Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 52 (2019)

Development of Plastic Scintillator Barrel for WASA at GSI
R. Sekiya,∗1 V. Drozd,∗2 H. Fujioka,∗3 K. Itahashi,∗4 S. Y. Matsumoto,∗1,∗4 T. R. Saito,∗2,∗5,∗6 K. Suzuki,∗7 andY. K. Tanaka∗2,∗8Fig. 1. Schematic configuration of a single slat of PSB.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8

Department of Physics, Kyoto University
GSI Helmholtz Center for Heavy Ion Research
Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center
High Energy Nuclear Physics Laboratory, Cluster for Pioneering Research, RIKEN
Helmholtz Institute Mainz
SMIII. Physikalisches Institut, Giessen University- 126 -100

Time Resolution σt [ps]We plan to conduct new experiments for hypernuclear spectroscopy1) and η ′ -mesic nuclei exploration2)
using the WASA central detector3,4) and the fragment
separator (FRS) at GSI. The WASA central detector
consists of a superconducting solenoid magnet, a plastic scintillator barrel (PSB), and a cylindrical drift
chamber for charged particle measurement and CsI
calorimeters for gamma-ray measurement.
We are upgrading the cylindrical part of PSB toachieve 100 ps in time resolution based on the experimental requirement. In the old conﬁguration, we had
only one photomultiplier tube with a light guide foreach scintillator slat of PSB. We improved the time resolution by detecting photons on both sides using multipixel-photon counters (MPPCs). Since the PSB will
be located inside the superconducting solenoid magnet, we chose detectors that can be operated under amagnetic ﬁeld. MPPC satisﬁes this requirement.
We made a single slat of PSB as schematicallyshown in Fig. 1. The size of the plastic scintillator
is 550 × 38 × 8 mm3 . We adopted Eljen EJ-230, which
has an attenuation length of 120 cm. We attachedfour MPPCs (Hamamatsu Photonics S13360-6050CS)
on each side of the plastic scintillator and electricallyconnected them in series. The eﬀective area of theMPPC is 6 × 6 mm2 . The MPPCs were then connected to ampliﬁers developed in Ref. 5). The ampliﬁed signals were recorded by a waveform digitizer(CAEN V1742) with a sampling frequency of 2.5 GHz.
In order to evaluate the time resolution, we irradiated the plastic scintillator with electrons from a 90 Srsource with an endpoint energy of 2.28 MeV. A slit witha gap of 2 mm was inserted between the plastic scintil-95

908580Overvoltage
4V756V8V

70−30−20−100

102030Source Position [cm]Fig. 2. The dependence of the time resolution σt on source
positions and applied bias voltages.

lator and the 90 Sr source. We performed mesurements
by changing the position of the source to investigate
the hit position dependence. The overvoltage was alsochanged to 4, 6, and 8 V.
We evaluated the time resolution of the plastic scintillator by analyzing the waveform data. In the analysis, we simulated a function of constant fraction discriminators by using a software to obtain the precision
arrival time of signals at the right and left ends of theplastic scintillator (T R and T L ). The time resolutionof this single slat of PSB (σt ) can be estimated byﬁtting a (T R − T L )/2 histogram with a Gaussian function and taking its standard deviation, neglecting thehit position distribution of the electrons.
Figure 2 shows the time resolution σt as a functionof the source positions. Diﬀerent colors correspond to
diﬀerent bias voltages. We achieved a time resolution
better than 100 ps for all the tested conditions. Wealso found that the time resolutions become better as
the overvoltage is decreased.
Based on the measurements described above, we
conﬁrmed that a single slat of PSB achieves the required time resoution. We are planning a more systematic evaluation of the PSB performance by using
mono-energetic proton beams.
References1) T. R. Saito et al., proposal GSI-SIS S447 (2017).
2) K. Itahashi et al., proposal GSI-SIS S457 (2017).
3) C. Bargholtz et al., Nucl. Instrum. Methods Phys. Res.
A 594, 339 (2008).
4) H.-H. Adam et al. (WASA-at-COSY Collaboration),
arXiv:nucl-ex/0411038 (2004).
5) P. W. Cattaneo et al., IEEE Trans. Nucl. Sci. 61, 2657 (2014).