Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Upgrade of proton detector NINJA
N. Chiga,∗1 T. Kobayashi,∗1,∗2 T. Saito,∗1,∗3 A.T. Saito,∗1,∗4 Y. Togano,∗1,∗4 T. Isobe,∗1 and H. Otsu∗1
The detector NINJA1,2) is a plastic scintillator array positioned inside the SAMURAI gap chamber tomeasure protons that cannot to exit the gap chamber with the standard SAMURAI setup. NINJA iscomposed an X-layer and a Y-layer. The X- and Ylayers consist of 18 slots with dimensions of 60(w)×720(l)mm2 × 10(t)mmt and 12 slots with 60(w)×1100(l)2 ×10(t)mmt , respectively. The X-layer is located 5 cm upstream of the Y-layer. Scintillation lightsignals from the plastic are transported to the MPPC
through a wave-length-shifter rod in the scintillator.
The details of the NINJA can be found in Ref. 2.
[2].
This report describes the upgrade of the readout circuits and the performance of NINJA. The readout circuit of NINJA was earlier the VME-EASIROC3) , witch
was utilized to digitize the signals from NINJA. VMEEASIROC supplies HV to MPPC. Since we determinedthat the connection of the external HV supplier was
the major noise source, the readout circuit has beenchanged to NIM-EASIROC,4) which has an internal
HV supplier. Since NIM-EASIROC is not equippedwith an internal TDC, the V1190A TDC was used (asan external TDC) to accept the leading and tradingedges of signals.
The timing resolution and the detection eﬃciencyof NINJA for protons were evaluated by using theprojectile-like protons produced in the 1 H(6 He,X) reaction in the SAMURAI13 experiment5) . The detection eﬃciency ε of the X-layer was evaluated asN (X ∩Y )/N (Y ), where N (X ∩Y ) represents the number of events occurring at both the X- and Y-layers,and N(Y) the event ﬁring the Y-layer. The Y-layer
eﬃciency εY was evaluated as N (X ∩ Y )/N (X). The
eﬃciency of NINJA for protons was evaluated to be98% for both layers.
The timing resolution was evaluated by using thetime diﬀerence between the X- and Y-layers, since thedistance between the two layers is small (50mm). Figure 1 shows the spectrum of time diﬀerence between
the X- and Y-layers. One channel in the X-axis corresponds to 100 ps in this case. The red curve indicatesthe result of ﬁtting using a Gaussian distribution. The
resultant sigma of the Gaussian is 6.5 ch, corresponding to 650 ps. Assuming that the time resolutions theX- and Y-layers are identical, the time resolution ofNINJA is estimated to be 460 ps. This resolution ismostly governed by one of the EASIROC circuits3,4) ,and it can be further improved sightly vis correctingof the walk eﬀect and adjusting the ﬁne oﬀsets of the∗1
∗2
∗3
∗4

RIKEN Nishina Center
Department of Physics, Tohoku University
Department of Physics, University of Tokyo
Department of Physics, Tokyo Institute of Technology- 177 -individual TDC channels. NINJA is currently ready tobe employed as the SAMURAI standard detector. It
will be used to detect protons emitted from reactions ofproton-rich unstable-nuclei beams, for example, for thestudy of exotic decay channels such as a d-bar (singlets state of the p+n system) decay from excited statesof nuclei. The momentum reconstruction of protons
will be performed by combining NINJA with a drift
chamber that is currently being developed, which will
be positioned immediately in front of NINJA.
This work was funded by ImPACT Program ofCouncil for Science, Technology and Innovation (Cabinet Oﬃce, Government of Japan).

Fig. 1. Spectrum of time diﬀerence between X- and Ylayers of NINJA.

References
1) N. Chiga et al.: KEK Proceedings 2016-1, O-1-1 (2015),
p.5.
2) N. Chiga et al.: RIKEN. Accel. Prog. Rep.49, 165(2015).
3) T. Shiozaki, Master Thesis, Department of Physics, Tohoku university (2015).
4) N. Ishijima, Master Thesis, Department of Physics, Osaka university (2014).
5) S. Chebotaryov et al.: in this report.