T555 : Development of TPC and HBD for the PHENIX Upgrade
B. Azmoun,1 T. Gunji,2 H. Hamagaki,2 M. Inuzuka,2 T. Isobe,2 Y. Morino,2 M. Naglis,6 S.X. Oda,2K. Ozawa,2 I. Ravinovich,6 S. Saito,2 T. Sakagachi,2 S. Sawada,3 I. Tserruya,6 Y. Yamaguchi,5S. Yokkaichi4
12Brookhaven National Laboratory, Upton, NY 11973-5000, USA
Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033,Japan
3KEK, High Energy Accelerator Research Organization, Tsukuba-shi, Ibaraki-ken 305-0801, Japan
4
RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan
5Waseda University, Advanced Research Institute for Science and Engineering, 17 Kikuicho, Shinjuku-ku, Tokyo
162-0044, Japan
6Weizmann Institute, Rehovot 76100, Israel

The PHENIX experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory has been performed to create and study extremely hot and dense matter. Although there
are some strong indications that a new state of matter is being created, we need additional measurements to conclude that the new matter is Quark Gluon Plasma. One of required measurements
is the measurement of low mass vector mesons (ρ, ω and φ) to study chiral symmetry restoration.
The dielectron channel is suitable for this measurement, however, there is huge background from π 0Dalitz decays and gamma conversions with the present detector configuration.
We are planning to install a Hadron Blind Detector (HBD) and a Time Projection Chamber(TPC) using Gas Electron Multipliers (GEMs) in the PHENIX detector to reduce the background.
The GEM is a metalized polyimide foil with holes and used for charge multiplication. High electricfields are realized inside the holes with relatively low voltage difference. The HBD is a sort ofgas Cherenkov detector and consists of a 50-cm long radiator with CF 4 gas directly coupled to a
triple-GEM detector element with a CsI photocathode and pad readout. A mesh locates before thefirst GEM foil to ensure a zero field above the photocathode, which rejects ionization charges. TheTPC uses GEM foils as a readout instead of wires to improve spatial resolution and adjacent track
resolution, and to reduce positive ion feedback into the drift region.
T555 experiment was carried out to evaluate the performance of the TPC and the HBD at theKEK-PS π2 test beam line.
Detection efficiency, spatial resolution, adjacent track resolution, particle identification capability
by energy loss of the TPC are measured using 1.0-GeV/c negative pion beams. In addition to suchbasic measurements, those dependence on gas (P10, Ar(70%)-C 2 H6 (30%) and pure CF4 ), drift length(20–290 mm), readout pad shape (rectangle and zigzag) and beam momentum (0.5–3.0 GeV/c) aremeasured. Figure 1 shows spatial resolution both in the pad-row direction and in the drift direction
as functions of the drift length. Spatial resolution of 80 µm in the pad-row direction, adjacent track
separation of 12 mm in the drift direction and detection efficiency of 99.3% are achieved. Energy loss
measurements show the good particle identification capability. Beam rate dependence of the TPC
characteristics are also checked. The TPC holds high detection efficiency and good spatial resolution
with high particle rate of 4800 cps/cm 2 , which exceeds the rate of RHIC. The test results show that
the TPC meets the requirements for operation in the PHENIX detector.
Measurements of the number of photons in the bandwidth (110–200 nm) per unit distance, N 0 ,and hadron rejection factor of the HBD are also objectives of T555 experiment. Figure 2 shows themean amplitude versus the drift field E D in the gap between the mesh and the first GEM with 1.0GeV/c negative pion beams. The amplitude decreases as sharply as expected when the polarity ofthe drift field is switched. The figure shows also the results of similar measurements performed with
an α-ray source. A direct measurement of N 0 was not possible due to the large UV absorption in theavailable gas system. We observed only 6–10 photo electrons for a 1.0-GeV/c electron. This number
is however consistent with the expected number of ∼35 p.e. after correcting it for the measured
absorption in the gas.

Spatial Resolution in Drift Direction [µ m]Spatial Resolution in Pad-row Direction [µ m]600500400300200
1000050100 150200 250300

Drift Length [mm]600500400
300
Ar(90%)-CH4 (10%) zigzag

200Ar(90%)-CH4 (10%) rectangle
Ar(70%)-C 2 H6 (30%) rectangle

100CF4 zigzag

0050

100 150200 250300

Drift Length [mm]Realtive amplitude

Figure 1: Spatial resolution of the TPC versus the drift length in the pad-row direction (left) andthe drift direction (right).

1.4

Charge collection

1.2

α-particles

1pions

0.8
0.6
0.4
0.2
0-3-2-10

1

2

3
ED (kV/cm)

Figure 2: Collection of ionization charge of the HBD versus the drift field E D in the gap between themesh and the first GEM.