Development of a prototype TPC with CF4 & GEM
readout for heavy ion collision
T. Isobe, H. Hamagaki, K. Ozawa, M. Inuzuka, T. Sakaguchi, F. Kajihara, T. Gunji, N. Kurihara, S. Sawada, andS. Yokkaichi

Abstract— This article describes the R&D of Time Projection
Chamber (TPC) for heavy ion collisions. A prototype TPC wasdeveloped for the R&D of CF4 gas. It has a capability of using
GEM readout.
The characteristics of CF4 is a small diffusion coefficient andlarge drift velocity. It is suitable for our purpose of heavy ioncollisions. Characteristics of CF4 gas, such as a diffusion coefficientand a drift velocity, were measured with the prototype TPC. Inaddition, a tracking resolution and sensitivities of double track
separation and particle identification were measured using MWPCand pad readout. We achieved position resolution of 100 µm with
2.5 mm square pads.
For future use of a TPC readout, we have produced Gas
Electron Multiplier (GEM) using a plasma etching method. Gainof the GEM was measured as a function of an applied voltage with
an X-ray source. A gain of 104 was obtained in argon mixture
gases.
Index Terms— TPC, CF4 , GEM.

II. I NTRODUCTION

N the field of the experiments of the relativistic heavy ion
collisions, it is important to develop a high precision tracking detector which cope with high multiplicity environment. ATime Projection Chamber (TPC) is one of the most capabledetector to use under extremely high multiplicity condition.
Recently, in order to detect electron pairs which are comingfrom γ conversions and π 0 Dalitz decays, a TPC which cansurround the vertex more closely with more large acceptancehas been required. Then, it is need to develop a new type of
TPC which has higher double track separation capability as
well as good position resolution.
Recently, requirements for chracteristics of TPC became very
hard, because a multiplicity of particles went up higher at theRHIC energy. Especilly, the vertex TPC which is proposed
an upgrade option of the PHENIX experiment [1] will meetan extremely high particle density. One of aims of this R&D
program is supplying basic information to the PHENIX upgrade
TPC. The PHENIX [2], [3] detector system is a complexof spectrometers to measure hadrons, photons, electrons andmuons. The system was made for study of hot and dense√nuclear matter, produced by Au+Au collisions at sN N =T. Isobe, H. Hamagaki, K. Ozawa, M. Inuzuka, T. Sakaguchi, F. Kajihara, T. Gunji, and N. Kurihara are with the Center for Nuclear Study,
graduate school of science ,The University of Tokyo, Tokyo, Japan. E-mail:isobe@cns.s.u-tokyo.ac.jp .
S. Sawada is with KEK, Tsukuba, Japan.
S. Yokkaichi is with RIKEN, Saitama, Japan.

200A GeV at the Relativistic Heavy Ion Collider (RHIC) in
Brookhaven National Laboratory (BNL). The TPC is proposed
to be installed as the vertex spectrometer which has fullcoverage in azimuth in the pseudo-rapidity |η| ≤ 0.50, andserves for electron identification in the momentum range ofp≤ 200 MeV/c. It should be mentioned that the capability of
double track separation is the most important requirement, since
the expected dNch /dy value for RHIC collider is about 800 for
the 5% most central collisions. According to this environment,
the requiremant of the double track separation capability is lessthan about 1 cm.
In order to satisfy the above requirements, CF4 is proposed
as a chamber gas because of its small diffusion and fast driftvelocity. When the electric field of 1 kV/cm is applied, thedrift velocity is expected to be faster than 10 cm/µsec andlongitudinal diffusion is expected to be less than 100 µm for
1 cm drift [5].
To develop such a kind of TPC, CF4 gas characteristics andadaequatus readout electronics should be checked carefully. For
this purpose, a prototype TPC and GEM foils were constructedin Japan and tested with cosmic-ray and testbeams at KEK.
II. D EVELOPMENT OF A PROTOTYPE TPC
A. Design of a prototype TPC
A TPC was developed to measure the characteristics of CF4
and the total TPC performance. Layout of the design is shownin Fig. 1. Dimension of the field cage is 16 cm × 16 cm ×36 cm. It is designed to apply electric field more than 900 V/cm
with multistage strips in drift region. It is suitable for evaluating
characteristics of CF4 gas. Fig. 2 shows the schematic view ofthe readout region. The diameter of the anode wire and the grid
wire is 20 µm and 100 µm, respectively. Anode wires, grid
wires and cathode pads are adhered at separate frames whosethickness is 4 mm. These frames are detachable for testing withother type of readout. In particular, the installation of GEM
foils for readout is considered. Cathode pads of 2.5 mm, 6 mm,9.5 mm, and 13 mm square are mounted in order to study thepad size dependence of position resolution.
B. Front End Electronics
Front End Electronics of the TPC consists of Pre-amplifierand flash ADC circuits. The Pre-amplifier is mounted nearby
the cathode pads and output the signals to a flach ADC.

44600364[mm]10200105105120Gain(*10^4)41grid frame
anode frame
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Schematic view of the TPC readout.

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CF4 data
P-10 data

20Pre-amplifier was developed to convert a charge signal toa voltage signal and to drive differencial outputs. The preamplifier has high gain of 1 V/pC, and large bandwidth of300 MHz.
A flash ADC (RPV-160 by REPIC Co., Ltd) module wasused for converting to digital information from voltage signals.
It has the dynamic range of 8bit and conversion rate of100 MHz. The 100 MHz sampling rate is needed for the readoutelectronics to measure gas characteristics, because the expected
rise time of signals are ∼10 nsec when pure CF4 is used andabout E/p = 1 (V/cm/Torr) of field is applied. The module isaccessed via VME bus using on board CPU module (RIO3 byCES Co. Ltd). A DAQ system to operate the modules was alsodeveloped.
III. BASIC TESTS OF CF4 GAS
Characteristics of CF4 and P10 (Ar(90%)-CH4 (10%)) weremeasured with this TPC. P10 was used as the reference.
Measurements were carried out with a N2 -laser (YKN-500 by
Usho Co. Ltd) and a 55 Fe radioactive source. The gas contained
about 100 ppm water molecule and 30 ppm oxygen during themeasurements. Tested items are as follows.
1) gain: The absolute value of gain was measured byilluminating 5.9 keV X-rays from a 55 Fe radioactive sourcewith intensity of 370 kBq. Fig. 3 shows the result of gain
measurement. This TPC has a gain over 104 for pure CF4 gas,
when the voltage of anode wire is +3.3 kV. CF4 needs higher
voltage to detect the suitable signals.

3.4
3.6
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anode V oltage (kV)104Fig. 2.

3.2Fig. 3. The measured gain as a function of anode voltage obtained with P10
and CF4 .

drift velocity (cmm/ sec)

Fig. 1. Drawing of the TPC. Dimension is 16 cm × 16 cm × 36 cm. Electricfield is made by the 116-tier strips divided by 1MΩ register. By applying thehigh voltage to the top plate (right plate in this cage), this field cage can make33 cm drift region up to 900 V/cm. As you see the left of this figure, thereadout is detachable for testing with GEM readout.

CF4P-100.2

0.40.60.8
1electrical field (kV/cm/atm)

Fig. 4. The measured drift velocity as a function of electrical field obtainedwith P10 and CF42) drift velocity and longitudinal diffusion: The drift velocity and the longitudinal diffusion of CF4 and P10 were
measured using a N2 laser. The wave length and the energyof the laser beam is 337.1 nm and 2.5 mJ, respectively. Thislaser generates ∼ 102 electrons at a beam focal point in CF4 . To
deal with single electron for this measurement, an intensity wasattenuated. The N2 -laser beam was incidented at two different
positions of drift region. The drift velocity in pure CF4 and P10
was calculated using the distance and arrival time difference oftwo positions. As shown in Fig. 4, the drift velocity of CF4 gas
is more than 10 cm/µsec, when the electric field is 900 V/cm.
From the fluctuation of the arrival time, longitudinal diffusionin pure CF4 and P10 was measured as shown in Fig. 5. The
longitudinal diffusion of CF4 gas is about 60 µm for 1 cm drift,
when the electric field is 900 V/cm.
IV. P ERFORMANCE
Basic performance, such as a tracking resolution and adouble track separation, was measured using the TPC. Also, aparticle identification capability from dE/dx measurements wasevaluated. It was carried out with electron, pion and proton
beam in the momentum range from 0.5 GeV/c to 2 GeV/c
at KEK. Drift chambers were used as reference tracker. Gas
Cherenkov detectors, Scintillation counters, and a Lead GlassCalorimeter were used for particle identification.

1200

in the drift direction (2.5 mm square pads)
resolution(µ m)longitudinal diffusion
P10 measurement data
P10 simulation data
CF4 measurement data
CF4 simulation data

resolution(µ m)diffusion for 1cmm( m)along the wire (2.5 mm square pads)

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C. Particle identification
The particle identification capability was evaluated withpions, protons and deuterons contained in test beams. Incident
particle was identified by gas Cherenkov detector and TOF
measurements. Fig. 8 shows the ADC distributions of padreadout for each particle, 0.5 GeV/c, 1.0 GeV/c, 1.5 GeV/c,and 2.0 GeV/c.
The values of dE/dx were evaluated from measurements foreach particle assuming that energy loss depends on only thevelocity. Results are shown in Fig. 9. The errors of each plots

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400300200200100050100150200250030050100150200250drift length (mm)300drift length (mm)Fig. 6. Position resolution as a function of the drift length. The left figures
show the resolution along the anode wire direction, and right figures show theresolution in the drift direction. The errors are defined by the static error foreach measurement.

Nidentified double tracks/Ninputted double tracks

In order to evaluate the double track separation capability,
measurements were carried out using converted electrons byplacing a lead plate in front of TPC. The thickness of the Leadplate is 5 mm, which corresponds with about 1 radiation length.
Fig. 7 shows the result of double track separation. The
horizontal axis is the difference in distance and the vertical axis
is the ratio of number of tracks obtained from measurements
over number of tracks estimated from GEANT simulation.
Double track separation capability in the drift direction wasevaluated to be better than 1 cm. It is expected to depend on
the bandwidth of front end electronics mainly.

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300200along the wire (9.5 mm square pads)

B. double track separation

input angle
0 (degree)
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900

400Fig. 5. The measured longitudinal diffusion as a function of electrical fieldobtained with P10 and CF4Position resolution was measured for three type of pads:
2.5 mm, 6.0 mm, and 9.5 mm square. The position resolution
was calculated using the positions of three adjacent layers. It
is defined by difference between the unit position obtained atsecond layer and the position using the trajectory obtained fromfirst and third layers.
Fig. 6 shows the measured position resolution as a functionof drift length for each input angles. A position resolution of
100 µm along the wire was achived with 2.5 mm square pad.
The resolution in the drift direction is same for each type ofpads. The resolution along the anode wire is worse as the input
angle is bigger, because induced charge distribute more broadly
at large input angle. The resolution doesn’t depend on the drift
length strongly, because of the small diffusion of CF4 .

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difference [cm]Fig. 7. Difference in distance of identified double tracks in the drift direction.
Horizontal axis is the ratio of identified double tracks in simulated doubletracks. It is not normalized.

are defined by the r.m.s. of each distribution. As a result, thistype of TPC is expected to serve for electron identification in
the momentum range of p≤ 200 MeV/c for 1 m track length.
V. D EVELOPMENT OF GEM
Gas Electron Multiplier (GEM) is a metal-clad polymer foilwith dence holes [4]. By applying voltage to the metal layersof a GEM, a drift electron passing through the hole causes aavalanche by an application of a difference of potential between
the two electrodes, which are transferred and collected byreadout system. Gains of the order of 106 can be realized withYield

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ADCADCFig. 8. ADC distribution of each particle, electron, proton, pion, and deuteron.
Each figure shows the distribution at each momentum.

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protonpion
electron

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110102341010momentum (MeV/c)

Fig. 10. The upper photograph shows a hole of GEM at CERN. The lower
photograph shows the one produced at Fuchigami Micro Co., Ltd. using theplasma etching method.

Fig. 9. Expected ADC sum plot for electron, pion, and proton beam. Theerrors are defined by the r.m.s. of each distribution.

triple-GEM configuration by applying relatively low voltage
compared with typical wire chambers.
The GEM as a front-end readout can reduce positive ion
feedback sufficiently. In addition, two dimensional readout is
expected to be available with GEM readout. Then, it is proposed
to use GEM as TPC readout.
We have produced GEM using the plasma etching method
at Fuchigami Micro Co., Ltd in Japan. The plasma etching
method to make holes on the Kapton layer is Reactive Ion
Etching (RIE). The foil is settled as one of the electrodes
in a chamber filled with oxide and fluoride gases. Kapton
molecules are chemically destroyed by the radicals producedin the plasma that established in the chamber. Then the new
GEM has cylindrical holes as shown in the lower photographin Fig. 10. The hole of the standard GEM produced at CERN
is shown in the upper photograph in Fig. 10.
A. Gain measurement
Fig. 11 shows the schematic view of the GEM test setup.
Gain measurements were carried out with two or three GEMlayers. The signal source was 55 Fe (X-ray, 5.9keV). P10 andAr(70%)-CO2 (30%) were used as chamber gas.
Amplification factor was measured as a function of theapplied voltage as shown in Fig. 12. VGEM means the applied voltage to each GEM. Compared with the double-GEM
configuration, higher gain can be achieved in the triple-GEM
configuration. The gain characteristics are very similar to thoseof the GEMs made at CERN.
This GEM will be mounted as the readout of the prototype
TPC. Capability of GEM-TPC will be tested with KEK testbeams scheduled in the 2004.
VI. C ONCLUSIONS
A prototype Time Projection Chamber (TPC) was developedfor experiments with heavy ion collisions. The characteristics ofCF4 gas was investigated with this TPC. Tracking resolution,
double track separation, and particle identification capability
were tested with beam at KEK. We achieved 100 µm of position
resolution with 2.5 mm square pad.
A Gas Electron Multiplier (GEM) was produced using theplasma etching method. We have checked the basic characteristics of GEM foils. We plan to test it for TPC readout.

GainFig. 11. Layout of the GEM test setup. This components are mounted in agas vessel made of aluminum.

(a) ArCH4Gain

VGEM (V)(b) ArCO2

VGEM (V)Fig. 12. The measured gain as a function of VGEM obtained with (a)P10 and(b)Ar(70%)-CO2 (30%). VGEM means the applied voltage to the metal layersof a GEM.

ACKNOWLEDGMENTS
We would like to acknowledge the suggestions of many people, especially for C. Woody and I. Tserruya. We acknowledge
the KEK beam channel group and experimental support group,in particular Dr. Ieiri. We acknowledge students and stuffs who
belong to Tsukuba Univ. for providing support.
R EFERENCES[1] http://www.phenix.bnl.gov/phenix/WWW/TPCHBD/[2] PHENIX Conceptual Design Report, BNL, USA, 1993, unpublished.
[3] PHENIX collaboration, Nucl. Phys. A638, 1998 565c.
[4] F.Sauli, Nucl. Instr. and Meth. A386, 1997 531.
[5] J.Va’vra,et al., Nucl. Instr. and Meth. A324, 1993 113-126.