Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, Japan

PASJ2017 THOL04

GRAPHITE THIN FILMS FOR ACCELERTOR APPLICATIONS
A. Tatami†, Y. Kawashima, M. Murakami, K. Murashima, M. Tachibana
Material Solutions Research Institute, Kaneka Corporation,
5-1-1 Torikainishi, Settsu, Osaka 566-0072, JAPANH. Hasebe, H. Okuno Nishina Center for Accelerator-Based Science,
RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, JAPAN
Abstract
Graphite thin films were prepared through a simple twostep carbonization-graphitization process from polyimidefilms and were applied as rotating charge stripping disks
for high intensity ion beam irradiation. These films demonstrated high mechanical robustness within the range of 300– 1000 rpm under vacuum conditions and worked remarkably well as charge stripping foils. Changing the thicknessof graphite thin films also enabled precise control of theion beam charge states. Further demonstrating its exceptional durability, a total of 3.31 × 1018 calcium ion particles
was stripped of their charge with the intensity of 10 eA.
The graphite thin film disks exhibit great potential for next
generation stripping foils by significantly extending stripping foil lifetimes under the highest intensity ion beam irradiation.

As shown in Table 1, the rotating graphite disks withstoodheavy ion radiation damage, which compared favorably tostationary amorphous carbon foils or glassy carbon disks.
The graphite disks also showed higher ion beam current
and charge stripping efficiency relative to the often-used
Beryllium disk. Furthermore, the lifetime of this graphite
disk was two times longer than that of Beryllium.
Table 1: History of Last Charge Stripping Foils in RIKEN
Nishina Center
ChargeStrippingFoils
ArizonaCarbon Foil
Beryllium
DiskGlassyCarbon
Graphite
SheetINTRODUCTION
Charge stripping foils are used at accelerators to change
the charge state of ion beams to appropriate values for acceleration. Due to its low density and excellent thermal andmechanical properties, carbon has been widely used forthis purpose. Common carbon foils are amorphous carbon
foils [1], Diamond-like carbon foils [2], CNT-carbon foil[3], and graphene foils [4]. Recent upgrades of accelerators,
however, have led to higher ion beam intensities, which inturn have increased the demand for carbon charge stripping
foils with extended lifetimes [3, 5-7]. To meet the requirement of long life charge stripping foils, the foils shouldhave high thermal conductivity to avoid evaporation ordamage from local beam heating. Other important characteristics include thermal stability, large thermal loading
area, and mechanical robustness.
Kaneka has been a supplier of graphite sheets, in which
the sheets are prepared from polyimide films annealed attemperatures of around 2900 ºC [8]. The sheets exhibit approximately 4 times higher thermal conductivity relative tothat of copper. Other key features include their high thermal stability of up to 2000 ºC under vacuum, and high tensile strength of about 40 MPa. All of these features provide
strong support for these sheets as candidates for highly durable charge stripping foils.
RIKEN Nishina Center has been testing these graphite
sheets as charge stripping foils since the autumn of 2014[9]. Graphite rotating disks were previously prepared foruranium ion charge stripping using two sheets of 35 mthickness.
___________________________________________

Maximum
Beam Intensity
(eµA)
2-3
121215Life Time(Charge)
7.12 × 1015
(71+)
1 × 1018(64+)
Not measured
2.19 × 1018
（64+）

Recent efforts at Kaneka have led to new techniques thatenabled the preparation of graphite thin films (GTF) withless than 10 m thickness. Precise thickness control was
found to be attainable through careful control of the starting polyimide film thickness. The thickness of an appropriate charge stripping foil strongly depends on the type of ion
and ion beam energy. For example, approximately 1.5 m(0.30 mg·cm-2) thickness is required for changing U35＋ ion
to U71+. The aforementioned capability of precise and facile
thickness control in GTF is therefore highly advantageous.
Two aspects that required testing on GTFs prior to their application as charge stripping foils were their mechanical robustness, under high rotation speed, and lifetime. In thispaper, we describe the rotating and charge stripping test of
the GTFs, as well as measurements on their lifetime underhigh intensity ion beams irradiation.

RESULTS AND DISCUSSIONS
Preparation and Characterization of Graphite
Thin Films [10]The thin polyimide films with thickness in the range of2 – 25 m, as shown in Fig. 1 (a), were subjected to carbonization at temperatures of up to 1400 ºC.

† Atsushi.Tatami@kaneka.co.jp- 159 -Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, JapanPASJ2017 THOL04

Figure 1: Preparation of graphite thin films (a) Kaneka polyimide film (b) Carbonized film (c) Graphite thin films (GTFs).

Figure 2: (a) Raman Spectra (b) Cross sectional SEM image and (c) TEM image of GTFs.
The resulting carbonized films shown in Fig. 1 (b) werethen subjected to graphitization at 2900 ºC to obtain GTFs
with approximately 1 – 9.8 m (0.23 – 2.2 mg·cm-2) thickness (shown in Fig. 1 (c)). The thickness and area of GTFs
was controllable by carefully selecting polyimide films
with the appropriate thickness and area, respectively.
The Raman spectra of GTF with thickness of 2.1 m asshown in Fig. 2 (a) exhibited a symmetric peak at around1585 cm-1 and an asymmetric sharp peak at around 2717
cm-1. These features indicate that the GTF was composedof highly oriented graphite. The cross-sectional SEM image in Fig. 2 (b) and TEM image in Fig. 2 (c) both show
highly oriented multilayer graphene layers in the depth
profile.

Rotation Test of Graphite Thin Film

real conditions. The result of the test is shown in Fig. 3 (b),where the disk after testing exhibits no damage nor deformation.

Charge Stripping Test with High Intensity Uranium Ion Beam [11]Charge stripping tests were performed for GTF disks(0.45 – 2.2 mg·cm-2) and Kaneka graphite sheet disks (14mg·cm-2). The rotation speed of disks was set at 300 rpm
and was increased up to 1000 rpm. The incident beam used
was U64+ with 10 eμA at a beam energy of 50 MeV/u and abeam diameter of about 4 – 5 mm; the beam intensity corresponds to a thermal load of 4.4 – 164 W. The charge distribution after the GTF disks was measured and shown inFig. 4.
The mean charge states of the GTF disks with thicknessof 0.45 (Red), 0.91 (Green), 2.2 (Blue) and 14 (Black)mg·cm-2 were 74+, 78+, 82+, and 87+ respectively. Thethicker charge stripping foils provided a higher fraction andnarrower charge states of uranium ion. Beam intensity fluctuation was not observed in the downstream in all GTF
disks.

A donut shaped GTF (0.23 mg·cm-2) with an outer diameter of 55mm in Fig. 3(a) was used to perform a rotating
test using the RIBF accelerator at RIKEN Nishina Center.
The thickness uniformity of charge stripping foils influences the fluctuation of beam intensity. Notably, thinner
disks are preferentially rotated at higher speeds for preventing undesired fluctuation during charge stripping. Therotation speed of the disk was therefore set at 300 rpm atthe beginning and was increased up to 1000 rpm to mimicLife Time Measurement at Calcium Ion Beam
[11]

Figure 3: Optical images of the GTF disk (a) before and (b)after rotation test.

Disk lifetime is a key characterization parameter for
stripping foils. A donut shaped GTF disk with an outer diameter of 110mm and a thickness of 2.2 mg·cm-2 was subjected to calcium ion beam. The disk was rotated at 300rpm and the Ca16+ ion was stripped into Ca20+ at an incident
energy of 45 MeV/nucleon. A total of 3.31 × 1018 calcium
particles with an intensity of 10 eμA were irradiated on theGTF disk and the thermal load of the disk was 6.4 W. Fig.5(b) shows the disk after irradiation. Slight footprints of theion beam were observed near the outer edge, in addition tosmall deformations within the disk. Although a minor beamswing was observed, these deformations, nevertheless, did- 160 -Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, JapanPASJ2017 THOL04

not affect the beam intensity at the downstream. Collectively, these results indicated that GTF disks possess a long
lifetime and the ability to strip the charges away from highintensity ion beams. Furthermore, the disk showed enoughmechanical robustness under high speed rotation while being exposed to high intensity ion beam irradiation.

Figure 4: Charge distribution of 238U64+ ion beam using
GTF disks.

Figure 5: GTF disks (a) before and (b) after calcium ion
beam irradiation.

Nucl. Instr. Meth. Phys. Res. A 328, pp. 168-172,1993.
[3] H. Hasebe et al., “Development of a new foil compounded from carbon nanotubes and sputterd-deposition carbon”, J. Radioanal Nucl. Chem. vol. 299, pp.
1013-1018, 2014.
[4] I. Pavlovsky and R.L. Fink, “Graphene stripper foils”,
J. Vac. Sci. Technol., B 30, pp. 03D106-1, 2012.
[5] W. Barth et al., “Carbon stripper foils for high current
heavy ion operation”, J. Radioanal Nucl. Chem. vol.
299, pp. 1047-1053, 2014.
[6] K. Kupka et al., “Intense heavy ion beam induced
temperature effects in carbon-based stripper foils”, JRadioanal Nucl. Chem. vol. 305, pp. 875-882, 2015.
[7] S. Korenev et al., “The use of graphene as stripper
foils in Siemens Eclipse cyclotrons”, in Proc. Cyclotron2016, Zurich, Switzerland, Sep. 2016, paper
TUP08, pp. 181-183.
[8] Kaneka Corporation; http://www.elecdiv.kaneka.
co.jp/english/graphite/[9] H. Hasebe et al., “History of solid disk improvementfor rotating charge stripper”, in 13th International
Conference on Heavy Ion Accelerator Technology(HIAT2015), Sep. 2015, Tokyo, Japan, paper
MOA1C01, pp. 17-19.
[10] A. Tatami, M. Tachibana, M. Murakami, T. Yagi, M.
Murakami “Preparation of multilayer graphene sheets
and their applications for particle accelerators”, in28th World Conference of the International Nuclear
Target Development Society (INTDS 2016), CapeTown, South Africa, Nov. 2016, O-3 in press.
[11] H. Hasebe et al., “Developing of rotating graphite carbon disk stripper”, in 28th World Conference of theInternational Nuclear Target Development Society(INTDS 2016), Cape Town, South Africa, Nov. 2016,
O-15 in press.

CONCLUSION
Graphite thin films were prepared though a simple twostep carbonization- graphitization process from polyimidefilms. Prepared GTFs were processed into rotating charge
stripping disks and showed enough robustness against both
high speed rotation and high intensity ion beam irradiation.
Furthermore, GTF disks exhibited long lifetime. These results indicated that this GTF disks have great potential for
charge stripping foils under highest intensity ion beam irradiation.

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