento del Acelerador, Instituto Nacional de Investigaciones Nucleares, Apartado Postal 18-1027, C.P. 11801, Mexico, DF, Mexicob Universidad Autonoma del Estado de Mexico, C.P. 50000, Toluca, Mexico

Received 22 August 2005; received in revised form 11 October 2005Available online 28 November 2005

Abstract

The RBS technique is applied to solve the problem of on-line monitoring of the carbon deposited on a thin backed foil under ion bom-bardment. An iterative method is used to reliably extract quantities such as number of projectiles and target thickness in spite of beamenergy changes and detector unstabilities. Experimental values for secondary electron yields are also deduced. Results are reported forthe thickness variation of thin carbon foils bombarded with carbon ions of energies between 8.95 and 13 MeV. A linear correlation of thisvariation is found with both, the ion fluence at target and the number of secondary electrons emitted. The correlation exists even though awide range of beam currents, beam energies and bombarding times was used during the experiment. The measured electron yields showevidence for a change in the emission process between the original foils and the deposited layer, possibly due to a texture change.� 2005 Elsevier B.V. All rights reserved.

PACS: 81.90.+c; 82.80.�d; 82.80.Yc

Keywords: Carbon buildup; Rutherford backscattering; Secondary electrons

1. Introduction

A common practice to measure nuclear reaction crosssections consists of bombarding a thin target with energeticions and detecting the reaction products of interest withappropriate devices. In order to get reliable absolutevalues, a precise knowledge of the target thickness is nor-mally required. The phenomenon of carbon buildup underbombardment with ions [1–4] might introduce unwantedcomplications, especially when the target itself is made upof carbon. The energy loss in the carbon layer depositedon the target affects the effective reaction energy and, inthe case of a carbon target, its thickness is also affected.

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2005.10.019

* Corresponding author. Address: Departamento del Acelerador, Insti-tuto Nacional de Investigaciones Nucleares, Apartado Postal 18-1027,C.P. 11801, Mexico, DF, Mexico. Tel.: +52 5553297241; fax: +525553297329.

E-mail address: efar@nuclear.inin.mx (E.F. Aguilera).

The potential problem of C-buildup was recognized,for example, in several works where fusion cross sections

for the 12C + 12C system were measured [5–9]. Cold trapsaround the target have been used in some works to mini-mize the carbon deposit [5,7,9] but the actual buildup, ifany, stays usually uncertain. The effects of C-buildup havebeen also monitored by systematic repetition of measure-ments at selected beam energies [6]. This procedure maygive good results but the corresponding correction requiresinterpolations whose validity depends on the repetition fre-quency. For measurements far below the barrier, where thecross sections are very small, frequent repetition becomesunpractical. In addition, since buildup occurs only at thebeam spot (see below), it is critical that repetition measure-ments be done with the beam hitting precisely the same tar-get spot, a goal that may not be easy to achieve in allsituations. For transmission targets, on the other hand,the combined product of target thickness and beam inten-sity can be monitored via the elastic scattering yield

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observed in a monitor detector at forward angles [8,10–12],by using known cross section values for this process. Goodabsolute normalizations are usually obtained this way butan energy shift may still be present since the actual targetthickness, including C-buildup, remains unknown.

Summarizing, there are still uncertainties in existing sub-barrier fusion data for 12C + 12C as to the effects of carbondeposit on the target. Indeed, the big discrepancies existentbetween different data sets might be mainly due to the lackof a good method for quantification of these effects. Eventhough the consequences might be more dramatic forexperiments using carbon foils, C-buildup does in factoccur as well for other kind of targets. Implementing sucha method should therefore be valuable for a wide variety ofexperiments aimed at measuring nuclear reaction cross sec-tions. Other areas where the phenomenon of C-buildupmight be important are ion implantation, ion depositionof materials and surface treatment by ion etching [2]; depthprofiling using resonant nuclear reactions [13]; nuclearreaction analysis (NRA) to quantify trace amounts of car-bon [4,14], etc.

The mentioned C-buildup phenomenon is not com-pletely understood yet. For low-energy ions, a ‘‘sub-planta-tion’’ model has been proposed [15–18] which treats thedeposition as a sub-surface growth process due to shallowimplantation of the impinging carbon ions, some of whichalso sputter and dilute the outermost layer of substrateatoms. However, higher energy ions would be implanteddeeper in the substrate and this mechanism could notexplain the surface buildup in this case. The model wouldbe also unable to explain carbon buildup produced underirradiation with non-carbon ions, a phenomenon that hasbeen actually observed [2–4].

It is not surprising that a carbon deposit may be formedunder irradiation with other ions since the analysis of resid-ual gases normally shows some molecules containing thiselement. Such an analysis, reported by Healy et al. [4],showed the main constituents of the residual vacuum tobe N2, CO, N, H2O, O2, O, CH4, OH, Ar, CO2 and hydro-carbons. The presence of hydrocarbon molecules in theresidual atmosphere of some vacuum systems is well under-stood. Backstreaming from diffusion or mechanical pumps,as well as the use of some common sealing materials, doactually produce hydrocarbon contamination [2,19,20].Vacuum systems equipped with turbo molecular pumpsare also not free of this kind of contaminant [3,4] associ-ated to the pump oil and the degradation of plastics. Bom-bardments with deuteron beams on different targets clearlyshow that these projectiles can induce carbon buildup [3,4].In [3], this phenomenon was additionally observed with Heand N beams.

Systematic studies of carbon buildup have shown,among other things, that the carbon growth occurs underirradiation only and takes place precisely at the positionof the beam spot [3,4]. Overall pressure of the target cham-ber has little or none effect but the introduction of a coldsurface near the sample minimizes the deposit [4] and so

does a temperature increase of the same [3,4]. Even thoughthe hydrocarbons from the residual atmosphere are themost probable carbon source [2–4], direct interaction ofthe beam with the residual gas is not believed to be themajor mechanism for carbon deposition. An analysis usingsecondary ion mass spectrometry (SIMS) proves that notonly deposit but polymerization also occurs at the beamspot [3]. The time development of these polymer filmswas actually studied in a semiquantitative way in a previ-ous work [2]. On the basis of these and other obervations,Blondiaux et al. [3] proposed a mechanism whereby thehydrocarbons of the residual gas near the sample are disso-ciated by the secondary electrons emitted by the target, andthen condensed (polymerized) at the beam spot on the tar-get surface. The plausibility of this hypothesis to explainsome qualitative aspects of the observations was discussed,but a simultaneous measurement of secondary electrons toestablish the correlation was not actually attempted.

In an effort to solve the discrepancies existing in the sub-barrier fusion cross sections for the 12C + 12C system, thec-ray technique [21] was recently used to make new mea-surements incorporating a method for on-line monitoringof the target thickness [22]. As usual in this technique,the carbon target is deposited onto a thick substrate (Tain this case); the method is then based on measuring, witha silicon surface barrier (SSB) detector, the Rutherfordbackscattering (RBS) spectra produced by the target back-ing. Preliminary fusion results have been presented in [22]and a more complete report will be published elsewhere[23]. In this work we describe the method and analyzethe RBS data taken with the SSB detector. The target-thickness variations obtained as a function of the fluence ofcarbon ions at the target are presented. In addition, simul-taneously measured values for the number of electronsemitted by the target are reported and their correlationwith the carbon growth is studied.

2. Experimental procedure

The data were obtained with 12C ions in the energyinterval from 8.95 MeV to 13 MeV, produced with theEN-Tandem Van de Graaff accelerator at the InstitutoNacional de Investigaciones Nucleares (ININ). The beamwas focused and collimated so that the beam spot on targethad a diameter of 3.5 ± 0.2 mm, determined by repeatedmeasurements of the blackened spot left by the beam ontargets and substrates. A C4+ beam was used for energiesin the region 10.75–13 MeV, while for lower energies thecharge state was 3+. Fig. 1 shows schematically the exper-imental arrangement.

Three carbon foils mounted onto a thick Ta backingwere used as the target in successive stages of the experi-ment. The foils, made by ACF-Metals, are nominally amor-phous (more accurately, nanocrystalline) carbon layers withnatural isotopic composition, vacuum-arc-deposited onto aparting agent on glass microscope-slide substrates at highvacuum. The reported foil density is 2.01 ± 0.02 g/cm3.

Fig. 1. Experimental set-up.

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Before floating and mounting the foils, the Ta substrate wascleaned with alcohol and then by resistive heating. Theoriginal foil-thicknesses were t1 = 18.3 ± 0.2 lg/cm2, t2 =21.6 ± 0.1 lg/cm2 and t3 = 28.5 ± 0.4 lg/cm2, as deter-mined from the fits to the backscattering spectra that shallbe described below.

Starting from 13 MeV, the energy was monotonicallydecreased in 150 keV steps. Targets 1, 2, 3 were used inthe energy ranges 9.85–13 MeV, 9.25–9.85 MeV and 8.95–9.25 MeV, respectively. At five selected energies (8.95,9.10, 9.85, 10.9 and 13.0 MeV) the target was rotated by180� and a measurement was done with the bare Ta back-ing exposed to the beam. The energy 9.85 MeV was donetwice, the first time with the backing of target 1 and thesecond one with the target 2 substrate. For clarity, the nota-tion C(Ta) will be used for the target to indicate the situa-tion where the C-foil was facing the beam and the notationTa when it was facing in the opposite direction, i.e. whenthe bare Ta side of the target was facing the beam.

Irradiation times for C + Ta, where C refers to the pro-jectile, were less than 1 h while for C + C(Ta) they variedfrom around 3 h for the higher energies to about 8 h forthe lower ones. The main criterion to stop a run in the lat-ter case was the statistics of the fusion measurements.Beam currents at target ranged between 10 and 60 parti-cle-nA, while counting rates at the detector varied between103 and 7 · 103 s�1. The target was electrically isolated andthe charge deposited on it by the beam was collected. Thedifference between this and the actual charge carried by thebeam determines the number of secondary electronsemitted.

An SSB detector was placed at 160� with respect to thebeam direction in order to measure the 12C ions elasticallybackscattered from the Ta backing. A slit collimator of1.5 mm · 6.3 mm was placed in front of the detector andoriented vertically in order to properly define the scattering

angle. The distance between slit and target was 68 mm,thus giving a solid angle subtended at the target of(2.04 ± 0.09) · 10�3 sr. An unexpected complication arosebecause the SSB detector presented a fluence-inducedpulse-height defect (PHD) which effectively changed thecalibration for the different experimental points. Thischange, however, occurred in a systematic way that couldbe precisely correlated with the ion fluence at the detector[24], thus neutralizing the complication. In fact, a partialdescription of the experimental procedure has been givenin [24] but we repeat it here for the sake of completeness,summarizing when possible and adding the details thatare only relevant to this work. Two SSB detectors wereactually used, one for the measurements with the first tar-get and the other one with targets 2 and 3. A PHD was alsoobserved for the second detector and treated in the waydescribed in [24] for detector one.

Previous to the experiment, a calibration spectrum wastaken using a triple a source (239Pu, 241Am, 244Cm). Thedetector resolution determined from this spectrum was30 keV FWHM for the 5486 keV alphas from 241Am. Thisvalue was obtained with no source collimation, i.e. someedge effects ought to be present.

The two Ge(HP) detectors in Fig. 1 were used to measurethe secondary c-rays emitted by the fusion–evaporationresidues [22,23]. As we shall see later, they were quite usefulto verify and quantify the presence of small amounts of car-bon on the (otherwise bare) Ta target.

3. Relevant aspects of thick target spectra

At the backward angle of the SSB detector only scatter-ing from the Ta substrate can be observed since the maxi-mum angle allowed by the kinematics for C + C scatteringis 90�. In addition, because of the low beam energies andthe high charge of the C and Ta nuclei, the regime wherenuclear forces are important is not reached and only theCoulomb force plays a role in the scattering process. Ruth-erford backscattering on thick targets has been studied fora long time [25] and is actually routinely used, for example,in sample characterization with ion beams.

The typical thick target spectrum shows a sudden rise atthe energy corresponding to scattering from surface nucleiand a plateau at lower energies, originated by scatteringfrom inner nuclei. The height of the plateau is directlyrelated to the number of projectiles, Np, and its slope isdetermined by the interplay between the energy loss pro-cess and the energy dependence of the cross section, as wellas by the energy calibration of the detector. To the extentthat kinematics, energy loss and Rutherford scatteringare well-known processes, the number of projectiles andthe detector calibration can be simultaneously determinedfrom one single spectrum. We achieved this by using thesimulation code SIMNRA [26] which incorporates the rel-evant features.

The thin C-foil added on the surface of the thick Ta sub-strate introduces additional energy losses that will lower

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the corresponding energies at the detector, thus shiftingdownwards the whole spectrum. The foil thickness, t, canbe calculated from this shift if the energy calibration is wellknown [27]. As mentioned above, no additional backscat-tering will be produced by the foil. The possibility of mul-tilayer targets is also incorporated in SIMNRA and fits canbe done using t as a free parameter. However, if both theenergy calibration and the foil thickness are unknown,ambiguities may arise in the fit since they both affect theenergy distribution of the backscattered particles and canpartially compensate for each other. A careful choice ofthe initial values of the fitting parameters becomes thennecessary in order to prevent the ambiguities. This wasactually the case for our experiment because the carbonbuildup phenomenon produced a variable target thickness,on one hand, and the energy calibration had to be varied inorder to simulate the observed fluence-induced PHD [24],on the other hand.

As an illustration, the SIMNRA fit to the 9.85 MeVC + C(Ta) spectrum taken with the second detector, isshown in Fig. 2. The final values of the relevant parametersin the fit were: detector calibration, E (keV) = 154 +6.56ch; number of projectiles times steradian, NpX =1.5 · 1012; target thickness, t = 1075.6 · 1015 at/cm2

(21.45 lg/cm2). Typically, the error in NpX is around orbelow 2%. Because of the uncertainty in X (the solid angle),a systematic error of �5% should be attached to Np andrelated quantities (ion fluence, number of secondary elec-trons, electron yield). Since the ion fluence involves in addi-tion the area of the beam spot, the maximum systematicerror in this quantity is estimated to be 13%. The uncertain-ties assigned to t will be based upon the iterative procedure

Fig. 2. Thick-target spectrum obtained for C + C(Ta) at 9.85 MeV andhLAB = 160�. The SIMNRA fit (- - -), overlapping with most of theexperimental points, gives a carbon-foil thickness of 21.45 lg/cm2. Toillustrate the sensitivity of the method, the simulation corresponding tohaving no foil is also shown (� � �).

described below. All SIMNRA calculations in this workused the stopping power values of [28] and no attemptwas made to include error bars associated to these values.We may point out, however, that since thickness-differences(buildup) rather than absolute thickness values are the rele-vant quantities in this work, a near cancellation of errors inthe stopping power should be expected.

4. Results and discussion

The method will be illustrated by presenting first theanalysis of the experimental data obtained with targetone. This actually corresponds to the most representativedata set since the majority of the points were taken withthis target. Later in this section, the results for all three tar-gets will be shown.

4.1. Comparative analysis of C + Ta and C + C(Ta) spectra

In reference to target 1, Fig. 3 presents the three experi-mental spectra taken with the Ta substrate facing the beam(Ta) along with the corresponding ones where the carbonfoil was exposed to the beam (C(Ta)), taken at the sameenergies. For a given energy, the Ta and C(Ta) spectra weretaken one after the other but otherwise they correspond tothe first (13 MeV), an intermediate (10.9 MeV) and the lastexperimental energy for target one (9.85 MeV), respectively.The SIMNRA fit to the 13 MeV Ta spectrum (not shownhere for the sake of clarity, but see [24]) gave the energycalibration adopted as the original detector calibration

Fig. 3. Thick-target spectra obtained for C bombardment of the baresubstrate (Ta) and the substrate with the carbon foil (C(Ta)) athLAB = 160�, at different 12C energies. The Ta spectra have been rescaledto the corresponding C(Ta) ones and, for the sake of clarity, arbitraryconstants have been added to the two top pairs, as indicated on the right-hand-side. The observed spectra shifts are caused by the energy loss in thecarbon foil.

Table 1Target thickness versus accumulated fluence at target 1 for the energieswhere the bare substrate was also irradiated

E (MeV) t 1015

(at/cm2)t

(lg/cm2)

PNp

1015 ionsFluence1015 ions/cm2

13.00 909 ± 11 18.3 ± 0.2 0.22 2.2910.90 1222 ± 20 24.0 ± 0.4 5.99 62.359.85 1509 ± 20 29.4 ± 0.4 13.37 138.92

Fig. 4. Target thicknesses obtained from the spectra shifts in Fig. 3,plotted versus the accumulated ion fluence at target. The solid line is theresult of a linear fit.

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for the C ions. This calibration differed by only 1.6% fromthe one obtained with the a source.

The shift between the Ta and the C(Ta) spectra corre-sponding to the same beam energy in Fig. 3 characterizesthe thickness of the carbon foil (t) for the given energy.Since these are spectra measured sequentially in time,uncertainties related to PHD variations or any possible sys-tematic error, are effectively minimized. Instead of keepingthe original detector calibration, spectra taken under differ-ent PHD conditions were simulated by changing corre-spondingly this calibration, as described in [24]. By usingthe kinematic and energy loss calculations incorporatedin SIMNRA, corresponding t values were deduced, withthe results shown in columns 2 and 3 of Table 1. Clearly,a large carbon buildup occurred during the targetirradiation.

Attempts have been made [3,4] to experimentally corre-late the buildup with quantities such as bombarding time,beam current, beam density and beam area, among others,but in all cases these were not independent variables. So,when the bombarding-time dependence was being ana-lyzed, the beam intensity was kept fixed, while the collectedcharge remained constant when studying beam-currentdependence, etc. We shall search for a correlation with f,the accumulated fluence at target, and show that, withinthe experimental uncertainties, this may be considered asan independent variable.

The accumulated number of projectiles at target,P

Np,was determined iteratively by accumulating the corre-sponding number of projectiles (Np) deduced with SIM-NRA for each spectrum, from the start of the experimentup to the point being analyzed. Counting from the begin-ning of the first measurement, the above t values corre-spond to the

PNp values indicated in column 4 of Table

1. By using the known diameter of the beam spot at target,the ion fluence f can be also calculated, as shown in column5. As mentioned above, a maximum systematic error of13% should be attached to the fluence. The results are dis-played in Fig. 4, where a linear relationship is stronglysuggested.

4.2. Analysis of C + C(Ta) data. Iterative procedure

By using the results of Fig. 4 as a guide, iterative SIM-NRA fits were made to the thick-target spectra for all theremaining experimental points taken with the same target,i.e. the ones corresponding to the C(Ta) target situation.

Once again, the detector calibration had to be adapted tothe different PHD conditions [24], but the previous resultsproved to be most useful to solve possible ambiguities. Thestraight line in Fig. 4 was used to estimate initial values forthe C-foil thickness in the SIMNRA fits. In addition, theinitial calibration for each fit was obtained by settingthe initial thickness as described above and starting fromthe calibration of the previous experimental point, makinga first fit where only the calibration was varied. Before this,the number of projectiles was fit to correctly reproduce theheight of the spectrum. Once having the initial values forthe three parameters, a next fit was performed where allof them were allowed to vary and, finally, the thicknessof the carbon foil was iteratively changed until self-consis-tent values were obtained.

Carbon buildup at the surface of the bare Ta was alsotaken into account within the iterative procedure by com-paring the yields of c-rays for C + Ta and C + C(Ta) forthe same bombarding energy. Picking c-lines characteristicof C + C fusion, the yield ratio normalized to number ofprojectiles gives the carbon proportion in the targets. Theaccumulated carbon deposited on the (originally) bare Taamounted to 1.3 lg/cm2, 1.5 lg/cm2 and 3.9 lg/cm2 afterthe runs at 13 MeV, 10.9 MeV and 9.85 MeV, respectively.

The consistency of the whole procedure was tested bycomparing the results obtained for the thickness at13 MeV, 10.9 MeV and 9.85 MeV (Fig. 4 or Table 1) withthose determined as described above, and by repeating theprocedure in reverse order, i.e. beginning first from the13 MeV point in decreasing-energy order and then fromthe 9.85 MeV data in increasing-energy order. The rela-tively small differences obtained after the procedure wascompleted, were used to assign corresponding error bars

Fig. 6. Dependence of the carbon buildup with the accumulated ionfluence at target, including results for all three targets.

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to the determined quantities for each measured point. Thefinal results for the dependence of the foil thickness withthe ion fluence at the target are presented in Fig. 5, wherethe points of Fig. 4 are also included for comparison pur-poses. The straight line resulting from the fit to all pointsis slightly different from the one in Fig. 4 but the differenceis consistent with the experimental errors. The importantresult is that the linear correlation suggested by the threepoints in Fig. 4 is confirmed in this more statistically signi-ficative sample.

Similar results were obtained for the experimental pointscorresponding to the second and third targets. TheC-buildup results for all three targets are presented inFig. 6 in the form of a thickness variation, Dt, as a functionof the accumulated ion fluence at target. Dt is simply calcu-lated by subtracting the independent term in the fit,Dt = t � A (see Fig. 5 for target 1). Target 2 seems to showa trend toward a lower slope, but the linear relationshipstill gives a good fit to all points within the experimentaluncertainties. The solid line shows the result of such a fit,which was constrained to go through the origin. The corre-lation coefficient is 0.993 in this case, with a standard devi-ation of 2.1. The buildup rate is 4.38 ± 0.03 atoms per ion,where the deposition area was calculated from the beam-spot diameter. Notice that the 13% systematic error inthe fluence should not affect the rate or its error.

Since the main goal of the present analysis was to quan-tify the carbon deposition on target for normalizationpurposes, no attempt was made to measure the target tem-perature at this stage. However, this can be estimated fromthe energy delivered by the beam, whose rate varied

Fig. 5. Dependence of the C-foil thickness with the accumulated ionfluence at target. The circles were obtained with the iterative proceduredescribed in the text and the values of Fig. 4 are also included (squares) forcomparison purposes. The solid line represents the result of a linear fit tothe data.

between 0.03 and 0.18 W for the different experimentalruns. These rates were quite accurately determined sincethe beam energy, the number of projectiles per runand the corresponding elapsed time are well-known quanti-ties. The generated heat spreads to the whole target holderby conduction and to the chamber walls by radiation.Taking into account the material (Al) and dimensions(area = 25 cm2) of the target holder, and using an effectiveabsorptivity of 0.3 for the target-plus-holder system, a meantemperature of 37 ± 7 �C was estimated for the targetduring the present experiment.

4.3. Correlation with electron emission

According to the hypothesis of Blondiaux et al. [3], men-tioned in Section 1, a natural variable to correlate withshould be the number of secondary electrons emitted bythe target. It is known that electrons are emitted both fromthe projectile entrance and exit surfaces of thin carbon foils[29,30]. In fact, measurements have been reported [30] ofthe electron yield c (no. of electrons per incoming projec-tile) for both surfaces (cB for backward and cF for forwardelectrons), as a function of the projectile atomic number Z,the incident charge state and the projectile energy. Becauseof the target backing, which prevents any emission in theforward direction, only cB is relevant for our experiment.

In this work, the experimental number of emitted elec-trons for each run, Ne, was extracted from the differencebetween the collected charge, Q, and that carried by thebeam, Npq

+e, being q+ the projectile charge state and e

the charge of the electron. In contrast to other works,where Faraday cups and targets are usually biased to facil-itate electron collection, our method has the advantage thatno additional electric fields are introduced which might

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affect the emission mechanism. Uncertainties related to thepossible escape of fast electrons, having enough energy tosurmount the given electric fields, are also eliminated in thismethod.

The backward electron yield, cB = Ne/Np, is plotted inFig. 7 as a function of the beam energy. Except for a fewfluctuations, the yield stays nearly constant around a cBvalue close to 8, even though the charge state of the beamwas changed from 4+ to 3+ at 10.6 MeV. These cB valuesare lower than those reported by Clouvas et al. [30] in thesame energy region and for the same charge states, as indi-cated with the black squares in the figure. A difference wasto be expected because of the different angles of incidenceof the projectile with respect to the surface normal(a = 0� in [30] versus 15� in our case). Since the yield goesas 1/cos(a) [31], however, the expected difference should bearound 3.5% but in the opposite direction, i.e. the yieldshould increase with a.

Since the electron emission may vary with the texture ofthe target (amorphous, single crystal and polycrystal), thisdiscrepancy could mean that the deposited layer has a dif-ferent texture than the original carbon films. Indeed, thestructure and properties of deposited films have beenshown to depend on the deposition parameters [32] and,for lower energies, ion bombardment of amorphous carbonfilms has been observed to induce the formation of dia-mond and graphite [33]. This presumed texture modifica-tion might also be the cause that the average change ofabout �1.5 in cB that was observed in [30] when changingthe beam charge-state from 4+ to 3+, is not reproduced inour data. A slight increase in cB with decreasing energy, ofabout 0.5 per MeV, was also observed in [30]. This energydependence for the electron yield does not seem to be

Fig. 7. Backward electron yields obtained in this work, as a function ofthe beam energy. For comparison, data from [30] are also shown. Theerror bars in our data do not include the 5% systematic error.

confirmed in our experiment, but here again, the fact thatthe experiment was carried out from high to lower energiesled to an increasing C-buildup with decreasing energy,which could contribute to hide any possible increment in cB.

The ‘‘spikes’’ observed in Fig. 7 can be also qualitativelyunderstood in terms of a possible texture change. The spikeat 9.85 MeV, for example, occurred precisely for the firstpoint measured with the second target, when the buildupwas probably not enough to appreciably change the surfacetexture. The previous point in the experiment, also at9.85 MeV, was the last point taken with the first target sothat a considerable amount of carbon had been depositedby then. The point at 9.7 MeV, on the other hand, wastaken right after the one giving the spike and the obtainedcB value seems to indicate that the buildup-related texturewas already dominant during the corresponding run. Sim-ilar reasoning could lead to explain the spikes at 9.25 and13 MeV.

The effect of the buildup on cB can be nicely seen in the9.1 MeV points. The corresponding run was subdivided inthree parts with independent RBS spectra taken for eachpart. Since the energy was the same for all three partsand all other experimental conditions were kept fixed, cor-responding changes in cB can be safely ascribed to effects ofthe irradiation, such as an increase in the buildup. In unitsof 1015 (at/cm2), the carbon deposit increased from 22.1 to36.9 for the first part, and then to 121 and 169 for the sec-ond and third parts, respectively. The corresponding valuesof cB were 8.22, 7.09 and 6.99, respectively. Clearly, theeffects of the deposit are more dramatic for small absolutevalues of the buildup, i.e. when the texture could be stillchanging from the one of the original foil to that associatedto the deposited layer.

The ratio of number of carbon buildup atoms to numberof emitted secondary electrons can be obtained from Fig. 8,

Fig. 8. Carbon buildup per emitted electron.

434 E.F. Aguilera et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 427–435

which is equivalent to Fig. 6 except that the accumulatednumber of emitted electrons per unit area replaces theion fluence in the horizontal axis. Results obtained withall three targets are included. The fact that the two figuresare so similar is a consequence of the near constancy of cB.The fluctuations observed in Fig. 7, produce relativechanges in the corresponding points of Fig. 8 which actu-ally helped to slightly improve the agreement between theresults of targets 1 and 2, especially for large fluences. Inany case, these results are indeed consistent with a constantbuildup per emitted electron, thus supporting the hypothe-sis of Blondiaux et al. The results of a linear fit goingthrough the origin are shown by the solid line in Fig. 8.A correlation coefficient of 0.995 was obtained in this case,with a standard deviation of 1.7 and a buildup rate of0.540 ± 0.003 at/el. Similar to the case for the at/ion rate,this value or its uncertainty are not affected by the 13% sys-tematic error in

PNe.

5. Summary and conclusions

The phenomenon of carbon buildup was studied fora backed carbon foil under bombardment with 8.95–13 MeV carbon ions. An experimental technique usingRBS is presented which allows the simultaneous determina-tion of the number of incident projectiles, the C-targetthickening produced by the buildup, and the number ofsecondary electrons emitted by the target under the ionbombardment. An iterative method is devised in order toprevent possible ambiguities in the analysis of the thick tar-get spectra. The technique does certainly solve the problemof on-line buildup monitoring during nuclear reaction mea-surements [23].

Within the experimental uncertainties, a linear correla-tion is found between the amount of carbon buildup andthe accumulated beam fluence at target. The correlationexists regardless of the vales taken by other experimentalvariables such as beam energy, beam current, collectedcharge or irradiation time.

A nearly constant electron yield cB � 8 is found, show-ing a few fluctuations with beam energy. As compared toprevious reports, this value is low and does not show anyconsistent variation with beam charge or energy. A possi-ble explanation is given in terms of a texture change ofthe deposited layer with respect to the original foil. Theobserved fluctuations in cB are consistent with thishypothesis.

The buildup rate per emitted electron is found to remainconstant within the experimental uncertainties. The linearcorrelation of the buildup with the emitted electrons is evenbetter than the one with the ion fluence. This supports thehypothesis set forward in a previous work [3] in the sensethat the secondary electrons emitted by the target areresponsible for dissociating the hydrocarbons of the resid-ual gas near the sample. Within this mechanism, charged orpolarized fragments would then be attracted to the target,which may be itself transiently charged, thus producing the

buildup. Partial carbonization of the polimerized hydrocar-bons could then occur.

An alternative mechanism could be proposed basedupon the adsorption phenomenon. In the absence of irradi-ation, the flux of hydrocarbon molecules incident on thetarget and the mean time of residence of the moleculeson the surface (prior to re-evaporation) combine to givea steady state situation with some surface coverage of Nadsorbed hydrocarbons per unit area. According to Hirsch[2], N will be typically reached with a time constant of a fewseconds, i.e. before ion bombardment can commence.Therefore, there is no contradiction with the observationthat carbon growth occurs under irradiation only [3,4].Upon ion bombardment, the outgoing secondary electronscould dissociate adsorbed hydrocarbons into free radicalsinducing polymer films firmly attached to the surface,much in the same way as electron bombardment does[20]. Repetition of this process under continued irradiationcould thus lead to the observed C-buildup. Notice that theeffects of cold traps and/or target heating, which affect theadsorption process, would be automatically included in amodel based upon this mechanism.

Both proposed mechanisms are consistent with thehypothesis that the secondary electrons play a principalrole in the process. Following this hypothesis and usingthe results of this work, we may say that for a carbon targetwith no nearby cold trap which is bombarded with ener-getic carbon ions, the carbon buildup at the front surfacecould be estimated from the measured rate of 0.540 ±0.003 at/el if the number of emitted electrons is measured.In case of using a different incident angle a, appropriatecorrections should be applied, as mentioned above. Forions with energies around 10 MeV, as in our experiment,and for a target kept at a temperature around 37 �C, thebuildup could be also estimated from the ion fluenceaccording to the obtained experimental value of4.38 ± 0.03 at/ion.

In using these estimations, however, one should beaware that different experimental conditions, such asheating the target [3,4], using a cold trap close to it [4] orincreasing the beam intensity beyond some etching thresh-old [2], might drastically change one or both values of thegiven rates. If the secondary electrons produce the buildup,using electron suppression biases should also change theat/ion ratio. It would be interesting to experimentally provethis point, which would imply that electron suppressioncould certainly help to effectively minimize carbon buildup.In addition, it would help to discriminate between the pre-vious hypothesis and the one proposed in [2] in the sensethat it is the ion impact (not the electrons) what polimerizesthe adsorbed molecules.

Future experiments should also measure the at/elbuildup ratio for other bombarding ions and different tar-get materials. A more extensive investigation of a possibleion-energy dependence of the buildup should be also con-ducted. Finally, a more direct experimental confirmationof the texture-change hypothesis to explain the variations

E.F. Aguilera et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 427–435 435

in the electron yield would be most valuable. It is hopedthat this work will stimulate both theorists and experimen-talists to look for a more complete explanation of theseinteresting phenomena.

Acknowledgements

Thanks are due to Pedro Villasenor and GaudencioLinarte for their valuable help in keeping the acceleratorrunning during the experiment. The authors also thankR. Policroniades, A. Varela, E. Moreno, H. Berdejo, J.Aspiazu, D. Lizcano, H. Garcıa-Martınez, A. GomezCamacho, E. Chavez, M.E. Ortiz, A. Huerta and R.Macıas, for their help in data taking. This work waspartially supported by the CONACYT (Mexico).

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