Center for Nuclear Study,

Performance of LP-MWDC for � beam at 35.2 MeV
H. Miyaa , S. Shimourab , A. Saitoc , T. Kawabatab , H. Kureib , S. Michimasab , K. Mikic ,
K. Nakanishib , S. Otab , Y. Sasamotob , T. Uesakab and H. Sakaic aDepartment of Physics, Rikkyo University
bCenter for Nuclear Study, Graduate School of Science, University of Tokyo
cDepartment of Physics, University of Tokyo
1. Introduction
We are developing Low-Pressure Multi-Wire Drift
Chambers with stripped cathodes and delay-line readouts
(LP-MWDCs) as tracking detectors [1], which will be used
in the SHARAQ beam line under construction at RIBF
[2, 3]. The LP-MWDC has characteristics that positions
of beam particles are determined with signals from anode
wires and cathodes, and the chamber gas is operated at the
low pressure of less than 20 kPa to reduce the effect of the
multiple scattering. Figure 1 shows a schematic structure
of the LP-MWDC. The strips of the cathode (CX, CY) are
connected to delay lines [4]. The anode plane (AX, AY)
consists of 16 anode wires and 17 potential wires. The LP-
MWDC has outputs of 2 cathodes and 32 anode wires.
Cathode(CY)
Anode (AX)
Cathode
Cathode
-HV
Anode (AY)
-HV
-HV
Cathode (CX)
96mm
-HV
96mm
-HV
Potential wire
Anode wire
Figure 1. A schematic structure of the LP-MWDC with stripped
cathodes and delay-line readouts.
In March 2008, we performed a test experiment to
study performances of the LP-MWDC for an � beam at
35.2 MeV. Here, we report basic characteristics of the LP-
MWDC, derived from on-line analysis.
2. Experiment
The test experiment was performed at E7B course in
RIKEN RIBF operated by RIKEN Nishina Center and
CNS, University of Tokyo. � An beam at 35.2 MeV was
supplied by the AVF cyclotron accelerator. The energy deposition
of � the beam in the LP-MWDC is comparable to
�£� a N beam 200� at MeV, which is a candidate for the first
experiment to be performed with the SHARAQ. Figure 2
shows a schematic view of the experimental setup. A plastic
scintillator with the size 50� of mm� 50 and the thickness
of 5 mm was used as a trigger counter. The LP-MWDC was
operated using an isobutane gas (i– � � ) at 4, 5, 10, 15
and 20 kPa. The timings from the anodes and the cathodes,
�����
and the pulse heights from the cathodes were measured.
-HV
60
α Beam
Gate valve
Al, ZnS
Vacuum Chamber
LP-MWDC
Plastic Scint.
Light Guide
PMT
Figure 2. A schematic view of the experimental setup.
The data of these timings and pulse heights were taken with
CAEN V1190A multi-hit TDC and CAEN V792N QDC,
respectively.
3. Results
Figures 3 (a) and (b) show the detection efficiencies of the
cathodes and the anodes for the gas pressure of 20 kPa (open
triangle), 15 kPa (closed square) 10 kPa (open square),
5 kPa (closed circle), and 4 kPa (open circle) as a function of
the applied voltage of the cathodes and the potential wires.
The efficiencies are defined as the ratio of the counted number
of the cathodes and the anodes to the counted number
of the plastic scintillator. Since the threshold levels for the
cathodes and the anodes are different due to electric noise
levels, the efficiency of the cathodes is less than that of the
anodes. The threshold levels of the cathodes and the an-
odes were evaluated to be �����
���
and ���
���
, respectively,
based on the energy deposition of the � -particle and the gas
multiplication factor [1].
Figure 3. The efficiencies of (a) the cathodes and (b) the anodes
for gas pressures as a function of the applied voltage .
A position measurement via the delay-line readout of the
cathodes was demonstrated by using a mask with 2 mm