Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Developments of time-of-ﬂight detectors for Rare-RI Ring
Y. Abe,∗1,∗2 D. Nagae,∗2 S. Okada,∗2 A. Ozawa,∗2 T. Yamaguchi,∗3 Y. Saitoh,∗2K. Sawahata,∗2 T. Suzuki,∗3 Y. Yamaguchi,∗1 T. Uesaka,∗1 and M. Wakasugi∗1
Construction of the Rare-RI Ring, which will be
used to measure masses of short-lived rare-RI with arelative precision of 10−6 , is underway at RIBF.1,2)
We are developing three types of time-of-ﬂight(TOF) detectors for installation in the Rare-RI Ring;
two of the three are placed at the entrance (start detector) of the ring and the third is placed inside the ringas the circulative ion detector (CD). The start detector provides the start signal of the TOF system for themass measurement. The CD provides a signal corresponding to each circulation. The CD is indispensable
for monitoring the motion of the particle inside thering at the beginning of the storage.
The required speciﬁcations for the start detector arei) a good timing resolution less than 100 ps because
the total TOF is about 0.7 ms, ii) a large eﬀective area
(100 mm × 50 mm) to cover the large beam size, iii)
small energy loss and energy straggling so as to notaﬀect the mass resolution of mass measurement in thering, and iv) no change in the charge state of the nuclei,achieved by passing them through a detector to avoid
reduction of the transmission eﬃciency in the ring. On
the other hand, the required speciﬁcations for the CD
are i) small energy loss to maintain the momentumof the nuclei within the momentum acceptance during
100 circulations, ii) a high detection eﬃciency, iii) alarge eﬀective area (100 mm × 50 mm) to match thelarge beam size, and iv) a good timing resolution toseparate each circulation (the typical time for one revolution is about 350 ns.) Furthermore, the CD should
be maintained in ultra high vacuum.
To mount the detector in a limited narrow space,
we developed a ”T-shaped” TOF detector, as shownin Fig. 1(a). The left part of the detector consistsof two 1” photomultipliers (R4998) coupled to the topand bottom parts of a 100 µm-thickness scintillator,
while the right part contains one 2” photomultiplier(H2431-50) coupled directly to the right side of thescintillator. It is noted that, in the ”T-shaped” TOF
detector, we can obtain the horizontal position information, which may be used to improve the timing resolution. In the case of heavy nuclei and changes in thecharge state, the 100-µm-thickness of the scintillator
is no suﬃciently thin. We thus consider introducing a
micro channel plate (MCP) detector, as has been usedat ESR3) and CSRe4) , which has a suﬃciently thin carbon foil. To cover the larger beam size at the entranceof Rare-RI Ring, we are developing the detector with∗1∗2∗3

RIKEN Nishina Center
Institute of Physics, University of Tsukuba
Department of Physics, Saitama University

a lager sensitive area.5)
As the CD, we developed a similar MCP-type detector, used at the Gas ﬁlled Recoil Ion Separator(GARIS). 6) When the beam passes through the thincarbon foil (60 µg/cm2 ), secondary electrons are generated in the foil. The generated electrons are transported to the MCP by only the electric ﬁeld. A
schematic view of the detector is shown in Fig. 1(b).
A mirror electric ﬁeld and an acceleration electricﬁeld are how they are created using wires. Wires(W+Au) with a 40-µm diameter are set at distanceof 8.0 mm from carbon foil with a 1.0-mm pitch, andwires (W+Au) for the triangular part are set with a
3.0-mm pitch.
(a)(b)
PMT : R4998
Light guideIon beam

Plastic
scintillator
PMT : H2431-50
PMT : R4998

W+Au wire(φ40 µm / 1 mm pitch)

W+Au wire(φ40 µm / 3 mm pitch)

C-Foil (60 µg/cm2)Ion Beame-MCP

Fig. 1. Schematic view of the (a) ”T-shaped” TOF detectorand (b) circulative ion detector (CD).

The experiment to check the performance of theTOF detectors was carried out at the secondary beamline, SB2 course,7) at HIMAC at the National Institute of Radiological Sciences (NIRS). A primary beamof 84 Kr was accelerated up to 200 A MeV and delivered
to the SB2 course. For the ”T-shaped” TOF detector,a timing resolution of σ ≈ 60 ps is obtained. The position resolution in the horizontal axis is around σ ≈2 mm. For the CD, a timing resolution of σ ≈ 130 ps
is obtained. The detection eﬃciency was about 72%.
This value is comparable with the eﬃciency for theisochronous mass measurement at ESR.8)
We will install these TOF detectors in the Rare-RI
Ring in the next ﬁscal year.
References
1) Y. Yamaguchi, et al.: Nucl. Instr. and Meth. B 317(2013) 629.
2) A. Ozawa, et al.: Prog. Theor. Exp. Phys. (2012)
03C009.
3) M. Hausmann, et al.: Nucl. Instr. and Meth. A 446(2000) 569.
4) B. Mei, et al.: Nucl. Instr. and Meth. A 624 (2010) 109.
5) D. Nagae, et al.: Nucl. Instr. and Meth. B 317 (2013)
640.
6) K. Morimoto et al.: RIKEN Accel. Prog. Rep. 46 (2013)
191.
7) M. Kanazawa, et al.: Nucl. Phys. A 746 (2004) 393.
8) N. Kuzminchuk: PhD thesis, Justus-Liebig Universität
Gießen, (2011).

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