Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Development of gating grid driver for SPiRIT TPC
T. Usukura,∗1,∗2 T. Isobe,∗2 H. Baba,∗2 and K. Ieki∗1The symmetry energy part of the nuclear equationof state (EOS) influences various phenomena in nuclear astrophysics, nuclear structure, and nuclear reactions. The behavior of nuclear symmetry energy canbe probed through a measurement of the π − /π + ratio
in heavy ion collisions. For this purpose, experiments
using the Time Projection Chamber (TPC) installed inSAMURAI magnet1) have been proposed.2) The TPC
is necessary to measure charged particles such as pions,
protons, and light ions in high multiplicity environment produced by heavy ion collisions. When we perform the experiments at SAMURAI, heavy ions passthrough TPC as well as light charged particles, resulting in gain reduction due to the production of a large
amount of ions from the avalanche process around anode wires. To avoid such a gain reduction, gating grid
wires are located prior to the avalanche region. Techniques to protect the avalanche region have been wellestablished.3) In the open gate mode, all the gating
grid wires are held at the same potential VG , admitting
electrons from the drift volume to enter the avalanche
region. In the closed gate mode, the gating grid is biased with a bipolar field (positive side:VG + ∆V , negative side:VG − ∆V ), which prevents electrons from thedrift volume to reach the avalanche region. The closed
gate prevents ions created in the avalanche processesof previous events from drifting back into the drift volume. A gating grid driver (GGD) was developed torealize such protection of the TPC. Figure 1 shows aprototype of the GGD. The design is based on the E907
TPC GGD.4)

Fig. 2. Position resolution along wire axis as a function offlash ADC sampling frequency for each case, with andwithout GGD.

Fig. 1. Prototype of gating grid driver.

The GGD performance was studied by using a Xebeam at HIMAC. Since we could not use the TPC for
SAMURAI, we used the BRAHMS TPC5) to checkthe GGD performance. A CsI target was located infront of the TPC so that light charged particles can be∗1
∗2

Department of Physics, Rikkyo University
RIKEN Nishina Center

measured as well as the Xe beam. T2K-TPC electronics were used to read out 256 (4 ×64) pads. Withoutswitching the gating grid wire using the GGD, we observe a gain attenuation at the beam rate of 10kpps.
On the other hand, such a gain attenuation can besuppressed by switching the gating grid wire. The
switching potential (VG ) is the same as that for theBRAHMS experiment.5) Though we can suppress thegain attenuation, the base line is fluctuated every timeby the gate operation, which generates a large noise on
the pad readouts with respect to the signal from theMIP particles. One of the main reasons of the noises
from the GGD is the different rise times in the positive
and negative sides although the sum of the positive andnegative side voltages (2VG ) should be controlled to beconstant. Since the noise shape caused by the GGD issimilar among different events, we calibrate the baseline shape with a pedestal run and reconstruct the hit
position after the subtraction of the baseline. Figure 2shows the position resolution along the wire axis of
the TPC after the calibrated baseline is subtracted.
The position resolution under the bad noise condition
created by the GGD is similar to the resolution withGGD. This implies that the noise created by the GGD
can be made insignificant by subtracting the baseline
as far as the position resolution is the same.

References
1) T. Kobayashi et al.: NIM B317, 294 (2013).
2) T. Murakami et al.: RIKEN Accel. Prog. Rep. 46, 150(2013).
3) P. Nemethy et al.: NIM 212, 273 (1983).
4) G. Rai et al.: IEEE Trans. Nucl. Sci. 37, 56 (1990).
5) M. Adamczyk et al.: NIM A499, 437 (2003).

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