I~VESTIGACIÓ:-l REVISTA MEXICANA DE FíSICA 47 (3) 242-244 JUNIO 2001

Surfacc bchavior of thin carbon strippcr foils under hcavy ion bcambombardmcnt

H. MUlODepartamento del Acelerador, Centro Nuclear de MéxicoApartado pos/o/ /8-/027, //80/ México. D.f':, Mexico

Recibido el7 de diciembre de 2000; aceptado el 19 de fehrero de 2001

Resuhs of ¡¡retimes and surfare irradimiul1hchavior uf (hin (...•..511g1cm'2)carhan foils prcparcd by a mixcd ion bcam spuucring rnclhod aregiven. Thc mean ¡¡retime of rhe foils \Vas 57.6 me (4,6 h), \vhich was ¡¡hOlll 15 (imes looger lhan that of conventional commcrcial foils undcr

3.2 ~1eV. Nc+ ion bcam bombardmcnt. Thc thin foil was abo durable unJcr embao build.up conditioo with normal non-bakablc ehambcr.

Keywords: Strippcr foil\: heav)' ions

Se mucstr<m los resultados de tiempos de vida y comportamiento superficial de películas de carbón preparadas por iones que arrancan elmaterial desde algunas superficies. El tiempo de vida de promedio de las películas es de 57.6 me (4.6 h). lo cual es 15 veces mayor que elde las pdículas comerciales convencionales. Las películas son también durahle ..•hajo condiciones de creciemiento en condiciones nonnalesde hornear.

DescripIOrl'S: Inlercambiadorcs de carga; iones pesados

PAes: 2Y.27.Eg

1. Introduction

A mixed ion beam sputtering method have been tlevelopedfor producing super long-lived self-supporting carbon strip-per foils [\, 2]. The carbon foils of 15 l/g/cm' in lhicknessmade by lhis method were shown to last about 60 times lon-ger than commercially available foils during a 3.2 McV Ne+ion beam bombardment. For the surface behavior of the car-bon foils of sputtering method, it was made clear that lheIifetimc of the carbon foils had a great dependence uponlhe thickness. Thickening and thinning behavior of carbonfoils under heavy ion bombardmenl can be understood withinthe microscopic theory of ion induced alterations in carbonfoils [3). The irradiation damage mainly contains slackening,thickening and shrinkage during irradiation. In Ollf measure-ments the sputtering process was remarkable and no thick-ness ¡ncrease had been observed for the thicker foils. Howe-ver. the thin foils showed differenl behavior under heavy ionirradiation in comparison to the thicker foils. The thin car-bon foils always sho\\'cd conslant average thickness or slightthickcning during irradiaton [2]. Thc increased encrgy lossunder ion irradiation has ofien been ascribcd to carhon build~up from the result gas. Many authors attribuled lhe observedenergy loss variation to foil thickening caused by carbon de-posilion H-6]. In this paper lhe innuence of carbon bllild-upfor the ¡ifelime of the thin carbon foils is rcported.

2. Carhon foil prcparation

Carbon foils produced by lhe mixed ion beam sputtcring met-hod were produced by the well known ion beam sputtcringtechnique [1.2.7]. Typical running parameters of the mixed

ion beam sputtering method are shown in Ref. l. The produc-tion Illcthod is capable of producing in the thickness range of2-80 Jlg/cm"l. Neon and krypton gases were introduced to aduoplasmatron ion source passing through each gas Ro\\' me-ter. Source gas purity was 99.999%. The gas mixlure usedwas ncon/krypton in the ratio 8/1 and acceleration voltagewas 10 kV. Focusing voltage (einzel) was 1.5 kV and IOlalion current was 2.D mA. Target material was Carbon graphite(99.999% p"rily. Poco Ca., USA). Vacllum gauge reading inthe preparation chamber was 10'1 Pa.

Carbon foils were deposited on the surface of a glass sli-de covcred with a release agent (Johnson and Sons Co .. Ltd.CREME COTE). An aluminum foil (3 cm x 3 cm, ~ "m inthickness) positioncd near the glass slide was weighted befo.re and after deposition for a thickness Illeasurement. This wasdone by mean, of an electric micro-balance (Meuler UM 30).AII foils werc mountcd on 17 mm x 14 mm stainless steel hol-ders having a hole of la mm diamcler.

3. Expcrimcntal

Por lifetime measurement, wc constructed and installed abcam line on the 4.75 lvtV Van de Graaff accelerator atlhe Tokyo Inslitule of Technology as menlioned in previouspapers [1,2J. The pressure in the irradiation chamber waskcpt abollt 10-.1 Pa. AII measurcments wcn: made usinga 3.2 MeV, 3.5 ¡lA Ne+ ion bcam over an are¡l of 9.5 111m2.

Transmitted ion bealll was collected in a Faraday cup 10c<1-ted downSlream from the foil and monitored by a currenlinlcgrator.

Thc ¡¡fetime uf a foil is givcn as the total charge incident(milli-coulomb) on the foil nccessary to callse its rupture as

SURFACE BEHAVIOR OFTHIN CARBON STRIPPER FOILS UNDER HEAVY ION BEAM BOMBARDMENT 243

TABLE 1. Lifetimcs of the carbon foils of 5 ¡lg/cm"}. in thidncssmade by (he mixcd ion bcam !'.puttcring rncthod with those com-mcrciallyavaliablc.

indicated by a sudden decrcasc in the transmitted current lolhe Faraday cupo Al intervals during irradiation, lhe change inIhe carbon foil was monitorcd by detecting scattcred neonions [ram lhe irradiated carbon foil by a salid statc detec-tor positioned al a laboratory angle of 22.5° [2]. In addition,commercial foils prepared by thermal evaporation (ArizonaCo. LId.,) of known Ihickncss (51,g/cm2) weee used as stan-dards foc determining lhe thickness of lhe other foils.

10

" -~-N 1= 1.2 "AE

~H" 6".:. -'r ~'l' '• ,o~:cf- 2

I IO

O 0.5 1.5 2 2.5 3 J.S

;'I;umber or incidenl iOlls IX 1017 atomsl

FIGURE l. Average thickness as a function ol' incident ions for lhecarbon foils made by (he mixcd ion heam sputtcring mClhod irra-diatcd by 3.5 ¡lA and 1.2 ¡lA. buth of similar thickness (5 w;/cm"l).In (he 1.2 ¡lA (12.5 ¡lA/cm2) case a carbon deposilion rale 01"-lOng/(cm2min) was dClcrmined.

Lifclimcs (me]

57.G 0105.2

3.70100.5

Nurnhcr of samples

44

Foils

Sputlcring methodCammercía!

Fram the experimental observalion of lhe color of thebombaeded foils, lhe lemperaluee of the foil irradiatedby 3.5 ¡lA beam at the beam spot was estimated about4000 C. In the case of 1.2 ¡lA. we could not estimate the sur~face lemperature because no color change \vas obscrvcd du-ring irradiation. In the case of 1.2 ¡LA we could not estimatethe surface lemperature beca use no color change was obser-ved during irradiation. Therefore. we calculated the surfacetemperaturc using the Stefan.Boltzmann law [11]:

whcre dE / pd:r is the stopping power taken from the tablesof Northc1iff ¡¡nd Schilling [12J, rjJ is lhe partic1e current dcn-sity in units of cm-2s-1• é is the total hemispherical emis-sivity of the foil. pd is foil thickness in units of ¡Lg/cm2

and a is the Stefan-Bo1tzmann constant. For our case thatlhe rjJ = 7.8 X 1013 cm-2s-1, Ne+, 3.2 MeV ion beam wilhpd = 51/g/cm2 we obtained T = 540.0 K (273.50 C). Whi-le, in lhe 3.5 l/A case T = 703.8 K (-130.80 C) was obtainedfram Eq. (1). Of course. this estimation is only quantitative.so this value must be measured accurately in further develop.rnents.

In the terminal of the Tandem Accelerator of the CentroNuclear de rvlexico carbon stripper foils of 5 and 10 ¡lglcm2

in thickness are used. in our daily operation with duoplasma.tron ion source or SNICS (Source of Negative Ions by Ce-siulll Sputtering) the vacuum gauge reading of the accelera-tion tube is 5.0 x 10-.1 ....., 10-3 Pa.

Gencrally, in order lo prevent lhe hydrocarbon build-up which is bringing out scrious multiple scattering.hydrocarbon.free vacuum chambcrs have been used by se-veral laboratories. However, bakable chamber and the highvacuum systcms cost a large amouot. From our data anne-aling of the surface of carbon foils during beam irradiation

4. Rcsults and discussion

l'lble I shows the Iifelimes of the caebon foils of 5 l/g/cm2

in areal density made by the mixed ion beam sputtering met-hod with those cornmercially availablc. The mean ¡¡fetime ofthe sputtering carbon foil was 57.6 me (3.6 x 1017 incidentions), which was about 15 limes ¡onger than that of a conven-tional cornrnercial foils. During the irradiation shrinkage ofthe carbon foil of sputtcring method was very 510w and radialstress I¡nes appeared al the pcripheral regioo of (he bC<lm spatin Ihe last phase of lhe bombaedment. The eupture of lhe foilalways loo k place neae the edge ofthe foil [2].

The thickness ¡ncrease causing increascd Illultiple scat-tering was reported by many authors [8-10]. In OUTvaclIum

cnvironment, \\/c had diffcrent kind of background gases su-eh as hydrocarbons. Howcver, carbon build-up is Iimitcd bysudace tcmperaturc ami uoes nol take place at sufficientlyhigh temperature [8]. Figure I shows the average thicknessas a function of incident ions for the carbon foils prepared bythe mixed ion bcam sputtcring method irradiated by 3.5 ILAand 1.2 ¡,A.

In the case of 3.5 ILA no thickness increase has been ob-served during beam irradiation. On the other hand. in the caseof 1.2 ILA, foil thickening and slight decrease in transrnittedintensity was observed. The mean lifetime of the C<.lrbonfoilwith high intensity beam was shown lo last 57.6 me whichwas about 5 times longer than thal of the foil with carbanbuild.up. The mean lifetime of the low intensity foils \''''as11.8 mC (2.7 h, 7A X lOt6 incident ions). In lhe bOlh high andlow intensity beam irradiation number of measured samplesW;'I!'; four. In addition, the mean lifelime of the commercialfoils with low intensity beam was about 1.5 me (.....,20 min.9A x 1015 incident ions). The shrinkage and the lotal concen-tration of a cOllllllercial foil during irradiation was so grcatthat the additjonaJ material s such as hydrocarbons drawn intoirradiated arca caused greatly increased beam scattering anda reduction in the transmittcd bcam.

,

[rjJ (dE/pdx) pd 1 _,] ¡

T= 2 +3001,Ea

(1)

Reo'. Mex. Fís. 47 (3) (2001) 242-244

244

may have important role lo hall! b~',~, -lle hydrocarbon build-up. even though, with normal non-bakablc chamber.

5. Concl usions

These results have shown 11131a grcal enllanCClllcnt of lheI¡felimes of lhe thin carbon strippcr foils prcparcd by lhe mi-xed ion beam sputtering mcthod. Improvclllcnls in Iifclimcof lhe ;) Jlg/cm2 carban foils al leas! a factor 01' 8 have hc-

1. H. ~luto el al .. NucJ. /1I51r. (l/Ili Alt'//¡. f/ H3 (1993) 291.

2. H. t\1UIO ¡'\lile!. Imjlr. (lIId Ml't1r. H 103 (1995) 249.

3. G. Dollingcr ano P. Maicr-Komor. Nlld. IIl.Hr. (md .\krh. ¡\ 2H2(1989) 223.

..1. P. Dohbcr~tcin ano 1.. Hcnkc. N/te!. IIIJfI: al/{I Mc'lh. 119 (197..$)148.

5. \V.5. Bickel ano R. Buchta. Phys. Scril't. 9 (1974) 14~.

6. P.D. Dumom el al., PhYJ. Scripl. 13 (1976) 1:!2.

11.MUTO

en achievcd by comparing with lhe conventional commercialfoils cvcn with carboll build-up conditiol1.

Acknowlcdgmcnts

Thanks are duc lo ProL 1. Aspiazu of lhe Tandem Labora-tory, Centro Nuclear de México and praL 1. Katayama ofIPNS. High Energy Accelerator Rcsearch Organization fol'thcir very lIscful discussions and encouragcments.

i. G. 51cttcn and P. Knudsen. Nud JIlS1r. and Me/h. 102(1972) 459.

8. A.E. Li\"ing~ton. G.H. Bcrry. and G.E. Thomas, Nucl. JIIStr. andM<,III. 148 (1978) 125.

9. U. Sanocr. H.H. Buko\l.'. and H.Y. Buu1ar. Nue!. "lSlr. {lmi Jleth.167 (1979135 .

10. N.R.S. Tait. Nue!. "1S1r. (¡/Id Meth. 18-1(1981) 203.

11. F. Nickcl. N/le!. "/J/r. lll/{i Meth. 195 (1982) 457.

12. L.e. Northclifr and R.E Sí:hiJling. Nucl. Data A 7 (1970) 233.

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