RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Kyoto Multiplicity Array for the SπRIT experiment
M. Kaneko,∗1,∗2 T. Murakami,∗1,∗2 T. Isobe,∗2 M. Kurata-Nishimura,∗2 W. G. Lynch,∗3 M. B. Tsang,∗3J. Barney,∗2,∗3 J. Estee,∗2,∗3 G. Cerizza,∗2,∗3 C. Santamaria,∗2,∗3 G. Jhang,∗4 J. W. Lee,∗4,∗2P.
L
P. Lasko,∗5,∗2 J. L
 ukasik,ukasik,∗5 P.
P. Pawlowski∗5,∗2 and Y. Zhang∗6,∗2 for the SπRIT collaboration
collaboration
The SπRIT experiment, which aimed to investigate
the nuclear equation of state at supra-saturation densities, was performed in the spring of 2016 at RIKENRIBF. The integral or momentum spectrum of π − /π +produced in central heavy-ion (HI) collisions can bea useful probe1) and was measured in this experiment. In order to identify multi fragments including
charged pions generated from HI collision and to reconstruct their momenta, a large time projection chamber(SπRIT-TPC2) ) and ancillary trigger detectors wereconstructed. These detectors were used in the SAMURAI spectrometer with a magnetic ﬁeld of 0.5 T.
The ”Kyoto Multiplicity Array” has a sensitivity
to sideward charged particle multiplicity which correlates with collision centrality. This detector consistsof 2×30 plastic scintillator bars that are in close contact with both sides of the SπRIT-TPC. The enclosureof the SπRIT-TPC has 1-mm-thick aluminum window
parts allowing light fragments from the reaction topass through and be detected by the external triggering system. The dimensions of each plastic bar are
450×50×10 mm3 , and the plastic bars are coated withoxidized titanium for light reﬂection. Each bar has ahole of about 1.5 mmφ centered along its length fora wavelength-shifting ﬁber as a light guide. The lightpropagated in the ﬁber of 1 mmφ will be detected by a
1.3-mm2 multi-pixel photon counter (MPPC) capableof use in a magnetic ﬁeld. Figure 1 shows the geometrical setup of detectors in the SAMURAI magnet.

Fig. 1. Schematic drawing of SπRIT-TPC and ancillarydetectors from beam-left downstream side. One-sided
Kyoto Multiplicity Array is indicated by black, and theother side is behind the SπRIT-TPC.
∗1
∗2
∗3
∗4
∗5∗6

Department of Physics, Kyoto University
RIKEN Nishina Center
NSCL and Dept. of Phys. & Ast., Michigan State University
Department of Physics, Korea University
Institute of Nuclear Physics, PAN, Kraków
Department of Physics, Tsinghua University- 172 -The ASD chip designed to readout photodiode typedetectors - EASIROC - is used for shaping MPPC signals. For handling the EASIROC and its digital outputs from discriminators, FPGA is integrated in one
board together with EASIROC (VME-EASIROC3) ).
The number of digital signals, as charged-particle multiplicity, is calculated by a combination of ROMs andadder circuits, which is implemented in the FPGA. If
the calculation result surpasses the user-set threshold,a trigger signal will be generated within about 52 ns forthe whole electronics. In addition to the online triggering function, multi-hit TDC with 1 ns time resolutionis also implemented in the FPGA for oﬄine analysis.
In the SπRIT experiment, triggered event multiplicity
was required to be greater than 4.

Fig. 2. Number of hits in each plastic bar in oﬄine analysis from beam-right upstream side. The coordinatescorrespond to the position of each bar, and the target
was located at X=0 mm and beam axis=–8.9 mm.

Although this was the ﬁrst operation in the magnetic
ﬁeld, the data for TPC with ancillary detectors wereacquired without major problems. The hit pattern obtained by the detector is presented in Fig. 2, where theeﬀect of the magnetic ﬁeld on charged particles can beobserved. While there are some ﬂuctuations, detailed
analysis is necessary to understand the bias for triggered events and the impact parameter information.
This work
work isis supported
supported byby the Japanese MEXTThisKAKENHI grant
the U.S.
grant No.
No.24105004;
24105004;
the DOE
U.S. underDOE
under Nos.
GrantDE-SC0004835,
Nos. DE-SC0004835,
DE-SC0014530,
Grant
DE-SC0014530,
and DEand DE-NA0002923;
the US
NSFNo.
Grant
No. PHYNA0002923;
the US NSF
Grant
PHY-1565546;
1565546;
and thePolish
NSC
Nos. UMOandthe Polish
NSC
Grant
Nos.Grant
UMO-2013/09/B/2013/09/B/ST2/04064
and UMO-2013/10/M/ST2/00624.
ST2/04064
and UMO-2013/10/M/ST2/00624.
References
1) Bao-An Li et al., Phys. Rev. C 71, 014608 (2005).
2) R. Shane et al., Nucl. Instr. Meth. A 784, 513 (2015).
3) T. Shiozaki: Master’s Thesis, Tohoku University (2015).