Ⅱ-9. Instrumentation

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RIKEN Accel. Prog. Rep. 49 (2016)

Construction of readout system for SPiRIT-TPC

T. Isobe,∗1 M. Kurata-Nishimura,∗1 H. Baba,∗1 M. Kaneko,∗2 T. Murakami,∗2 W.G. Lynch,∗3 J. Barney,∗3

J. Estee,∗3 S. Tangwancharoen,∗3 G. Jhang,∗4 and J.W. Lee∗4 for the SPiRIT Collaboration

SPiRIT Time Projection Chamber (TPC) is be-ing constructed for the study of density dependentsymmetry energy using heavy RI collision at RIKEN-RIBF1). A novel readout system, General Electronicsfor TPC (GET)2), for the signal coming from the TPChas been employed for the SPiRIT-TPC3).

At the end of summer 2015, 48 AsAd boards for theamplification and digitization of signal were mountedon the TPC as shown in the Picture 1. During theinstallation of AsAd boards, the performance of thesystem was carefully checked. After we mounted halfof the electronic components, we found that there werelarge gain deviations between different pads. This wasa serious problem for the TPC, as TPC needs a uni-form gain so that it can achieve good tracking andproper triggering. The reason for this was the instabil-ity in the power that was provided to the AGET ASICchips on the readout board, which can be improvedby changing the components on the AsAd board. Af-ter this modification, we mounted all the boards onthe TPC again and checked the uniformity of gain bypulsing the ground wire, which generates a potentialaround the amplification region. As shown in Fig. 2,the uniformity of the gain is good enough for the re-construction of charged particle trajectories.

Fig. 1. GET electronics mounted on SPiRIT TPC. Each

AsAd board is shielded with Al cover.

The GET system employs NARVAL4) as DAQ sys-tem, GANIL run control (RC), and GANIL user in-terface (UI). We employ the GANIL RC and graphicalUI for controlling SPiRIT system. In order to con-trol RIBF-DAQ along with GET in the same frame-

∗1 RIKEN Nishina Center∗2 Department of Physics, Kyoto University∗3 National Superconducting Cyclotron Laboratory, Michigan

State University∗4 Department of Physics, Korea University

Fig. 2. Gain uniformity of TPC pads. 5.3V signal is

pulsed through the ground wire of TPC. Out of

108×112=12096 pads, 3 pads show lower gain while

other pads show good gain uniformity.

work, a software of the SOAP server that communi-cates with GANIL RC was developed. As demon-strated in the graphical UI shown in Fig. 3, a usercan control the CoBo boards, which are back-end elec-tronics of GET, as well as RIBF-DAQ system namedBabirl/GTO. Fig. 3 also demonstrates the DAQ datathroughput on the disk of ∼100 MB/s/CoBo, whichcorresponds to 1.2 GB/s in total.

Fig. 3. Graphical UI of SPiRIT DAQ. Babirl, which is

RIBF common DAQ system, and GTO can be con-

trolled through this UI.

The commissioning of the readout system includingSPiRIT-TPC was carried out during 2015 fall as re-ported in this annual report.

References1) S. Rebecca et al., Nucl. Inst. and Met. A 784, 513

(2015).2) E. Pollacco et al., Physics Procedia 37, 1799 (2012).3) T. Isobe et al., RIKEN Accel. Prog. Rep. 48, 204 (2015).4) X. Grave., Proc. 14th IEEE NPSS RT Conf, 65(2005).

Multiplicity trigger array for the SπRIT experiment

M. Kaneko,∗1 ∗2 T. Murakami,∗1 T. Isobe,∗2 M. Kurata-Nishimura∗2 for the SπRIT collaboration

In the SπRIT (SAMURAI pion Reconstruction andIon Tracker) experiment, it is important to identifythe charged pions that are generated from heavy ioncollisions of high centrality and reconstruct their mo-menta to determine the π+/π− ratio, which containsinformation on the nuclear symmetry energy. To accu-mulate data that focuses on symmetry energy, it is alsoimportant for the trigger system to have sensitivity tothe event centrality. As it is well known that the im-pact parameter correlates with multiplicity of the col-lision, higher-centrality collisions are extractable pref-erentially by triggering events with high multiplicity.While detail analysis on charged particle multiplicitywill be necessary through SπRIT-TPC,1) the impactparameter within at least 5 fm as a central collisionmust be survived at the stage of triggering.

We have developed an trigger detector sensitiveto multiplicity for the SπRIT-TPC data acquisition,which is called the ”multiplicity trigger array”. Thisdetector consists of 30 extruded scintillator bars thatare in close contact with both sides of the TPC, whichis shown in Fig.1 as enclosed with an orange dottedline, using 60 bars in total. The walls of the TPC are1mm thick aluminum windows allowing light fragmentparticles from the reaction to pass through and be de-tected by the external triggering system. The dimen-sions of each scintillator bar are 450 mm*50 mm*10mm, which are coated with oxidized titanium to im-prove light reflection. Each bar has a hole of about1.5 mmϕ centered along its length for a wave lengthshifting fiber. The collected light will be detected by1.3 mm2 MPPC attached to the ends of the fiber.For readout electronics, the VME-EASIROC2) is

used. This module was developed by Tohoku and KEKgroup in 2014, for the readout of multi-MPPC detec-tor systems. EASIROC (Extended Analogue SiliconPhotoMultiplier ReadOut Chip) is used for 32 photo-diodes readout developed by Omega in France.3) Eachchip has a parallel circuit of preamplifiers, shapers, anddiscriminators. VME-EASIROC has two EASIROCchips on the board and is capable of reading out 64MPPCs. The parameters of the EASIROC are vari-ably controlled by the onboard FPGA communicatingvia an SiTCP connection to another computer. Wecan also obtain the information of the MPPC signal bythe ADC onboard chip and the MHTDC logic imple-mented in the FPGA. The calculation logic for mul-tiplicity is located on the FPGA, which is done bycounting the number of discriminator signals that sur-pass the threshold value. After the calculation, the

∗1 Department of Physics, University of Kyoto∗2 RIKEN Nishina Center

result will be compared with the user set threshold ofmultiplicity and generate a trigger signal within about52 ns for the whole VME-EASIROC module.

In the fall of 2015, the SπRIT-TPC was commis-sioned by performing an experiment using a 79Se beamimpinging on Al and Sn targets. The SπRIT-TPC wassetup outside the SAMURAI magnet and surroundedby the multiplicity trigger array. The histogram inFig.2 shows charged particle multiplicity with the con-dition of coincidence between at least one hit in themultiplicity trigger array and the beam start counterlocated just upstream of the TPC. It is clear that mostof the events had low multiplicity originating due tonon-central collisions. In perspective of the real exper-iment, analyzing commissioning data is crucial to findthe optimal trigger conditions for rejecting peripheral-type collisions.4)

Fig. 1. One-sided view of multiplicity trigger array.

Fig. 2. Multiplicity plot of charged particle detected by

multiplicity trigger array in the 79Se commissioning run.

References1) R. Shane et al.: Nucl. Instr. Meth. A 784, 513 (2015).2) T. Shiozaki: Master’s thesis, Department of Physics,

Tohoku university (2014).3) Omega group. EASIROC DATA SHEET. (2011).4) M. Kurata-Nishimura et al.: in this report.