RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Development of new ionization chamber for low energy heavy ionsN. Chiga,∗1 A. Hirayama,∗1,∗2 K. Chikaato,∗1,∗3 K. Nakano,∗1,∗4 J. Suwa,∗1,∗4 S. Kawakami,∗1,∗5
∗1,∗2
∗1
∗1
∗1Takeuchi,∗1,∗2
D. Suzuki,Suzuki,∗1Sumikama,,∗1
andS. Takeuchi,D.
T. Sumikama
and H.
H. Otsu
Otsu∗1A new multiple-sampling ionization chamber (MUSIC) was developed for the ImPACT experiment1)
in which measurements at 50MeV/nucleon were performed. A charge is read from pads divided into ﬁfteen
units in the beam direction. All fragments producedby the secondary reaction at the F8 focal plane werestopped inside of the MUSIC, and a ranges and theenergy loss of the incident particles were measured ineach segmented pad. To measure the fragments of different ranges at the same time, the length of the eﬀective area in the beam direction is required to be approximately 750 mm, although the length of a previousMUSIC2) at F11 was 480 mm. The previous MUSIC
had many anode and cathode ﬁlms on the beam trajectory; therefore, a large energy loss in the ﬁlms near theBragg peak cannot be measured. In order to overcome
these demerits, the new MUSIC, which has an eﬀective
length of approximately 750 mm and consists of only
counter gas in the eﬀective area, was developed. The
size of the new ionization chamber is summarized inTable 1.
Figure 1 shows a ﬁeld cage and a readout pad with
Frisch-grid. A high voltage of 4 kV is applied between top and bottom plates, and the electrons are
drifted to the top (anode) side. An entrance window with vapored Au electrodes on a Mylar ﬁlm withuniform thickness of 4 μm was adopted to uniform athicknessof substance.
substance. Fig.
thickness ofFig, 22 shows
shows aa contour
contour plotof the electric-ﬁeld gradient inside the detector byGARFIELD3) . The electric-ﬁeld gradient inside thecage was zero. The Frisch-grid was used to decrease
the eﬀect of charges induced by ions for readout pad.
The Frisch-grid is made in consideration of the aperture to the pad and the uniformity of the electric ﬁeld.
The drifted electrons pass through the Frisch-grid andare detected in a readout pad. The readout pad show
the backgammon electrode structure4) for position sensitivity. The size of one unit is 50.5 × 280 mm. The
readout pad consists of 15 units of electrodes, and thetotal size of the eﬀective area is 757 × 280 mm. The
MUSIC was successfully operated in the ImPACT experiments in autumn 20161) . Analysis of particle identiﬁcation is in progress.
This work was funded by ImPACT Program ofCouncil for Science, Technology and Innovation (Cab∗1
∗2
∗3
∗4
∗5

inet Oﬃce, Government of Japan).
Table 1. Speciﬁcations of ion chamber.
Eﬀective area (X × Y × Z)
260 × 170 × 757 mm3
Electric ﬁeld cage size
369 × 194 × 780 mm3
Gas box size
700 × 440 × 1000 mm3dŽƉƉůĂƚĞZĞĂĚŽƵƚƉĂĚ&ƌŝƐĐŚͲŐƌŝĚ^ŝĚĞŽĨĨŝĞůĚĐĂŐĞŽƚƚŽŵƉůĂƚĞ

Fig. 1. Photograph of a ﬁeld cage and a readout pad witha Frisch-grid.

Top plate: 0 kV

Frisch-grid

Side of ﬁeld cage

Bottom plate: -4kV

Fig. 2. Contour plot of the electric-ﬁeld gradient calculatedusing GARFIELD.

References
RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Department of Fundamental Sciences, Niigata UniversityDepartment of Advanced Energy Engineering Sciences,Kyushu University,
Department of Applied Physics, Faculty of Engineering,
University of Miyazaki,- 164 -1) T. Sumikama et al., in this report.
2) H. Otsu et al., RIKEN Acc. Prog. Rep. 42, 163 (2009).
3) R. Veenhof:, GARFIELD, a drift-chamber simulation
program, CERN (1994).
4) R. Allemand, G. Thomas: Nucl. Instr. Meth. 137, 141(1976).