I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

HiCARI: High-resolution Cluster Array at RIBF
K. Wimmer,∗1,∗2 P. Doornenbal,∗2 N. Aoi,∗3 H. Baba,∗2 F. Browne,∗2 C. Campbell,∗4 H. Crawford,∗4H. De Witte,∗5 C. Fransen,∗6 H. Hess,∗6 S. Iwazaki,∗3 J. Kim,∗2 A. Kohda,∗3 T. Koiwai,∗7,∗2 B. Mauss,∗2B. Moon,∗2 T. Parry,∗8 P. Reiter,∗6 D. Suzuki,∗2 R. Taniuchi,∗9,∗2 S. Thiel,∗6 and Y. Yamamoto∗3The RIKEN Nishina Center and the RIBF have been
conducting in-beam γ-ray spectroscopy experiments at
the forefront of nuclear structure research. Studies on
exotic nuclei have focused on experiments employing
beams with the largest possible isospin asymmetry. Low
beam intensities require thick secondary reaction targets to achieve reasonable luminosity for measurements
as well as a high eﬃciency γ-ray detector. The NaI(Tl)detector array DALI2 1) features a very high detection
eﬃciency, ∼20% for 1 MeV γ rays, but it has very limited resolution for the transition energies (approximately
10% FWHM for typical in-beam experiments). This detector is ideally suited for ﬁrst spectroscopy experiments
for nuclei at the limits of the known nuclear chart, as well
as for studies featuring low level densities or selective
population of states such as magic nuclei or Coulomb
excitation. The detailed spectroscopy of complex level
schemes and measurements of excited state lifetimes are
not possible with the DALI2 array.
To overcome these limitation for in-beam γ-ray spectroscopy experiments at the RIBF, the HiCARI project
was initiated. The HiCARI array comprises segmented
high-purity germanium detectors. This provides excellent energy resolution to in-beam experiments by improving both the intrinsic energy resolution of the detector material itself and the position resolution for theinteraction points of the γ ray with the detector material, which is required for the Doppler reconstructionof transition energies. The high resolution also allows
determining the lifetimes of excited states by analyzing the shape of the peak in the Doppler reconstructed
spectrum. For the HiCARI array, eight Miniball triple
cluster detectors,2) 8(4)-fold segmented Clover detectors
from IBS Korea (two clusters) and IMP China (four clusters), the RCNP quad-type 36-fold segmented tracking
detector, and a triple segmented tracking detector P3
from LBNL Berkeley, which is also 36-fold segmented,
are available. The ﬁnal array installed at the F8 focus
for experiments employing the ZeroDegree spectrometercomprises six Miniball triples, four Clovers, and the twotracking detector modules; it is shown in Fig. 1.
The readout electronics is based on the digital data acquisition of the GRETINA array.3) The signals of eachcentral contact and segment electrode are digitized with∗1∗2∗3∗4∗5∗6∗7∗8∗9IEM-CSIC
RIKEN Nishina CenterRCNP, Osaka University
Nuclear Science Division, Lawrence Berkeley National Laboratory
Instituut voor Kern- en Stralingsfysica, KU Leuven
Institut für Kernphysik, Universität zu Köln
Department of Physics, University of Tokyo
Department of Physics, University of Surrey
Department of Physics, University of York

Fig. 1. HiCARI array located at the F8 focus. The beam
enters through the beam pipe from the left. The forwardring (θlab ∼ 22–55◦ ) comprises six Miniball detector modules (MB). The tracking detectors (P3 and Quad) are located in the horizontal plane, with the Clover detectors(CL) arranged on top and bottom of the θlab ∼ 60–85◦
ring.

a 100 MHz sampling rate and recorded. The energy
is derived using a trapezoidal ﬁlter algorithm. For thetracking detectors, the energy and three-dimensional position information of individual interaction points within
the segments is derived from the GRETINA signal decomposition analysis.
Before the experimental campaign (for details on individual experiments, see contributions in this volume),a series of characterization and commissioning measurements was performed. In-beam measurements using anintense 82 Ge beam of approximately 260 MeV/nucleon
impinging on Be and Au targets of various thicknesses
were used to characterize the in-beam eﬃciency and resolution of the HiCARI array. The response to the highintensity, low-energy radiation from atomic processes in
the collision of beam and target was also investigated,
and the eﬀect of diﬀerent shielding materials was explored.
References

1) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res. A
763, 596 (2014).
2) N. Warr et al., Eur. Phys. J. A 49, 40 (2013).
3) S. Paschalis et al., Nucl. Instrum. Methods Phys. Res. A
709, 44 (2013).

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