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

Ⅱ-9. Instrumentation

Detector system for
the KEK isotope separation system
S. Kimura,∗1 H. Ishiyama,∗2 N. Imai,∗3 S.C. Jeong,∗2 H. Miyatake,∗2 Y. Hirayama,∗2Y.X. Watanabe,∗2 M. Oyaizu,∗2 M. Mukai,∗1 Y.H. Kim,∗4 and A. Ozawa∗1

∗1
∗2
∗3
∗4

Department of Physics, University of Tsukuba
Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK)
Center for Nuclear Study, University of Tokyo
Department of Physics, Seoul National UniversityFig. 1. Schematic view of the detector system.
   


The KISS has been developed to study the β-decay
properties of neutron-rich nuclei around neutron number N = 126 which determine the r-process pathand form the third peak of the elemental abundance(A∼195).1,2) The detector system of the KISS requires
high detection eﬃciency for low-energy β-rays because
the nuclei of interest have small Q-value around 2
MeV. In addition, the system should be operated ina low-background environment because of the low production rates of these nuclei. Although the tolerable
count rate of the background depends on the production rates, our ultimate goal is to set around several
tens of counts per day, allowing access to these nuclei.
The detector system of the KISS consists of β-ray
telescopes, Ge detectors, and a tape transport system.
A schematic view of the detector system is shown inFig. 1. For particle identiﬁcation, three Ge detectors
are employed to detect K-X rays emitted from the nuclei of interest. To eﬃciently count low-energy β-rays
with low background, the β-ray telescopes are composed of three double-layered thin plastic scintillators;
the thicknesses of the ﬁrst and second layers are 0.5mm and 1 mm, respectively. The solid angle of theβ-ray telescopes is 90% of 4π.
The β-ray telescopes worked as designed. The energy spectra of the scintillators were in good agreementwith Geant4 simulations. A comparison of energy deposit in the ﬁrst layer between the simulation and themeasured result is shown in Fig. 2. The measured efﬁciency of the β-ray telescopes for low-energy β-rays
emitted from a 90 Sr/90 Y source was 55.4(15)% with anenergy threshold of 20 keVee for the ﬁrst layer and 30
keVee for the second layer.
The background rate of the β-ray telescopes wasmeasured to be 5 cps. The origins of the backgroundwere considered to be cosmic rays and electrons scattered by γ-rays from natural activities. Initially, toreduce the cosmic rays, a veto counter system was installed, which consisted of plastic scintillators. The
conﬁguration of the plastic scintillators was designed
based on the Geant4 simulation, and the designed
value of the veto eﬃciency was 92% for 1 GeV muon.
The veto counter reduced the previous background rateby 1.1 cps. In addition, to reduce the room background of γ-rays from natural activities, we installed
    Fig. 2. Energy deposit distribution of the first layer for βrays from the 90 Sr/90 Y source.

shields with low-activity Pb blocks and reduced thebackground rate by 2.4 cps. Finally, by raising theenergy threshold of the second layer to 70 keVee without signiﬁcant sacriﬁce of the detection eﬃciency for
β-rays of interest, we reduced the background rate by
0.3 cps. In total, we reduced the background rate to
1.2 cps from 5 cps.
The main component of this background rate is environmental γ-rays and accidental coincidence rate inan extra active area of the scintillators. We will replace
the present telescopes with new ones consisting of gas
counters and thin plastic scintillators. Gas counterswill be constructed with a small amount of materials
to reduce the Compton scattering of γ-rays.
References
1) S. C. Jeong et al.: KEK Report 2010-2.
2) Y. Hirayama et al.: In this report.

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