Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Construction of PHENIX Silicon Pixel Tracker
H. Torii,∗1 Y. Akiba,∗1 H. Asano,∗1 T. Hachiya,∗1 M. Kurosawa,∗1 T. Moon,∗1 H. Nakagomi,∗1 C. Pankcake,∗2H. Sako,∗1 S. Sato,∗1 T. Sumita,∗1 A. Taketani,∗1 and the PHENIX VTX group∗1∗2

RIKEN Nishina Center
Department of Physics, Stony Brook University- 229 -electron and hole pairs are created. If we assume theaverage distance from pixel pads to such the electrons
is 100µm that is in the middle of pixel sensor thickness, the 14,000 electrons distributes on pixel pads likegaussian distribution with 100µm sigma from a naive
estimate. Because one pixel pad has 50 × 425µm, twopixel pads is expected to be more than the thresholdon average.
3×10

Number of events

The PHENIX experiment is upgraded with a central
silicon vertex tracker1) to enhance its physics capabilities in nearly full azimuthal coverage over |η| < 1.2.
The main goal of the upgrade is to track the production vertex of charged particles with the resolution of50µm. The central vertex tracker consists of four layers, two layers of silicon pixel type and two layers ofsilicon strip type. The tracker has been constructed in2009 and has been producing fruitful physics results.
During the operation before 2012, the heat cycle during
its operation resulted in about 30% of inactive area on
the constructed silicon pixel type. In this paper, wewill report the status of repairing such the damaged
pixel-type tracker and new construction.
The silicon pixel tracker consists of 10 ladders forinner layer and 20 ladders for outer layer. Each ladder is made of mainly three components; a supporting
thermo-plate, four hybrid sensor, and a readout bus.
The hybrid sensor is made of a silicon sensor plate andfour readout chips, with 32 × 256 readout pixels whose
each size is 50 × 425µm. All the three components
are glued each other by epoxy resin with the thicknessof 100µm ± 30µm while the relative distance between
two hybrid sensor is kept within 15µm accuracy, whichis smaller than the estimated electron-drift diffusionof 20µm. They are electrically connected by 25µm
alminum bonding-wire. The wires are encapsulated bysoft silicon resin in order to prevent any damage froman accidental contact during construction and from vibration caused by the electrical altenative current of
10MHz through the wire and the PHENIX magnetic
field. For the damaged ladder, the electrical connection
together with the silicon encapsulation are redone after
old bonding-wires and old silicon resin are cleaned.
While entire construction procedure, the readout
test by the PHENIX readout system2) using electrical pulse and 0.546MeV beta-ray from Sr90 are performed three times; before the encapsulation, before
the transportation from Wako to RHIC, and before theinstallation. The main goal of the test is to ensure theelectrical connection between the sensor hybrids andthe readout bus and to measure the ratio of the active
pixels to all pixels. The threshold during the test is set
at about 3,000 e− while the nominal noise of the hybrid
sensor is expected to be 200e− . When a minimum ionization particle penetrates the silicon sensor with thethickness of 200µm, 14,000 electrons are estimated tobe created and collected by pixel pads. The distribution on the pixel pad surface depends on where theEntries

500000

Mean2.083

RMS

1.672

510150104310102

100101050

00551010151520252025
Number of pixels

Fig. 1. Number of fired pixels per event in 500k beta-ray
events by Sr − 90 radio active source.

Figure 1 depicts the number of fired pixels per hit.
The average of the number of pixels is 2.1, which isconsistent with the above estimate. This fact enables
us to measure the accurate rate of the active pixel toall pixel by this beta-ray test. Because some beta-ray
enters the silicon sensor with angles due to scatteringin materials between the radioactive source and thesilicon sensor, some gamma-ray produce wider distribution, that is also observed in the figure.
After reparing procedure of 15 ladders, the selected
ones were installed3) with <10% inactive pixels, whichis caused by due to faulty bump-bonding between thesilicon sensor plates and readout chips. For futher improvement, new construction procedure with improved
bump-bonding already started. All hybrids sensor4)
and readout buses are under gluing. At least seven
new ladders will be ready for installation before next
run of RHIC starting in the beginning of 2015.
References
1) A. Taketani, Nucl. Instrum. Meth. A541(2005)137-143
2) K. Fujiwara, K. Czech. J. Phys. 55(2005)1639-1643
3) M. Kurosawa et al.: In this report.
4) T. Sumita et al.: In this report.