PHENIX VTX Group Designs, Constructs Precision Vertex Tracking System
By Eric Mannel
As the PHENIX experiment moves from the discovery phase to the detailed exploration of quark matter in the coming decade, understanding heavy-flavor production will provide key information about the earliest stages of heavy ion collisions. Understanding the production characteristics, such as yield and spectra, of charm particles (D mesons, J/Y, and J/Y') and beauty particles (B mesons, U) in heavy ion collisions will help provide this information. In order to make measurements of heavy-flavor production, precise vertex tracking is required to identify the decay particles from charm and beauty particles through the displacement of their trajectories from the primary collision. A B meson with a lifetime of 1.5 ps (1 ps = 10-12sec), will travel 500mm on average from the collision point, while a D± meson with a lifetime of 1.0ps will travel 300mm on average from the collision point. The use of a high precision tracking detector will allow the separation of charged tracks that come from the primary collision point and those that come from the decays of charm and beauty particles. In particular, since one of the decay particles of B mesons is the J/Y, precision vertexing will make it possible to separate J/Y's that are produced in the primary collision from those originating from B decays.
The PHENIX VTX group has designed and started the construction of a precision vertex tracking system (VTX) for the central arms using state-of-the-art silicon detectors. The VTX will provide 2p in azimuthal coverage and -1.3 < h < 1.3 coverage in rapidity with a distance to closest approach resolution better then 50mm.
The detector consists of 4 barrels, with the inner two barrels using pixel detectors based on the ALICE/LHCb1 design, and the outer 2 barrels using a novel stripixel design developed by the BNL Instrumentation group. A drawing of the detector is shown in Figure 1. The 4 barrels of pixel detectors reside in the central portion of the VTX enclosure, and the two end cap regions or "Big Wheels" are used to house the readout electronics for the VTX. The total length of the detector is 80 cm, with the central barrel region having a radius of 20 cm.
The two pixel barrels are made from pixel ladders, with each ladder containing 4 silicon sensor modules. The silicon sensor modules are made from 200mm thick silicon with 32×256×4 pixels. Each sensor has an active area of 1.23×5.44cm2 with each pixel has an active area of 50×425 mm2, Four ALICE/LHCb1 readout chips, thinned to a 150mm thickness are bump bonded to the silicon to make a complete sensor module. Two sensor modules are then attached to a pixel read out bus measuring 1.5×25cm2, with two pixel buses forming a complete pixel ladder. The readout of the ALICE/LHCb chips on the pixel bus is controlled by a SPIRO module located in the "Big Wheel" which transmits the data via optical fibers to the pixel Front End Module (FEM) which acts as the interface to the PHENIX DAQ system. Design and fabrication of the PIXEL ladders and readout electronics is the responsibility of PHENIX Collaborators from RIKEN, LLR-Ecole Polytechnique and SUNY Stony Brook.
The outer two barrels are built from stripixel modules. These modules use an innovative stripixel design developed by the BNL Instrumentation group. The stripixel detector is designed to provide 2 dimensional readout from a single side of a silicon detector. Each sensor is 625mm thick silicon measuring 3.43×6.36cm2 divided into 2 separate sensitive segments. The silicon is formed with 80×1000mm2 pixels that have two interleaved serpentine metal strips. A metal strip connects one set of serpentines in each pixel that are in straight line to form one readout direction to form the X view, with a second metal strip connecting the cells at a 4.6 degree angle with respect to the "X" view to form a "U" view. The design is shown schematically in Figure 2. The effective strip size is 80mm 3000 m.