Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

Performance of the SπRIT Time Projection Chamber
G. Cerizza,∗1,∗2 J. W. Lee,∗2,∗3 G. Jhang,∗2,∗3 J. Barney,∗1,∗2 J. Estee,∗1,∗2 T. Isobe,∗2 M. Kaneko,∗2,∗4
M. Kurata-Nishimura,∗2 P. Lasko,∗2,∗5,∗6 J. L ukasik,∗5 W.G. Lynch,∗1 A.B. McIntosh,∗7 T. Murakami,∗2,∗4∗2,∗5∗6∗2
P. Pawlowski,
K. Pelczar, H. Sakurai, C. Santamaria,∗1,∗2 R. Shane,∗1 M. B. Tsang,∗1 S.J. Yennello,∗7Y. Zhang,∗2,∗8 and the SπRIT and NeuLAND collaborations
Of the remaining events, 85% have a reconstructed vertex within 5 mm from the target position along thebeam axis. The average track multiplicity per event is
45 with the distribution shape aﬀected by the multiplicity selection criteria in the multiplicity trigger arrays. Preliminary reconstruction eﬃciencies due to thegeometry of the TPC have been estimated from Monte
Carlo simulation to be above 90% for charged particles
of momenta up to 2000 MeV/c in the range [−60, 60]
and [−70, 70] degrees for polar and azimuthal angles,
respectively. The expected momentum resolution isestimated to be a few percent by using GEANT4 simulation to model the TPC. The lower limit in the reconstructed pion momenta is about 30 MeV/c. With
the ongoing improvement of the tracking and ﬁtting
algorithms of the SπRITROOT analysis package theﬁnal eﬃciencies will be determined at a later stage.
The success of the ﬁrst commissioning of SπRIT-TPC
inside the SAMURAI magnet and the pion ratio experiments performed in May 2016 are the basis of future
proposals for the physics program of constraining thesymmetry energy term in the nuclear-matter equationof state (EOS) at super-saturation density.
This work is supported by the U.S. DOE underGrant Nos. DE-SC0004835, DE-SC0014530, DENA0002923, US NSF Grant No. PHY-1565546, theJapanese MEXT KAKENHI grant No. 24105004, andby the Polish NSC Grants UMO-2013/09/B/ST2/04064
and UMO-2013/10/M/ST2/00624.

The goal of the SAMURAI Pion-Reconstruction
and Ion-Tracker Time-Projection Chamber (SπRITTPC)1) project is to constrain the symmetry energy
term in the nuclear-matter equation of state (EOS)
at super-saturation density. We propose the comparison of π − /π + production ratio among various isospin
asymmetry systems through several combinations ofunstable Sn beams and stable Sn targets inside theSAMURAI magnet. A commissioning run using a132
Sn beam impinging on a natural tin target was performed in April 2016, immediately before the pion ratio experimental campaign of May 2016. This included
four beams (132,124,112,108 Sn) on two diﬀerent targets(124,112 Sn) and a cocktail beam consisting of p, d, t,3He, 4 He, and 6 Li particles used to calibrate the gain of
the detector. For the commissioning experiment, theSπRIT-TPC was placed inside the SAMURAI magnet and lined up at 0◦ with respect to the beamline.
The secondary 132 Sn beam impinged on a 0.5 mm natural tin foil target mounted on a ladder in front of
the TPC ﬁeld cage window. The complete version of
the SπRITROOT analysis framework was oﬃcially deployed and tested for the ﬁrst time. The three plots on
the right side of Fig. 1 show the pad-plane view (with12096 pixels) of examples of collision on target (topand middle) and active-target (bottom) events. Twospiral trajectories can be observed in the ﬁrst two panels and are identiﬁable as rare low-energy π − particles,
which curve in the opposite direction from the positively charged ions. In the bottom panel, an activetarget collision event between the 132 Sn beam and thegas molecules ﬁlling the volume of the TPC is shown,
where the reaction vertex can be visually identiﬁed inside the TPC. In all of them, the saturation eﬀects ofthe electronics caused by high-Z particles and the beampassing through the target can be observed in the center region of the pad plane (red pads). The left plotof Fig. 1 shows the particle identiﬁcation spectra ofparticles emitted from the 132 Sn+nat Sn collisions anddetected in the TPC. The reaction vertex was extrapolated from reconstructed tracks event by event. About
30% of the events have one reconstructed vertex inside
the TPC volume in active-target-like collision events.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8Fig. 1. Commissioning of the TPC with the 132 Sn radioactive beam: particle identiﬁcation map (left) and padplane track projections for three sample events (right).

NSCL and Dept. of Phys. & Ast., Michigan State University
RIKEN Nishina Center
Department of Physics, Korea University
Department of Physics, Kyoto University
IFJ PAN, Kraków
Jagiellonian University, Kraków
Cyclotron Institute, Texas A&M University
Department of Physics, Tsinghua University

Reference1) R. Shane et al., Nucl. Instr. Meth. A 784, 513 (2015).

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