by Ken Oyama. Published: 14 January 2011

Three additional Transition Radiation Detector (TRD) supermodules were added to ALICE in the last two weeks of 2010. As the modules were inserted into the spaceframe - the support structure that holds the beampipe and the central ALICE detectors - the installation had to be closely watched by our survey team to ensure that any movements and deformations within the frame stayed within acceptable tolerances of less than 1-2 millimeters. With most of the ALICE subdetectors already in place, the installation of the new modules, around densely packed detector elements and services, posed a considerable challenge.


A TRD module (from the 2006 installation)

The TRD is an essential detector in ALICE which performs electron identification especially at high momenta, and in the high multiplicity environment of lead-lead collisions. It detects transition radiation X-rays, which are emitted when very energetic, charged particles cross the border between two media of different dielectric constants – in this case, between air and polypropylene. The TRD uses this information to identify electrons.

The TRD is also important for the ALICE (di-)electron trigger and jet triggers. Highly integrated signal processing electronics on the detector, together with advanced global tracking units, achieve the finding of di-electrons and jets in less than ten microseconds after the interaction. This trigger capability is useful in seeing rare signals - such as Upsilon and high momentum jets.

The detector is composed of drift tracking chambers and a transition radiator. The drift time is approximately 2.5 microseconds. The transition radiation emitted from the radiator passes into the drift region, and overlaps with the normal energy loss by ionization of the charged particle. It is absorbed in the Xenon gas and then observed as an enhanced signal, in addition to the energy loss. By detecting this signal enhancement, electrons are identified with 90 per cent efficiency. The rejection capability for contaminating particles - mostly pions - is 99 per cent.

ALICE’s TRD system consists of 18 supermodules, cylindrically surrounding the Time Projection Chamber. Each of these supermodules has either 24 or 30 chambers, each with around 2300 readout channels. In total, there are more than 1.15 million channels across the whole of the TRD.

The research and development for the TRD started in 1997. During the first proton-proton and lead-lead runs of 2010, we operated with the seven TRD supermodules which were installed inside ALICE. During this period four additional supermodules were also manufactured. The first weeks of the long technical stop, after the first LHC heavy ion run, saw the installation of three of these additional supermodules; and already, in the beginning of January, 10 supermodules were successfully operated together.

The current plan is to finish the production of the rest of the supermodules in 2011, to be ready for installation during the 2012 shutdown.

Ken Oyama is a physicist at the University of Heidelberg.