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Vacuum 86 (2012) 899e902

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Vacuum

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Effect of impurities in charge-exchange carbon foil on foil thickness reduction

Yasuhiro Takeda a,*, Toshiharu Kadono b

aHigh Energy Accelerator Research Organization (KEK), Oho1-1, Tsukuba-shi, Ibaraki-ken 305-0801, JapanbDepartment of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

a r t i c l e i n f o

Article history:Received 8 October 2010Received in revised form23 March 2011Accepted 5 April 2011

Keywords:Stripper foilCarbon foilStrippingCharge-exchangeGasses

* Corresponding author. Tel.: þ81 298 864 5576.E-mail address: yasuhiro.takeda@kek.jp (Y. Takeda

0042-207X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.vacuum.2011.04.029

a b s t r a c t

Carbon thin foils are commonly used as a charge stripping material in particle accelerators. Depending onthe original foil thickness, changes in thickness during beam irradiation vary: thin foils (w10 mg/cm2)thicken by build-up, medium thickness foils (w15 mg/cm2) remain unchanged, and thick foils (w20 mg/cm2) become thinner. The thickness reduction differs even under identical manufacturing processes andconditions.

The factor causing foil thinning is unknown. On the basis of the low sputtering rate of carbon, it can besaid that impurities contained in the foil cause foil thinning.

Carbon foils contain impurities such as water. These impurities dissociate and combine with carbonand then evaporate. Taking this into consideration, we examined the gas composition during beamirradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen con-tained in the foil plays a role in foil thinning.

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1. Introduction

Self-supporting carbon foils are used as stripper foils to peel offthe electrons of charged particles in order to increase the energy ofaccelerated particles and achieve charge conversion in an acceler-ator. Because of recent higher accelerating currents, the short lifespan of conventional stripper foils has become a serious issue, andthe development of long-life foils that can withstand high currentsis now under way [1,2]. However, this extension of life has led toa change in the thickness of striper foils, which was not seen in caseof shorter lifetimes. Therefore, we investigated thickness changesof stripper foils in a systematic manner using the beam of the Vande Graaff accelerator of the Faculty of Science of Tokyo Institute ofTechnology and found out that effects vary with differences in foilthickness. As indicated in Fig.1, when foils were radiatedwith a Neþ

beam of 3.2 MeV, 10 mA, under vacuum of 1�10�4 Pa, the thicknessof thin foils (10 mg/cm2) increased (build-up) while that of mediumfoils (w15 mg/cm2) remained unchanged (constant) and that ofthick foils (20 mg/cm2) decreased (thinning) [3,4]. In the case ofbuild-up, hydrocarbons produced by the dissociation of residualgas molecules were deposited on carbon foils. However, the causeof thinning is not yet clear.

When foils are radiated with beams, heated gas is released andsurface atoms sputter from foil surfaces. In the case of an ion beam

).

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of 400 eV, Neþ, for example, the sputtering rate of the atoms ofmetals in general or that of copper in particular is 1.55 atoms/ion,whereas that of carbon atoms is very smalldas small as 0.1 atoms/ion. Therefore, the decrease in foil thickness by sputtering is almostnegligible. On the other hand, in the case of the gases released,impurities contaminated in the foil are emitted or those that havepenetrated the foil reach the surface by diffusion and are thendesorbed. Ion excitation caused by ion irradiation causes chemicalreactions. For example, hydrogen and oxygen producemethane andcarbon monoxide by reaction with carbon. The composition of theresidual gas should be examined to determine the cause of thin-ning, which we attempted in this study. Further, we aimed todetermine the source of decrease in foil thickness by studying therelations between the components of gases released from foils andsuch reductions.

2. Experiment

Ion excitation caused by ion irradiation causes chemical reac-tions. For example, hydrogen and oxygen produce methane andcarbon monoxide by reaction with carbon. We examined gascomposition during beam irradiation to determine which impuritycauses foil thinning.

The experiments were conducted in the Van de Graaff labora-tory at the Faculty of Science of Tokyo Institute of Technology. Abeam of 3.2 MeVNeþ ions (8 mm 4 and 10 mA), accelerated by a Vande Graaff accelerator, was applied to carbon foils placed in a scat-tering chamber. A turbo molecular pump (TMP; 550 l/s) for vacuum

Foil

thic

knes

s (

g/cm

2 )

Integrated irradiation (mC/cm2)

30

20

10

0

Build-up

Constant

Thinning

Broken

Broken

Broken

Fig. 1. Different features of foil thickness by beam irradiation. Thickness of a foil ofapproximately 7 mg/cm2 was build-up, and the foil was damaged as a beam was irra-diated. Thickness of a foil of approximately 15 mg/cm2 was constant until the foil wasdamaged. Thickness of a foil of approximately 30 mg/cm2 decreased, and the foil wasdamaged with beam irradiation. As shown here, three patterns are possible, dependingon the foil thickness.

BEAM

SSD

Q-mass

Carbon

foilsTransfer

rod

Faraday

cup

TMP

View

Port

View

PortView

Port

Movable

Fig. 2. Experimental setup (vacuum chamber). By bombarding a beam on a carbon foilset at the center and using an SSD set 1.5 m downstream of the foil to measure scat-tering particles, foil thickness was calculated. In order to observe the composition ofthe gas emitted from the foil, the composition of the residual gas was measured byusing Q-mass while the chamber was kept evacuated.

Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902900

pumping, a quadrupole mass spectrometer (Q-MASS) for residualgas measurement, and a solid-state detector (SSD) for foil thicknessmeasurement were placed in the scattering chamber, facilitatingobservation of changes in residual gas and in foil thickness withtime. The carbon foils, five of which were placed in the scatteringchamber, were affixed to stainless steel frames having 15 mmdiameter holes (Fig. 2). Long-lived carbon foils, 20e30 mg/cm2 thickand formed by ion beam sputtering, were used in the experiments.The foil thickness was measured by measuring the particlesdispersed by Rutherford scattering with the SSD, which was placed22.5� from and 1.5 m downstream from the foil. The thicknesses ofcommercial foils were based on known standard thicknesses; foilthicknesses were determined by counting carbon particle massesscattered from the measured foils. The residual gas was measuredusing an ANELVA QIG-066 partial pressure vacuum gauge. Therange of mass numbers measured was 1e66 amu; the residual gaswas calculated on a computer on a change-with-time basis. Thevacuumwas maintained at 1�10�4 Pa at the time of measurement.

3. Results and discussion

A total of ten samples were radiated in the experiments. Fig. 3shows photographs of foils before and after irradiation. Fig. 4shows an example of change in foil thickness in comparison withthe radiation value. The vertical axis represents foil thickness (mg/cm2) and the horizontal axis represents the integral of beam

Fig. 3. Photographs of foils be

radiation values (mC/cm2). Two effects are apparent. First, thethickness of the foil decreased significantly just as beam irradiationcommenced. Second, the change in foil thickness started slowingdown when the radiation value exceeded a certain level (500 mC/cm2). The thickness of the foil decreased to 1/4the1/5th of thatbefore irradiation; the foil was eventually destroyed.

Fig. 5 shows the time variations of major components of theresidual gases. The vertical and horizontal axes represent thepartial pressure (Pa) and the integral of beam radiation values (mC/

fore and after irradiation.

0

10

20

30

40

50

005100010050

Foi

l thi

ckne

ss (

g/cm

2 )

Integrated irradiation (mC/cm2)

Broken at 1440 mC/cm2

Fig. 4. Variation foil thickness with irradiation for the initial of 26 mg/cm2. Two effects are apparent. First, the thickness of the foil decreased significantly just as beam irradiationcommenced. Second, the change in foil thickness started slowing down when the radiation value exceeded a certain level (500 mC/cm2).

Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902 901

cm2), respectively. COþ and N2þ were assumed to be proportional

to the amounts of COþ (mass number 28) and N2þ (mass number

28) released; the separation of COþ and N2þ was calculated from

their count ratios. The peak in Fig. 4 corresponds to the time pointat which irradiation was started. No release of carbon aloneoccurred. Hydrocarbon atoms and carbonic oxides were primarilyreleased, their discharge increasing to about twice to eighteentimes the background. After about 3 mC/cm2, the gas released atthe time of commencement of irradiation was attenuated byvacuum pumping and shifted to an equilibrium state.

Analyses of the graphs of the released gases indicate that (1) theamount of hydrocarbon gas released remains almost unchangedfrom commencement to completion of beam irradiation and (2)a large amount of carbonic oxides is released immediately after

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

-1 0 1 2 3 4 5

mC/cm2

Pa

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0 200 400 600 80

mC/cm

Pa

STRAT a b

Fig. 5. Time variation of major components of residual gases. The figure shows a zoomed gpoint when the foil was broken (b). When the irradiation was started, a large amount of gaswas observed.

irradiation, but this amount gradually decreases. In particular, theamount of released CO decreases when the foil thickness changes.We performed composition analysis by Rutherford backscattering(RBS) to determine the cause of foil thinning.

To understand the conditions of foil elements, we analyzedthem before and after irradiation with RBS by using a beam (Heþ,2 MeV, 500 nA, and 1.5 mm 4) produced by the Van de Graaffaccelerator at Kyoto University. Fig. 6(a) shows the spectrumobtained before a beam was irradiated, and Fig. 6(b) shows thatobtained after irradiation. The vertical and horizontal axes repre-sent the number of counts and the number of channels in propor-tion tomass numbers, respectively. Peaks of C and Owere observed.

As the foils were formed in a vacuum, the amount of oxygenmixed must have been small. Therefore, the conceivable reason for

0 1000 1200 1400

2

H2

CH4

H2O

N2

CO

O2

CO2

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1420 1440 1460 1480

mC/cm2

Pa

Foil broken

raph of the time point when beam bombardment of a foil was started (a) and the timewas emitted. When the foil broke, no significant change in the amount of gas emission

a

b

Fig. 6. RBS spectrum taken with 2 MeV Heþ beam. (a) shows the spectrum obtained before beam irradiation, and (b) shows that obtained after the irradiation. Peaks of C, O, Si, Cuand W were observed. The Si, Cu and W are impurities. (Si: Oil of vacuum pump, Cu: filament guide of ion beam sputtering, W: filament of ion beam sputtering). It was observedthat the peaks of O were far fewer than those of any other element after irradiation.

Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902902

the mixing of oxygen is that a large amount of water was used topeel off foils from the boards. The water absorbed in the foils wassplit into hydrogen and oxygen by beam irradiation, and then, theoxygen bonded to carbon and evaporated as carbon oxides, asdescribed by the following chemical formula.

CþH2O/ COþH2

Thus, the amount of CO released had a large effect on thedecrease in foil thickness, since 3.2�10�8 Pam3/m2s of CO wasreleased, even though the amount of vacuum pumped wasexcluded.

A comparison of foil before and after beam irradiation revealsthat the width of the carbon peak of the latter remains sharp,indicating that the foil became thinner. On the other hand, muchfewer peaks of O were observed, fewer in fact than those of anyother element after irradiation, indicating that O evaporated fromthe foil. The measured count number of oxygen decreased to aboutone-fourth (1920 counts before to 541 counts after irradiation); theamount of this released gas was much larger than that of otherelements, which did not change. Therefore, the carbonic oxides

produced on foils appear to be amajor cause of the decrease in theirthicknesses.

4. Conclusions

In this study, we performed a detailed analysis of the gasesreleased from foils to clarify the close relationship between theamount of carbon oxides released and the decrease in foil thicknessduring their exposure to beams. Oxygen content decreased witha decrease in foil thickness. We found that the presence of oxygen isthe main cause of the decrease in the thickness. To retard thisreduction, it is necessary to eliminate the oxygen contained in foils.

References

[1] Sugai I, Takeda Y, Oyaizu M, Kawakami H, Hattori Y, Kawasaki K, et al. Nucl InstrMeth A 2002;480:191.

[2] Takeda Y, Irie Y, Sugai I, Takagi A, Oyaizu M, Kawakami H, et al. Vacuum 2010;84:1448e51.

[3] Sugai I,TakedaY,OyaizuM,KaweakamiH,HattoriT,KawasakiK, etal. Proc.1st Symp.Beam science and technology for an emergent network (BESTEN). Tokyo: 2001.

[4] Sugai I, Oyaizu M, Takeda Y, Kawakami H, Hattori Y, Kawasaki K. Nucl Instr MethA 2008;590:32.