When a charged particle crosses the boundary between two media with different dielectric properties (that is to mean, with different index of refraction), it can emit an electromagnetic radiation, known as transition radiation, which was first observed by Ginzburg and Frank in 1946. The nice feature of this radiation is that its intensity is approximately linear with the Lorentz γ factor (=E/m₀c²) of the particle.

Practically, this radiation only becomes useful for detectors in the case of ultrarelativistic particles (γ ≥ 1000), when the emission is mainly along the particle direction, and in the energy range of X-rays. Thus, a Transition Radiation Detector (TRD) can be employed as a mean of identifying particles measuring their γ, or as a threshold detector, to distinguish between particles which emit or not transition radiation. It is for the latter that PAMELA's TRD will be employed.

Since soft X-rays, in the useful range between 2 and 20 KeV, are radiated with about 1% probability per boundary crossing, practical detectors must use radiators with several hundreds interfaces. A classical detector for transition radiation is composed of several similar modules, each consisting of a radiator and an X-ray detector, which is usually a wire chamber operated with a xenon-rich mixture, in order to efficiently absorb the X-rays.

In the case of the TRD for PAMELA it has been chosen a carbon fiber radiator, 60 g/l in density, which has, compared with other materials, a high emitting efficiency and a low absorption, thanks to its high atomic number Z.

As for the detecting devices, a gas detector was chosen rather than a solid state one, to minimize the energy loss of the particles. The detector has a modular design, whose basic component is a cylindrical proportional chamber (straw tube), 4 mm in diameter and 28 cm long, made of 30 μm thin copper-coated kapton foil. Inside, a 25 μm in diameter, tungsten anode wire is stretched to a tension of about 60 g. Each tube is filled with a gas mixture (80% xenon, 20% CO₂) and operates in semi-sealed mode. A 1500 l on-board storage of gas (at STP) will ensure a lifetime of the detector of at least 3 years. Between the anode wire and the cathode straw wall an operating voltage of about 1400 V is maintained.

The tubes are arranged in layers of 16 straws aligned sideways. Two layers, glued in a close-pack configuration, form a module of 32 straws each. These modules are then housed in a special frame to form a sensitive plane. The full TRD is made of 9 of such planes: 5 planes with 4 modules each placed on top of 4 planes with 3 modules each, in order to maximize the acceptance. Hence the total number of straws is 1024. The space between the straws is filled with the radiator layers. A double radiator layer is placed on top of the instrument, for a total of 10 layers.

All these features will ensure, for the TRD of PAMELA, a pion rejection factor of 5% at an electron efficiency of 90%. The TRD will then provide hadron/lepton rejection, helping the calorimeter for particle identification in the energy range not covered by the ToF.

The Department of Physics of the University of Bari and the local INFN section, are responsible for the TRD.