View source for Transition radiation tracker (ATLAS)

[[Category:ATLAS]]
[[Category:Detector]]
[[Category:Tracking]]
The ATLAS '''transition radiation tracker''' (TRT) is the outermost [[tracker]] of the [[Inner detector (ATLAS)|inner detector]]. It is embedded in a 2 T solenoidal magnetic field provided by the [[ATLAS central solenoid]]. It consists of around 300000 [https://en.wikipedia.org/wiki/Polyimide polyimide] [[Straw tube|straw tubes]]. Each is 4 mm in diameter, with a thin gold-plated tungsten wire stretched through the middle. The tubes are filled with a mixture of 70% xenon, 27% CO<sub>2</sub>, and 3% O<sub>2</sub>. The signal is created when a charged particle creates ionisation in the gas. Charge carriers are accelerated towards the wire by a 1500 V potential, leading to [[gas amplification]] by an avalanche of secondary ionisation, and eventually to an electrical signal in the wire, which is read out. The space between the tubes is filled with polymer fibres (in the [[barrel]]) or foils (in the [[Endcap|endcaps]]). Highly relativistic particles produce [[transition radiation]] on the many material boundaries this creates. The soft X-ray transition radiation photons, typically of around 10 [[keV]] energy can be absorbed by xenon atoms in the tube gas, amplifying the ionisation signal. As the amount of transition radiation produced depends on the relativistic factor <math>\gamma = E / m</math>, the signal strength ("high-threshold" versus "low-threshold" hit) can be used to help distinguish the light electrons from hadrons, up to a particle energy of around 150 [[GeV]].