The Heavy Flavor Tracker (HFT) was installed in the STAR experiment for the 2014
heavy ion run of RHIC. Designed to improve the vertex resolution and extend the measurement
capabilities in the heavy flavor domain, the HFT is composed of three different silicon detectors
based on CMOS monolithic active pixels (MAPS), pads and strips respectively, arranged in four
concentric cylinders close to the STAR interaction point.
The two innermost HFT layers are placed at a radius of 2.7 and 8 cm from the beam line,
respectively, and accommodate 400 ultra-thin (50μm) high resolution MAPS sensors arranged in
10-sensor ladders to cover a total silicon area of 0.16m2
. Each sensor includes a pixel array of
928 rows and 960 columns with a 20.7μm pixel pitch, providing a sensitive area of ∼ 3.8 cm2
.
The architecture is based on a column parallel readout with amplification and correlated double
sampling inside each pixel. Each column is terminated with a high precision discriminator, is read
out in a rolling shutter mode and the output is processed through an integrated zero suppression
logic. The results are stored in two SRAM with ping-pong arrangement for a continuous readout.
The sensor features 185.6μs readout time and 170mW/cm2 power dissipation. The detector is aircooled, allowing a global material budget as low as 0.39% on the inner layer. A novel mechanical approach to detector insertion enables effective installation and integration of the pixel layers within
an 8 hour shift during the on-going STAR run.
In addition to a detailed description of the detector characteristics, the experience of the first
months of data taking will be presented in this paper, with a particular focus on sensor threshold
calibration, latch-up protection procedures and general system operations aimed at stabilizing the
running conditions. Issues faced during the 2014 run will be discussed together with the implemented solutions. A preliminary analysis of the detector performance meeting the design requirements will be reported.
