RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-8. Accelerator

Improved-flatness beryllium disk stripper for uranium acceleration
at RIKEN RIBF
H. Hasebe,＊1 H. Okuno,＊1 H. Kuboki,＊1 H. Imao,＊1 N. Fukunishi,＊1 M. Kase,＊1 and O. Kamigaito＊1
We have successfully provided a stable uranium beamduring long-term operation in November 2012 (totally 1.18
× 1018 particles in 37 days) using a rotating beryllium disk
stripper with a thickness of 0.1 mm as the second stripper.
The number of irradiating particles and disk conditions aresummarized in Table 1 along with those of other disks
described below. This first used Be disk is denoted as Disk
1 in the Table. However, emittance growth due to thenonflatness of the disk exceeded the accepted levels for
subsequent cyclotrons (IRC, SRC)1).
To realize flatter disks, we prepared a Be disk subjected
to diamond polishing (Disk 2) in March 20132). Also, disk
thickness was reduced from 0.1 mm to 0.085 mm, which
was suited for the IRC injection energy. Therefore,transmission efficiencies of the IRC and SRC wereimproved. The Be disk was still usable even after the totally
9.29 × 1017 U-particle irradiation during the 30-day beamtime operation3).
Disk 2 was used again for the U beam time in March
2014. The lifetime of this disk ended and was determined tobe additional 21 days (Fig. 1: Right). The total number of U
beam particles was 1.68 × 1018 during 51 days (including 30days in 2013) as written in Table 1. Many cracks wereobserved along the beam irradiation traces.
The Be disk was replaced with a new one (Disk 3), which
was identical to Disk 2 (0.085-mm-thick, diamondpolished), for the remaining beam time. Irradiation with anadditional 8.83 × 1017 U particles was carried out in 17 days.
Beam transmission efficiency was improved, but since thethermal load to the disk was increased from 90 W to 230 W
because of the increased beam intensity, the disk wasgreatly deformed. The difference in the deformation isshown in Fig. 1 (Right: Disk 2, Left: Disk 3).

Table 1. Summary of four Be disk used.

Be Disk 1
Not polish
0.1-mm thickBe Disk 2
Diamond polish
0.085-mm thickBe Disk 3
Diamond polish
0.085-mm thick
Be Disk 4
Diamond polish
0.085-mm thick
φ110mmSpecial processing

Irradiation 　current
Total beam particle
4 - 5 eµA18

1.18x10DaysState
Many cracks

37

Still usable
Slight beam fluctuation

4 - 12 eµA
18

1.68x1051(30+21)

Distortion and Many cracks
Not usable
Slight beam fluctuation

12 eµA
178.83x10

Distortion, Slightly cracked

17

Still usable
No beam fluctuation

8 eµA

Slightly Distorted

179x10No crack

20

Still usable
No beam fluctuation

In October 2014, we introduced the Be disk with aspecial design2) (Disk 4) to reduce the thermal deformation.
Due to this improvement, this Be disk survived after the U
beam time with approximately 9 × 1017 U-particle
irradiation·in 20 days. Main changes were as follows: 1)
The outer diameter of the disk was 110 mm (from 120 mm),2) 12 areas with cuts existed around the disk circumference,3) the disk holder was made of copper (from aluminum)with an outer diameter of 65 mm, and 4) 12 holes with
2-mm diameter existed around the holder.	  Beamavailability was further improved by the suppression of thebeam fluctuation. The conditions of U beam irradiation arelisted in Table 1 (Disk4). This Be disk is still usable andexhibits no problem. Figure 2 shows the special-design Be
disk: (left) new and (right) after usage.

(a)(b)
Fig. 2. (a) New specially designed Be disk and (b) afterirradiation. Deformation was moderated.
Fig. 1. Polished Be disks after irradiation (Right: Disk 2,Left: Disk 3).

*1 RIKEN Nishina Center

References
1)
2)
3)

H. Hasebe et al., RIKEN Accel. Prog. Rep. 46, 133 (2013).
PASCAL CO., LTD.
URL: http://www.pascal-co-ltd.co.jp/home.html
H. Hasebe et al., RIKEN Accel. Prog. Rep. 47, 144 (2014).

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