Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Improved beryllium disk stripper for uranium acceleration at RIKEN RIBF
H. Hasebe,＊1 H. Kuboki,＊1 H. Okuno,＊1 H. Imao,＊1 N. Fukunishi,＊1 M. Kase,＊1 and O. Kamigaito＊1In 2012, we first attempted to use a rotating beryllium(Be) disk with 0.1 mm thick as the second charge stripper
for uranium (U) acceleration1). The Be stripper was
successfully provided stable high-intensity U beam (severalelectric μA on average) during a beam time of 37 daysusing a single disk with no exchange. The lifetime of thestripper was extended drastically compared to before. The
total number of U particles irradiated on one foil/disk
increased from 7.12 × 1015 (carbon foil in 2011) to 1.18 ×
1018 (Be disk in 2012). A remaining problem wasimprovement of the thickness uniformity for improving thetransmission efficiencies of the subsequent cyclotron IRCand SRC. In addition, the Be disk with a slightly thinner
thickness of 0.085 mm was found to be better to match theinjection energy of the IRC.
A thinner Be disk, with a thickness of 0.085 mm, wasfabricated by Pascal Co., Ltd.2), who proposed a special
machining method. They reduced the Be disk thickness of0.15 mm to the desired thickness of 0.085 mm by only
diamond-polishing both sides of the disk; diamondpolishing was used because in the previous study, we hadfound that the standard buff finish process made thickness
uniformity worse.

The polished new Be disk was used for a beam time from
April to May 2013. The outer diameter of the disk was 120mm, and the thickness was 0.085 mm, with a tolerance of 0.005 mm. The arithmetic average roughness (Ra) was lessthan 0.01 μm. A U64+ beam at 50 MeV/nucleon wasirradiated on the Be disk, which rotated at 1000 rpm. Figure
1 shows the polished new Be disk before installation. Figure
2 shows a photograph of the disk after the beam time. As in
the previous beam time, the outer circumference of thebeam-irradiated part (black band in Fig. 2) was deformed
when the irradiated U beam intensity was increased toseveral electric μA. However, unlike the last beam time, nocracks were observed.
The improvement in the thickness uniformity is shown infigure 33). The figure shows the beam intensity trends asmonitored by Phase Probe (PP)-G01 (G01: downstream ofSRC). The vertical and horizontal axes indicate the beam
intensity and scan time, respectively. Signals stay at“beam-on level” in the figure if beams are provided fromthe SRC, whereas signals drop to the bottom “beam-offlevel” if no beam is available. The upper and lower parts are
the trends measured in November 2012 and May 2013,
respectively. The red square parts denote a single rotation
period of the Be disk (60 ms). Availability of the U beams
was improved from 90.7% to 98.8%. A total of 9.29 × 1017
U particles were irradiated on the Be disk over 30 days. It
has been shown that the polished new Be disk is now readyfor practical use.

Fig. 1. Polished new Be disk.

Fig. 3. Signals of phase probe monitor (PP-G01). Verticalaxis: beam intensity. Horizontal: scan time.
References
Fig.2. Slightly deformed Be disk after irradiation.
＊1

RIKEN Nishina Center1) H. Hasebe et al., RIKEN Accel. Prog. Rep. 46, 133 (2013).
2) PASCAL CO., LTD.
URL: http://www.pascal-co-ltd.co.jp/home.html
3) R. Koyama et al., Nucl. Instr. and Meth. A 729 (2013) 788-799.

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