Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Status of silicon pixel detector for PHENIX experiment toward
RHIC Run-14
M. Kurosawa,∗1 Y. Akiba,∗1 H. Asano,∗2 K. Boyle,∗1 J. Bryshawskyj,∗3 C. Chen,∗1 T. Hachiya,∗1 M. Lentz,∗4B. Miljko,∗5 T. Moon,∗1 H. Nakagomi,∗6 R. Nouicer,∗4 C. Pancake,∗7 H. Sako,∗8 S. Sato,∗8 P. Stankus,∗5T. Sumita,∗1 A. Taketani,∗1 H. Torii,∗1 M. Wysocki,∗5 and the PHENIX VTX group
The silicon vertex tracker (VTX) was installed in thePHENIX experiment in 2010, and it successfully collected approximately 5 billion events of Au+Au colli√
sions at sN N = 200 GeV in the 2011 RHIC run (Run11) at Brookehaven National Laboratory (BNL). The
main function of the VTX is the separation of heavy
flavor (HF) hadrons, charm and bottom, with the measurement of the distance of closest approach of single
electrons from their decays. The nuclear modification
factor and azimuthal anisotropy for HF were measured,and analysis results have already been reported1) .
The VTX is composed of two inner silicon pixel detectors and two outer silicon strip detectors. The silicon pixel ladder is the basic component of a silicon
pixel detector. The ladder consists of four silicon sensor modules, two readout buses, and a cooling support.
The I/O pads of the silicon sensor module are electrically connected to that of the readout bus via aluminum bonding wires that are encapsulated in epoxy
resin. The silicon sensor module is an assembly of a silicon pixel sensor and four readout chips bump-bondedwith 25-µm-diameter bumps to the silicon sensor. During Run-11, a small fraction of bump bonds were defected due to thermal stress. In addition, some bonding wires were broken due to a thermal stress causedby the difference in the thermal coefficient between theencapsulation and the readout bus. The active areaof a VTX was decreased to 60% because of defectedbump bonds and broken bonding wires. The solutions
for these issues are to change the operation temperature from 0 degree to the room temperature to avoid
thermal stress, and to use a different type of encapsulation. The operation temperature was changed fromthat used in the physics run in 2012.
After the shutdown of the physics run in 2013,
the VTX was dis-assembled at BNL to replace theencapsulation with a different type of encapsulation.
All pixel ladders that had broken wires were sent toHAYASHI WATCH-WORKS CO., LTD. where pixel
ladders had been mass produced. In total, 15 pixel ladders had been repaired with the yield of almost 100%
during 6 months. A small fraction of dead area (< 1%)
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8RIKEN Nishina Center
Department of Physics, Kyoto Univ.
Department of Physics, City Univ. of New YorkBrookhaven National Laboratory
Oak Ridge National Laboratory
Department of Physics, Tsukuba Univ.
Department of Physics, Stony Brook University
Japan Atomic Energy Agency

was remained because of the damage to I/O pads during repair process.
The repaired ladders were electrically tested at
RIKEN before being shipped to BNL, and the final electrical test was performed at BNL before thereassembly of the VTX. The following are the testitems.
(1) Current consumption of a pixel ladder.
(2) Optimization of a reference voltage for DACs on
the readout chip of a pixel module.
(3) Bias voltage dependence of the leakage current
for a silicon pixel sensor.
(4) Response of the pixel ladder to a β source (90 Sr).
No issue was found in all tests. Figure 1 shows thetypical response of a pixel ladder to the β source.

Fig. 1. Response of pixel ladder to a β source. The horizontal and vertical axes represent z and φ direction in
the PHENIX coordinate, respectively. Low gray-levels
represent low number of hits.

Since some pixel ladders had faulty bump bonds,
there is a small fraction of unfixable dead area(< 1%)
in the pixel ladder, and the configuration of pixel ladders in the VTX affects physics data. The optimization of the performance of the pixel detectors had been
done by arranging the configuration based on the test
results at BNL.
The reassembly of the VTX had been successfully
performed at the end of 2013. As a results of the repair work, the active area of the VTX was significantly
improved to about 90%. Final optimization of operation parameters for pixel ladders is ongoing. We willcollect 200 GeV Au+Au collision data with around 10times more statistics than that collected in Run-11.
References
1) R. Nouicer et al.: Nucl. Phys. A 904-905 (2013) 647c652c- 228 -