RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-9. Instrumentation

Development of D2 gas-filled drift chamber for spectroscopy
measurements of pionic atoms in inverse kinematics

We report the development of a D2 gas-filled drift
chamber for spectroscopy measurements of pionic
atoms. We plan to perform missing-mass spectroscopy
measurements of deeply bound pionic atoms in inverse
kinematics of the (d,3 He) reaction. We will employ aD2 gas-filled multi-wire drift chamber (MWDC) as anactive target in a magnetic field of ∼ 1 T. A heavy
ion beam is injected into the chamber inside which thereaction occurs. The reaction angle and the energy of
forward scattered 3 He are measured using the MWDCand silicon strip detectors, which are installed inside
the MWDC. The energy of the 3 He is approximately
60 MeV. The feasibility of the measurements was investigated through simulations in a previous study.1)
According to the results of the simulations, a position
resolution of 500 µm and enough gain to measure 60
MeV 3 He are required for the MWDC.
To check the performance of the D2 gas-filled
MWDC, we fabricated a prototype of the detector. Forthe MWDC, hexagonal wire geometry, in which a sensewire is surrounded by six potential wires, is adopted.
The side length of the hexagon is 6 mm. The MWDC
consists of 10 planes, five of which are tilted (two Uplanes at 18◦ and three V planes at -18◦ as shown inFig. 1) to measure the vertical position of charged
particles. The outer cells serve as guard wires against
chamber walls to eliminate field deformation.
Figure 2 shows the measurement setup. Inside theMWDC, an α source (241 Am, 4.3 kBq) was placed anda silicon strip detector (12 strips, 37.5 × 50 mm2 , 300µm thick) was installed on the opposite side. The trigger was initiated by the signal of the silicon detector
and the timing of the MWDC signal was recorded using
a TDC. In this measurement, the chamber was filledwith 1 atm H2 gas instead of D2 gas as the first step.
Tracking analysis was performed and position resolution was evaluated when a HV of -1420 V was applied
to the potential wires. The α-ray detection efficiency
was higher than 99% for all planes except for the firstone. Figure 1 shows a typical event display. The drift
length in each cell is expressed as a circle and the hit
strip in the silicon strip detector is painted black. The
track was successfully reconstructed as indicated by
the red line in Fig. 1. The position resolution of theplanes with vertical and tilted wires was evaluated tobe 100 – 150 µm and 100 – 250 µm. The resolution is
sufficient for the experiment if the magnetic field does
not deteriorate the resolution considerably.
∗1∗2∗3x position [mm]Y. N. Watanabe,∗1,∗2 H. Fujioka,∗1,∗3 R. S. Hayano,∗2 K. Itahashi,∗1 T. Nishi∗1,∗2 and Y. K. Tanaka∗1,∗2NoName__66
Entries
1
Mean x0
Mean y0
RMS x0RMS y050

X X’X X’X
Planenumber     

40
30

U U’
 V V’V  

20100-10-20-30-40! source

-50-80-60

SSD-40-20020

406080z position [mm]Fig. 1. An example of an event display. There are potential
wires on the vertices and sense wires at the center of thehexagons. The circles represent the drift length for eachwire and the straight line is the deduced track.

! source

SSD

Fig. 2. A schematic drawing of the measurement setup.
An α source was placed in front of the first plane anda silicon detector was placed on the opposite side.

In the measurement, we observed the position dependence, i.e., the plane dependence of gas gain. In
the fifth – eighth planes, facing blank regions, the gas
gain was more than five times larger than that in theother planes when HV was -1420 V. This was due tothe geometrical configuration of the wires, and it wasqualitatively confirmed through Garfield2) calculations
that the electric field strength in these planes was effectively 5% higher. Geiger-mode behavior in theseplanes caused a rapid increase of current and made itdifficult to apply higher voltage. We found that extra
guard wires were needed facing the blank regions. Under the current condition, the detection efficiency for
60 MeV 3 He in the low-gain planes was estimated tobe less than 20%.
As the next step, we will improve gain uniformity by
applying discrete voltage to each plane.
References

RIKEN Nishina Center
Department of Physics, University of Tokyo
Department of Physics, Kyoto University

1) K. Okochi et al.: Accel. Prog. Rep. 46, 79 (2013).
2) R. Veenhof: Garfield, (http://garfield.web.cern.ch/garfield)- 236 -

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