RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

The Veto Collimator for the SπRIT-TPC
Y. Zhang,∗1,∗2 J. Barney,∗3,∗2 M. Kaneko,∗2,∗4 M. Kurata-Nishimura,∗2 P. Lasko,∗5,∗6,∗2 J. L ukasik,∗5
∗5,∗2
∗1,∗2
∗3,∗2
∗3,∗2
∗2
∗7,∗2
P. Pawlowski,
Z.G. Xiao,
G. Cerizza,
J. Estee,
T. Isobe, G. Jhang,
W.G. Lynch,∗3
∗4,∗2
∗6
∗3,∗2
∗3
∗4,∗2
∗6
∗3,∗2
T. Murakami,
Murakami,
K. Pelczar,Pelczar, C.
C. Santamaria,Santamaria,
M.B. Tsang,
Tsang and the SπRIT Collaboration
T.
K.
M.B.
for the KATANA4) array) and then to the leading edge
discriminators of the trigger logic system. Any of thefour scintillators ﬁring above the threshold produce aveto signal preventing a trigger for the TPC.
610

VC_TopVC_Bottom
VC_Left
VC_Right

510Counts

The Veto Collimator, VC, is an ancillary detectorof the SπRIT-TPC1) (SAMURAI pion Reconstruction
Ion-Tracker Time Projection Chamber), the main device used to study the nuclear Equation of State atRIKEN. It is located 22 cm upstream of the targetand provides a veto condition for beam particles which
do not hit the target and upstream reaction products.
The VC consists of four BC408 scintillators with thesize of 90×50×6 mm3 . Attached to each scintillator
is an MPPC (S10931-100P) which is a new type ofa photon-counting device made up of multiple APD(Avalanche PhotoDiode) pixels operating in Geigermode2) . The active area of this MPPC is 3×3 mm2 .
The BC408 is a premium plastic scintillator with thehighest light output and with the maximum emission
at the wavelength of 425 nm, which is compatible tothe peak sensitivity wavelength of the MPPC2,3) . The
gain of the MPPC amounts to about 2.4×106 .

104
310102101020406080 100 120
Amplitude [mV]

140160180

Fig. 2. The energy spectra of fragments hitting the VC.

Fig. 1. Mechanical sketch of the veto collimator.

The mechanical sketch of the VC is shown in Fig. 1.
Four scintillators, shown in purple, are mounted on adjustable support slots so they form a hole in the center
where the beam particles can go through towards thetarget downstream. To ensure that beam particles passthrough the hole will hit the target, the size is slightly
smaller than the 30×40 mm2 target.
For each scintillator, the analog signal produced bythe MPPC is sent to a preampliﬁer (same as those used∗1
∗2
∗3
∗4
∗5
∗6
∗7

Department of Physics, Tsinghua University
RIKEN Nishina Center
NSCL and Department of Physics and Astronomy, Michigan
State University
Department of Physics, Kyoto University
IFJ PAN, Kraków
Jagiellonian University, Kraków
Department of Physics, Korea University- 170 -During the experiment the MPPCs were biased at71.51, 71.52, 71.41 and 71.40 V and the thresholds
were set to 145, 155, 130 and 135 mV for the top,bottom, left and right paddles, respectively. The energy spectra of fragments hitting the four VC paddles
are shown in Fig. 2. The presented data come fromthe commissioning runs of 132 Sn at 300 MeV/nucleon,
for which the VC was operating in a triggering modeand thus the veto thresholds were not eﬀective. The
peaks at high amplitudes are due to the beam particles.
The counts at smaller amplitudes correspond to slower
and/or lighter upstream reaction products as well as tothose backward emitted from the target and from thedownstream material. The VC performed well during
the two experiments and the commissioning run with
the SπRIT-TPC in 2016, suppressing upstream reactions and oﬀ-target events.
This work is supported by the National Science
Foundation of China under Grant No. U1332207, bythe U.S. DOE under Grant Nos. DE-SC0004835, DESC0014530, DE-NA0002923, US NSF Grant No. PHY1565546, by the Japanese MEXT KAKENHI grant No.
24105004 and by the Polish NSC Grants UMO-2013/
09/B/ST2/04064 and UMO-2013/10/M/ST2/00624.
References
1) R. Shane et al., Nucl. Instr. Meth. A 784, 513 (2015).
2) http://www.datasheetlib.com/datasheet/707358/
s10931_hamamatsu-photonics.html
3) http://www.crystals.saint-gobain.com
4) P. Lasko et al., arXiv:1610.06682.