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

Ⅱ-9. Instrumentation

Development of single-crystal CVD diamond detector
for time-of-flight measurements
Y. Sato,＊1 T. Shimaoka,＊2 M. Kobayashi,＊3 M. Takaki,＊3 H. Miya,＊3 M. Matsushita,＊3 S. Michimasa,＊3 K. Yako,＊3S. Shimoura,＊3 M. Tsubota,＊2 J. H. Kaneko,＊2 H. Murakami,＊1 H. Sato,＊1 K. Yoshida,＊1 T. Kubo＊1
Production and identification techniques for radioactive
isotope (RI) beams are important for nuclear physics
experiments involving exotic nuclei that are very unstable.
RI beams are generally produced as secondary beams fromhigh-energy heavy-ion reactions using in-flight fragment
separators. In these devices, a charged-particle detector witha fast response and high radiation hardness is required tomeasure the time of flight (TOF) of ions for identifying theRI beams in high-dose environments.
Diamond is one of the most promising materials forfabricating a charged-particle detector for performing
accurate timing measurements under high-intensity beamconditions. Fast signals for superior timing resolution canbe obtained because of the high breakdown electrical fieldstrength and high carrier saturation velocities of the
diamond crystal. Additionally, high radiation hardness tolarge charged-particle fluxes can be expected because of thehigher displacement energy of diamond.
In this study, we fabricated a single-crystal (sc) chemicalvapor-deposited (CVD) diamond detector and evaluated theintrinsic timing resolution of the detector. Figure 1 shows
the block diagram of the electronic circuit used forevaluating the timing resolution. Two diamond detectors
were placed in the E7B beam line at the RIKEN accelerator
facility. The detectors were fabricated using an sc-diamondof approximately 4×4 mm2. In brief, the detector has a layer
structure of Pt/sc-diamond/Ti/Au. The thickness of thediamond crystals are 90 μm and 139 μm for the front-side
and rear-side of the detector, respectively.
8.6 MeV/nucleon-7Li beams penetrated two diamonddetectors. The pulses induced from the detector were readout using a broadband amplifier with an analog bandwidthof 1 MHz–2 GHz (CIVIDEC C2 Broadband Amplifier).
The amplifier signals were processed with a leading edge
discriminator, a time-to-amplitude-converter (TAC), and 8
k-multi-channel analyzer (MCA) for TOF measurement.
The timing resolution of the measurement system wasestimated to be 7.8 ps () using a constant time difference
generated by the fast-pulser signal and cables.
Figure 2 shows the measured TOF spectrum. The widthof the peak was estimated to be 45 ps (). The intrinsic
timing resolution of the detector was evaluated by dividing
the width of the peak by √2 and subtracting the timingresolution of the measurement system. Here, the same
performance of the two detectors was assumed. The
intrinsic timing resolution was estimated to be 31 ps ().
RIKEN Nishina Center
Graduate School of Engineering, Hokkaido University＊3
Center for Nuclear Study, University of Tokyo＊1＊2

Fig. 1: Block diagram of the electronic circuit used in theTOF measurement. The start signal and stop signal werecreated by output signals from the rear-side and thefront-side detector, respectively. The bias supply is left out.
The time offset of ~11 ns was created using cables. L.E.
discriminator: leading edge discriminator, TAC:
time-to-amplitude-converter, MCA: multi-channel analyzer

Fig. 2: Time of flight spectrum between two diamonddetectors. The intrinsic timing resolution (int) wasevaluated using distribution of the peak (45 ps ()) and thetiming resolution of the measurement system (7.8 ps ()).
This value is superior to the values reported by GSI: 57 ps
and 39 ps for 600 MeV/nucleon-58Ni and 1
GeV/nucleon-238U, respectively1,2). On the other hand,
NSCL/MSU achieved a resolution of ~15 ps for 87
MeV/nucleon-78Kr3).
In a future study, we will attempt to fabricate acharged-particle detector using a higher quality sc-diamondand evaluate the intrinsic timing resolution. On the otherhand, the use of a large size sc-CVD diamond (20 mm×20mm) was considered to fabricate the process for
manufacturing, large size detector.
References

1) E. Berdermann, et al.: Nuclear Physics B 78, 533 (1999).
2) E. Berdermann, et al.: XXXVI Int. Winter Meeting on Nucl.
Physics, Bormio 1998.
3) A. Stolz, et al.: Diam. Relat. Mater., 15, 807 (2006).

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