Introduction to the TRT Readout Drivers (RODs)

The Transition Radiation Tracker (TRT) is a straw-tube based tracking chamber that makes up part of the ATLAS Inner Detector. The Inner Detector is designed to provide precision tracking and momentum measurements for charged particles such as electrons, muons, pions, kaons and protons. The entire detector typically provides more than 40 measurements on the trajectory of each track. The tracker operates within a 2 Tesla magnetic field which causes the charged particles to bend and results in a helical trajectory. The curvature of the helix allows the momentum of the track to be measured.

The Inner Detector is made up of 3 detectors -- the Pixel Detector, Silicon Strip Detector and the TRT. Each detector consists of a central cylindrical barrel (centered on the interaction poing of the beam) and layers of wheel-like end caps. The Pixel Detector consists of 3 layers of 50x400 micron pixels, the Silicon Tracker (SCT) consists of 4 layers of silicon strip detectors in the barrel region and 9 in the end-caps, and the TRT consists of 73 layers of straw tubes in the barrel and 160 layers in the end-caps.

The Transition Radiation Tracker

The straws in the TRT are 4mm in diameter, made from aluminum plated with a 31 micron diameter wire running down the middle, filled with a of Xenon, C02 and Oxygen. When a charged particle passes through the gas within the straw, it ionizes the gas. The outer wall of the straw is held at a large negative voltage wirh respect to the anode wire, so the ionization electrons drift towards the wire. The full time taken for an electron to drift from the outer edge of the straw is approximately 50 nS, (50 billionths of a second). The presence of charge on the anode wire is a signal that a particle passed through that particular straw. More accurate position information can be obtained by measuring the time of arrival of the earliest signal, which corresponds to ionization from the passing particle at its closest distance to the central wire.

The 4mm diameter straws are spaced approximately 7mm apart, so a typical particle passing through the detector leaves hits in just of half of the layers of straws -- typically about 36 hits. The inter-straw space is not empty, however, but is filled with a radiator material. In the barrel, this is a mesh of polypropelene fibers. Particles will traverse many layers of material, and those at extreme relativistic speeds have the possibility of emitting Transition Radiation. Only particles with velocities very close to the speed of light (Lorentz boost above approximately 1000) will emit this radiation. For tpyical particles in an ATLAS event, only electrons are traveling fast enough to radiate -- and this is the second purpose of the Transition Radiation Tracker -- distinguishing electrons from other more common particles.

The TRT Data Acquisition System

There are approximately 300,000 straws in the TRT. Out of the 40M beam crossing per second, each yielding up to 20 proton-proton interactions, ATLAS can read out 100,000 per second. For each of these 100k events, the data from each of the 300k straws is read out. The total data volume from just the TRT is approximately 2 Tbits/second, or enough information to fill a typical hard drive, every second. The Data Acquisition System handles the flow of this data. One of the key components of this system are the Readout Drivers (RODs). These are responsible for receiving all this data, checking it for errors, compressing the data and formatting it to be passed to the next stage of the system for further analysis and possible storage for offline analysis. In addition, the RODs detect failures in the on-detector electronics.