Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 49 (2016)

Development of sensor prototype for sPHENIX Silicon Tracker
Y. Akiba,∗1 I. Nakagawa,∗1 and G. Mitsuka∗1sPHENIX is a new detector at RHIC1) , which will
be used for measuring jets, direct photons and Upsilon. The detector has electronmagnetic and hadronic
calorimeters to measure jets and a high precision
tracker. The tracker is contained inside a 1.5 T superconducting solenoid which was previously used forBabar experiment at SLAC.
We aim to construct a large silicon tracker forsPHENIX2) and are working on the research and development of the silicon detector. The tracker consists ofthree tracking stations (from inner to the outer, calledS0, S1 and S2) covering −1 < η < 1 and ∆ϕ = 2π.
The basic building block of the silicon tracker is asensor module, which consists of a sensor, two HighDensity Interconnects (HDIs) for read-out, and mechanical support. We are currently developing a prototype of the silicon module used for the S1 layer. In
this paper we report the status of a prototype silicon
strip sensor for the S1 silicon module. The status of
HDI is reported by another paper3) .

Fig. 1. Conceptual design of the S1 strip sensor.

Figure 1 illustrates the conceptual design of the S1sensor. The sensor is a single sided, AC coupled sensor. The active area of the sensor, 96 mm (z) × 45.5mm (ϕ), is divided into 10 × 6 blocks. Each block has
128 short strips that are 58 µm in pitch and 9.6 mm
long, and run parallel to the z (beam) direction. InFig. 1, the strip runs horizontally. The read-out linesof the strips, indicated by red lines in the ﬁgure, runperpendicular to the strips and bring the signals to theread-out chips placed at the upper and the lower edgeof the sensor. The upper and the lower 3 blocks areconnected upwards and downwards, respectively. This
arrangement reduces the channel count to save the costof the detector. Although signals in the 3 strips are
combined and cannot be distinguished in the read-out
chip, the oﬄine track reconstruction program should
distinguish the correct hit strip by requiring a good χ2for tracks ﬁtted to all layers simultaneously. The probability of misidentifying the hit strip is low owing to∗1RIKEN Nishina Center- 199 -Fig. 2. A drawing of the S1 prototype sensor.

the expected low channel occupancy. The occupancy
is ≃ 0.2% even in central Au+Au collisions.
FPHX chip, which is used for FVTX silicon detectorof PHENIX, is utilized to read-out the sensor. The
read-out pad pitch of the sensor is thus matched tothat of FPHX chip (75 µm). FPHX chip has low power
consumption, about 64 mW per chip, which reduces
the need for cooling for the sensor module. A FPHX
chip has 128 ch and is used to read-out 3 blocks thatare combined together. A total of 20 FPHX chips, 10at the upper edge and 10 at the lower edge, are used
to read-out one sensor.
The sensor will be manufactured by Hamamatsu
Photonics Co. (HPK). Figure 2 is a part of a design
drawing of the sensor made by HPK. Two sensors will
be fabricated on one 6-inch wafer. The size of the sensor is maximized to fabricate two sensors on a 6-inchwafer.
We will produce two types of sensors in the same
design. One is 320-µm thick, which is the standard
thickness of wafers that HPK uses. The other one is240-µm thick, which is made by thinining the 320 µm
sensor. The thinner sensor results in lower scatteringof charged particle and thus improve the momentum
resolution of tracks. However, the signal generated bya hit will be reduced. We will evaluate the S/N ratiofor MIP particles for both types of sensors to determine
which one we will use for the production.
The design of the sensor at HPK has been complete
and they are manufacturing the sensors. We expect
the delivery of the sensors by early 2016.
References
1) sPHENIX preliminary Conceptual Design Report(2015).
2) I. Nakagawa, in this report.
3) G. Mitsuka, in this report.