Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 WEOH01

DEVELOPMENT OF THE THIN ROTATED CARBON-DISKFOR URANIUM ACCELERATION
H. Hasebe†, H. Okuno, H. Imao, N. Fukunishi, M. Kase, O. Kamigaito
Nishina Center for Accelerator-Based Science, RIKEN, Wako, Japan
A. Tatami, M. Tachibana, M. Murakami
Material Solutions New Research Engine, Kaneka Corporation, Settsu, Japan
Abstract
Highly oriented graphite sheets provided by Kaneka
Corporation have been applied as charge stripper disks forheavy-ion acceleration at RIKEN RI Beam Factory since2014. The graphite sheets have a 10-μm-order thickness
and have withstood the heat load of the increasing beamintensity. Kaneka Corporation has also developed thegraphite thin film (GTF) disks and has successfully provided larger GTF disk with a thickness of 1.5 μm. We applied them to the first stripper in uranium acceleration andinvestigated after beam irradiation by SEM. Their performance during beam operations and the lifetime will be discussed.

INTRODUCTIONRIKEN RI Beam Factory has two charge stripping sections for the uranium (U) acceleration. The first stripping
section is essential because the incident U35+ beams are
stripped into the greatly enhanced charge state around twotimes higher. The conventional carbon foil (C-foil) stripper
can efficiently enhance the mean charge state up to U71+,
however, the radiation damage of the increased beam intensity shortened its lifetime in several hours. We developed the carbon nanotube sputter deposition carbon (CNTSDC) foils [1] for the rotating cylinder stripper device toextend the lifetime by enlarging the irradiated area. Thefoils successfully worked for the beam operations until
2011, and the lifetimes of the CNT-SDC foils were 2~5 C
representing by the total irradiated charge, which was 100times longer than static C-foils. However, unavailability ofthe CNT sheet and the continuously increasing beam intensity forced us to replace the CNT-SDC foil stripper to theHe gas stripper device in 2012 [2]. The He gas stripper has
both longer lifetime and better thickness uniformity. But,if we can solve the lifetime issues, C-foil stripper has a lotof advantages in terms of 1) higher charge states, 2) higher
beam transmission because orifices are unnecessary, 3)less residual radiation, 4) easier maintenance and replacement, and 5) shorter system length.

GRAPHITE THIN FILM DISK FOR
ROTATING STRIPPER
We have applied the high-density highly oriented graphite sheets to the rotating stripper device at the second
charge stripping section for U acceleration since 2014. We
have successfully supplied high intensity U beams forlong-term operations. The sheets after the beam operation
had no significant damage in appearance, however, microscopic damages was observed by SEM [3]. Kaneka Corporation also developed thinner graphite thin film (GTF)
disks [4] the thickness less than 10 μm for the second stripping section.

Characteristics of the GTF
The GTF are prepared from the heat-treated polyimide
films at temperatures up to 3000 °C. The GTF characteristics are listed in Table 1. A prominent feature of the GTF
is the very high thermal conductivity of 1500 W·m−1·K−1
in the plane direction. In addition, the GTF has a higher
thermal diffusivity than copper or aluminum, and has ahigh density and uniform thickness.
Table 1: Characteristics of the Kaneka GTF
Typical values

Units
In plane(XY axis)
Thermalconductivity Normal toplane(Z axis)
Thermal diffusivity
DensityTensile strength
Bending
Electrical conductivity

1500
−1

Wm K

−1

5
cm2 s−1g cm−3
MPaCycles
S cm−19.0
2.040>1000013000

GTF disk at the first stripping section in U beamtime
We installed the rotating stripper device just upstream ofthe He gas stripper device. The dimensions of the GTF disk
were 110 mm in diameter and 0.36 mg·cm-1 in thickness(about 1.5 μmt). We used this GTF disk as the first charge
stripper for the U beam operation in October 2018 (Fig. 1(a)). The operation period was four days with the maximum
U beam intensity of 45 eμA (U35+). The disk rotation speed
was 160 rpm and totally 5.8×1016 particles were irradiated.
The used GTF disk was shown in Fig. 1 (b). No significant
damage was observed, while slight deformations were observed in the irradiated area.

___________________________________________† hasebe@riken.jp-9-Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, JapanPASJ2019 WEOH01

As shown in Fig. 1 (b), we couldn’t observe any visible
damage in the GTF disk.

SEM images of the GTF disk surface
Figures 3 (a) and (b) show the no-irradiated part and thebeam-irradiated part, respectively. The magnification factors are 1000 times (left) and 5000 times (right). We foundthat many bumps and some peeled areas in the beam-irradiated surface in Figure 3 (b).
(a)

(b)

Figure 1: (a) GTF disk during U beam irradiation. The irradiated spot was brilliant. (b) The GTF disk after the beam
operation.

Charge distribution(a)

The charge distribution of the U beam after the passage
through the GTF disk was shown by solid squares in Fig.
2. The energy of the incident U beam was 10.75 MeV/u.
The mean charge state was 71 with its fraction of 16%. Thedistribution obtained with the He gas stripper was also plotted in Fig. 2 (by solid diamonds). The mean charge using
He gas was 64 with the fraction over 20%, owing to theclosed shell effect in U ion.
(b)
0.25
0.2
Fraction

Figure 3: SEM images of the GTF disk surface. (a) no-irradiated part and (b) beam-irradiated part.

HeGTF

0.15
0.1

SEM images of the GTF disk cross section

0.05
058

63

68
73
Charge-number

78

Figure 2: Charge distributions of the U beam at 10.75
MeV/u after passing through the GTF disk (solid squares)
and the He gas stripper (diamonds).

Figure 4 shows the SEM images of the GTF disk crosssection. The upper (a) and lower (b) images show the noirradiated part and the beam-irradiated part, respectively.
The magnification factors are 2500 times (left) and 10,000
times (right). The thickness did not change after the irradiation. While many fine layers can be observed at the noirradiated part, the structures become hard tobe distinguished at the beam-irradiated part.

OBSERVATION SCANNING ELECTRONMICROSCOPY
The GTF disks was observed by a scanning electron microscope (SEM) with the electronic scanning micro analyzer (EPMA). A part of the beam irradiation mark was cut
off with scissors and was placed onto the SEM sample
stage. The surface and the cross section of the GTF disks
were observed in two cases where the beam was irradiatedor not. To observe the cross section, a GTF disk piece wasattached onto an Aluminum block and was placed upright.

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Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, JapanPASJ2019 WEOH01

(a)

(b)
Figure 4: SEM images of the GTF disk cross-section: (a) noirradiated part and (b) beam-irradiated part.

SUMMARY
The U acceleration at RIBF has applied Kaneka GTFdisk as the first charge stripper. This GTF disk stripper has
successfully worked and provided stable high-intensity
beams for a long operation period. We couldn’t observe any
visible beam-irradiation damage in the GTF disk. The surface and the cross section of the GTF disks were observed
by SEM. The fine layer structures became hard to be distinguished at the beam-irradiated part. The reason for thedamage seems to be weakening of the bond strength or thelattice defects caused by the beam irradiation. It is noticed
that the GTF disk thickness did not change by beam irradiations. Further analysis by Raman spectroscopy or TEM
may make them clear in details. It is concluded that
Kaneka’s GTF disk is suitable for the charge strippers.

REFERENCES[1] H. Hasebe et al., “Development of a new foil compoundedfrom carbon nanotubes and sputter-deposition carbon”, J. Radional. Nucl. Chem., Vol. 299, Issue 2, pp. 1013–1018. doi
10.1007/s10967-013-2635-3[2] H. Imao et al., “Charge-stripping system for 238U ion beamwith recirculating He gas”, J. Radional. Nucl. Chem., Vol.
299, Issue 2, pp. 941–944. doi 10.1007/s10967-013-2649-x[3] H. Hasebe et al., “Development of High-density Highly Oriented Graphite Stripper”, Euro. Phys. J., to be published asProc. 29th World Conference of the INTDS 2018.
[4] A. Tatami et al., “Graphite thin films for accelerator applications”, in Proc. 14th PASJ, p. 159 (2017).

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