Ⅱ-9. Instrumentation

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collaborationand the SπRITThe SπRIT (SAMURAI pion Reconstruction andIon Tracker) experiment utilizes the SπRIT Time Projection Chamber (TPC)1) as the main detector. The
TPC operates within the uniform magnetic ﬁeld of theSAMURAI magnet. Within its ﬁeld cage, the TPC
provides a uniform electric ﬁeld anti-parallel to themagnetic ﬁeld. When particles travel through the ﬁeldcage, they ionize electrons from the counter gas, leaving tracks of electrons. These drift electrons move upwards to the detection region, which consists of wire
planes which amplify the electron signal via avalanche
process and a 2-D pad plane to readout the ampliﬁed
signal. The avalanche process produces many positiveions, which can alter the electric ﬁelds in the detector.
Both the electrons and positive ions cause aging eﬀects
on the wires. To avoid space charge problems and aging eﬀects, a gating grid wire plane is used to close thedetection region to prevent drift electrons from reaching the avalanche region unless a trigger has been provided. The gating grid operates with an average voltage Va that matches the electric ﬁeld from the ﬁeldcage. In the open state, all wires are maintained at
the average voltage Va . In its closed state, the voltages of alternate wires are oﬀset by voltage ΔV fromVa to voltage Vh , which is ΔV higher than Va , and voltage Vl , which is ΔV lower than Va . Drift electrons will
be attracted to the wires at Vh and repelled from thewires at Vl , causing them to terminate at Vh . The time
it takes to transition between closed and open must beminimized, as electrons which reach the detection region before the gating grid opens will not be detected.
To control the opening and closing of the gating grid,a gating grid driver was developed at MSU to quickly
drive the voltages from closed to open, and then backto closed again.
Key features of this gating grid driver include using a low-impedance transmission line, as well as using two pairs of N-type and P-type MOSFET switchesto drive both the opening and closing of the gatinggrid. For the opening of the gating grid, the Vh and Vl
wires are shorted together using an N-type and a Ptype MOSFET switch, bringing them to the common
voltage of Va . These switches are denoted as N 1 and†∗1

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Condensed from the article in arXiv:1612.06708
National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy, Michigan State University
RIKEN Nishina Center
Department of Physics, Tsinghua University
Department of Physics, Kyoto University
College of Nuclear Science and Technology, Beijing Normal
University- 169 -P 1 in Circuit A of Figure 1. To return the gating gridto the closed state, the switches N 2 and P 2 are used
to quickly recharge the wires to Vh and Vl .
In the recent SπRIT experiment, the gating grid
driver was used to drive the gating grid of the SπRIT
TPC. Both the TPC and the gating grid driver arelocated inside the SAMURAI magnet. According toGARFIELD calculations, in a magnetic ﬁeld of 0.5T,
ΔV should be less than 10V to achieve 90% electrontransparency, eﬀectively opening the gating grid. Thiscondition is achieved within 350 ns of providing thesignal to open. The gating grid with the driver worked
succesfully, with only a small pickup correction evidenced by the changing of Va , which can be removed
from the data at in oﬄine analysis. The analysis of
the performance of the gating grid is ongoing in oﬄineanalysis.

Fig. 1. The gating grid driver circuit design. Circuit A
refers to the left side, while Circuit B refers to the circuit inside the red-dashed area.

This work is supported by the U.S. Departmentof Energy under Grant Nos. DE-SC0004835, DESC0014530, DE-NA0002923, US National Science
Foundation Grant No. PHY-1565546 and the Japanese
MEXT KAKENHI (Grant-in-Aid for Scientiﬁc Research on Innovative Areas) grant No. 24105004.
Reference1) R. Shane et al.: Nucl. Instr. Meth. A 784, 513 (2015).