Heavy quark measurements are a key component of the heavy ion program at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL) for the systematic characterization of the dense medium created in heavy ion collisions, the so-called Quark-Gluon Plasma (QGP). The STAR experiment at RHIC uses a Time Projection Chamber inside a magnetic field as its main tracking detector, but with its 1 mm pointing resolution, the TPC is not able to resolve the decay vertex of these short lived mesons. Before the 2014 run a new silicon based micro-vertex detector called “Heavy Flavor Tracker” (HFT) was installed in STAR, which allows measuring decay vertices of open heavy flavor particles with very short lifetimes by direct topological reconstruction. The HFT consists of 3 different silicon detector subsystems arranged in 4 concentric cylindrical layers around the beam pipe: the “Silicon Strip Detector” (SSD); the silicon pad sensor based “Intermediate Silicon Tracker” (IST); and two layers of the Monolithic Active Pixel Sensor (MAPS) based “PiXeL” detector (PXL). The SSD and IST detectors’ aim is to guide charge particle tracks found in the TPC to the innermost PXL detector in a high hit density event.
The version of MAPS sensors used for the PXL detector is “Ultimate” from the Strasbourg IPHC PICSEL group. The readout electronics design allows interfacing to the sensors for readout and control, and was designed to fit into the existing STAR infrastructure with respect to Trigger, DAQ, and Slow Controls. The readout rate matches the TPC readout rate with little additional dead time.
The detector layers are thinned silicon sensors on a flex cable. At the end of each ladder are readout buffers and cable drivers that send zero-suppressed data over 2 m twisted-pair cable to the “Mass Termination Board” (MTB). The MTB contains terminations for the cables as well as power regulators with over-current protection circuitry. Each MTB services 4 ladders and there are a total of 10 MTB’s in the PXL detector. The sensor data is then transmitted 11 m to one of 10 Readout (RDO) Boards in the low Radiation Area. The FPGA based RDO board receives the data, buffers and formats it and sends it over 100 m optical fibers to one of ten fiber readout receiver card (RORC) channels in the DAQ room. The RDO card is also the interface to the trigger system and receives the triggers from the trigger/clock distribution board. The configuration and control interface between the RDO card and the sensors is done using the JTAG standard. The interface to DAQ is accomplished over the bi-directional Detector Data Link (DDL) standard adopted by STAR from ALICE, consisting of the “Source Interface Unit” (SIU) on the RDO boards, connected via optical fiber to the “Readout Receiver Card” (RORC) in the off-the-shelf DAQ PCs in the DAQ room.
This talk describes the details of the hardware, firmware, and software associated with the readout electronics for PXL.