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

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  • ca

    en 3ngo,

    onlchaumthic

    innnedpurig t

    sed asrder targe coratings becot can wis exteils, wh

    build-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

    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 MeVNe 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

    * Corresponding author. Tel.: 81 298 864 5576.

    Contents lists availab

    Vacu

    els

    Vacuum 86 (2012) 899e902E-mail address: yasuhiro.takeda@kek.jp (Y. Takeda).of 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 Ne

    beam of 3.2 MeV, 10 mA, under vacuum of 1104 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 of

    residual 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-1. Introduction

    Self-supporting carbon foils are uthe electrons of charged particles in oaccelerated particles and achieve chator. Because of recent higher accelespan of conventional stripper foils hathe development of long-life foils thais now under way [1,2]. However, tha change in the thickness of striper fo0042-207X/$ e see front matter 2011 Elsevier Ltd.doi:10.1016/j.vacuum.2011.04.029stripper foils to peel offo increase the energy ofnversion in an acceler-currents, the short lifeme a serious issue, andithstand high currentsnsion of life has led toich was not seen in case

    of 400 eV, Ne, 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 theGasses tained in the foil plays a role in foil thinning. 2011 Elsevier Ltd. All rights reserved.Charge-exchange irradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen con-Effect of impurities in charge-exchange

    Yasuhiro Takeda a,*, Toshiharu Kadono b

    aHigh Energy Accelerator Research Organization (KEK), Oho1-1, Tsukuba-shi, Ibaraki-kbDepartment of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Ho

    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 foilStripping

    a b s t r a c t

    Carbon thin foils are commthe original foil thickness,thicken by build-up, medicm2) become thinner. Theconditions.The factor causing foil th

    said that impurities contaiCarbon foils contain im

    and then evaporate. Takin

    journal homepage: www.All rights reserved.rbon foil on foil thickness reduction

    05-0801, JapanBunkyo-ku, Tokyo 113-0033, Japan

    y used as a charge stripping material in particle accelerators. Depending onnges in thickness during beam irradiation vary: thin foils (w10 mg/cm2)thickness foils (w15 mg/cm2) remain unchanged, and thick foils (w20 mg/kness reduction differs even under identical manufacturing processes and

    ing is unknown. On the basis of the low sputtering rate of carbon, it can bein the foil cause foil thinning.ties such as water. These impurities dissociate and combine with carbonhis into consideration, we examined the gas composition during beam

    le at ScienceDirect

    um

    evier .com/locate/vacuum

  • chamber, were afxed to stainless steel frames having 15 mm

    radiation values (mC/cm2). Two effects are apparent. First, thethickness of the foil decreased signicantly 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 the

    Foil

    thic

    knes

    s (g/

    cm2 )

    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, depending

    BEAM

    SSD

    Q-mass

    Carbon foils

    Transfer

    TMP

    View Port

    View 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) 899e902900diameter 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 1104 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 withpumping, 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, ve of which were placed in the scattering

    on the foil thickness.the 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 rod

    Faraday cup

    View Portresidual gases. The vertical and horizontal axes represent thepartial pressure (Pa) and the integral of beam radiation values (mC/

    fore and after irradiation.

  • cm2), respectively. CO and N2 were assumed to be proportionalto the amounts of CO (mass number 28) and N2 (mass number28) released; the separation of CO and N2 was calculated fromtheir 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)

    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 (He,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 oxygen

    0

    10

    20

    30

    40

    50

    005100010050

    Foil

    thic

    knes

    s (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 signicantly just as beam irradiationcommenced. Second, the change in foil thickness started slowing down when the radiation value exceeded a certain level (500 mC/cm2).

    b

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

    STRAT a1.E-07

    1.E-06

    1.E-05

    1.E-04

    1.E-03

    0 200 400 600 80

    mC/cm

    Pa

    Fig. 5. Time variation of major components of residual gases. The gure shows a zoomed gpoint when the foil was broken (b). When the irradiation was started, a large amount of gaswas observed.mixed must have been small. Therefore, the conceivable reason for

    1.E-07

    1.E-06

    1.E-05

    1.E-04

    1.E-03

    1420 1440 1460 1480

    mC/cm2

    Pa

    Foil broken 0 1000 1200 14002

    H2CH4H2ON2COO2CO2

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

  • aorelame

    Y. Takeda, T. Kadono / Vacuum 86 (2012) 899e902902b

    Fig. 6. RBS spectrum taken with 2 MeV He beam. (a) shows the spectrum obtained befand W were observed. The Si, Cu and W are impurities. (Si: Oil of vacuum pump, Cu: 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.

    CH2O/ COH2

    Thus, the amount of CO released had a large effect on thedecrease in foil thickness, since 3.2108 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

    that the peaks of O were far fewer than those of any other element after irradiation.beam irradiation, and (b) shows that obtained after the irradiation. Peaks of C, O, Si, Cunt guide of ion beam sputtering, W: lament of ion beam sputtering). It was observedproduced 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.

    Effect of impurities in charge-exchange carbon foil on foil thickness reduction1. Introduction2. Experiment3. Results and discussion4. ConclusionsReferences

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