Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 48 (2015)

A NaI(Tl) detector array for measurements of γ rays from fastradioactive isotope beams †S. Takeuchi,∗1 T. Motobayashi,∗1 Y. Togano,∗2 M. Matsushita,∗3 N. Aoi,∗4 K. Demichi,∗5 H. Hasegawa,∗5 andH. Murakami∗1
The recent development of fast radioactive ion (RI)
beams requires a new type of γ-ray detector array forin-beam spectroscopic studies. Experiments are performed in inverse kinematics by using fast RI beams
with a high velocity of v/c ≃ 0.3 − 0.6 that causesa large Doppler shift for γ rays emitted. In order toextract the transition energy in the rest frame of theprojectile, precise measurements of emission angles ofγ rays as well as a good intrinsic energy resolution
are required. Another requirement for γ-ray detection
is high eﬃciency, because the secondary-beam intensity for nuclei far from stability is typically low. TheRIKEN RIBF provides the world’s highest intensity
exotic beams to study unstable nuclei. To capitalizeon the performance of RIBF, we have constructed anew γ-ray detector array called DALI2 (Detector Array for Low Intensity radiation 2) for in-beam γ-ray
spectroscopy experiments.
The design of the DALI2 array follows a concept
similar to the original array DALI1,2) , which was developed for experiments at the old facility at RIKEN thatprovides light exotic beams with v/c ≃ 0.3. In experiments performed at the new RIBF facility providing
higher-velocity exotic beams with v/c ≃ 0.6, the performance of DALI is not optimized. Therefore, DALI2
was designed to fulﬁll the required conditions for experiments performed at the RIBF facility by improving
the angular resolution and the detection eﬃciency. In
order to compromise on requirements such as intrinsic
resolution, detection eﬃciency, and cost, we adoptedNaI(Tl) as the detector material. The DALI2 arrayconsists of a large number of detectors, 160-186, which,depending on the experimental conditions, are at various distances from the target. As shown in Fig. 1, thedetectors are arranged to form twelve layers that areset perpendicularly to the beam axis, and a detector
matrix covers the forward angles. Each layer consistsof 6-14 detectors and the forward matrix consists of 64detectors. In this standard conﬁguration, DALI2 cancover a polar angle between 15◦ and 160◦ .
The performance of DALI2 was examined by using measurements with standard γ sources and by
performing the Monte Carlo simulations with theGEANT3 code3) . Simulations reproduce measurements
†∗1
∗2
∗3
∗4
∗5Fig. 1. Schematic view of DALI2 in its standard configuration consisting of 186 NaI(Tl) crystals.

well, including results obtained by in-beam experiments. A typical full-energy-photopeak resolution of
10% (FWHM) and 20% eﬃciency are achieved for 1MeV γ rays emitted from moving nuclei with v/c ≃ 0.6
without applying add-back analysis. This resolution
is satisfactory for spectroscopy of low-lying states ineven-even nuclei. The high eﬃciency enables γ-γ coincidence measurements even for beam intensity as low
as 1 Hz. The DALI2 array has been applied successfully to a variety of experiments at the old RIKEN
facility and more recently at the new RIBF facility.
This will be used in many more experiments to studynuclear structures of exotic nuclei at RIBF. For further spectroscopic studies of heavy or odd-mass nuclei,
the SHOGUN array with superior energy resolution isplanned4) .
References
1) T. Motobayashi et al.: Phys. Lett. B 346, 9 (1995).
2) T. Nishio et al.: RIKEN Accel. Prog. Rep. 29, 184(1996).
3) GEANT3, CERN Program Library Long Writeup
W5013, 1993.
4) P. Doornenbal et al.: RIKEN Accel. Prog. Rep. 42, 182(2009).

Condensed from the article in Nucl. Instrum. and Methods
in Phys. Res. Sect. A, 763, 596 (2014)
RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
CNS, University of Tokyo
RCNP, Osaka University
Department of Physics, Rikkyo University- 209 -

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