Design of the forward straw tube tracker for the PANDA experiment
Monday, 9 January, 2017
In the PANDA experiment, for momentum analysis of forward scattered charged particles, a large gap dipole magnet and the Forward Tracker (FT) will be used. The main requirements for the FT include a high rate capability corresponding to particle fluxes reaching up to 2.5x10^4 cm^-2 s^-1 close to the beam pipe and a total counting rate of about 5x10^7 s^-1. A momentum acceptance extending down to at least 3% of the beam momentum and a momentum resolution better than 1.5% is expected. To meet these requirements, a tracking system based on 10 mm in diameter straw tubes, made of a thin (27 um) aluminized Mylar film, has been designed. It consists of 6 tracking stations, each comprising 4 planar double-layers of the straws with a total material budget of only 2% X0. The straws are made self-supporting by a 1 bar over-pressure of the working gas mixture (Ar/CO2). This allows to use lightweight and compact rectangular support frames for the double-layers and to split the frames into pairs of C-shaped half-frames for an easier installation on the beam line. The double-layers are built of separate modules consisting of 32 straws arranged in two staggered layers. The modular construction allows for fast repair and/or replacement of the modules suffering from aging effects or broken straws during the detector lifetime. The read out of the FT is based on the newly developed PASTTREC ASIC (0.35 um CMOS) providing configurable gain and shaping time, an ion tail cancelation and a baseline holder circuits appropriate for the high rate applications. The drift time as well as a Time-Over-Threshold (TOT) of the detector signals is measured using Trigger Readout Boards v3 (TRBv3) containing 64 TDC channels implemented in FPGA and serving also as nodes of the readout platform and data processing. Tests of prototype modules, performed with proton beams at high counting rates of up to 1 MHz/straw, demonstrated a tracking capability with a good spatial resolution of 150 um per straw and the applicability of the TOT technique for the identification of the particles species by means of their specific energy losses.