Editing ATLAS experiment (section)

====Transition Radiation Tracker====
The Transition Radiation Tracker (TRT), the outermost component of the inner detector, is a combination of a [[straw tracker]] and a [[transition radiation detector]]. The detecting elements are drift tubes (straws), each four millimetres in diameter and up to 144&nbsp;centimetres long.  The uncertainty of track position measurements (position resolution) is about 200&nbsp;micrometres. This is not as precise as those for the other two detectors, but it was necessary to reduce the cost of covering a larger volume and to have transition radiation detection capability.  Each straw is filled with gas that becomes [[ion]]ized when a charged particle passes through.  The straws are held at about −1,500&nbsp;V, driving the negative ions to a fine wire down the centre of each straw, producing a current pulse (signal) in the wire.  The wires with signals create a pattern of 'hit' straws that allow the path of the particle to be determined.  Between the straws, materials with widely varying [[index of refraction|indices of refraction]] cause ultra-relativistic charged particles to produce [[transition radiation]] and leave much stronger signals in some straws.  [[Xenon]] and [[argon]] gas is used to increase the number of straws with strong signals.  Since the amount of transition radiation is greatest for highly [[special relativity|relativistic]] particles (those with a speed very near the [[speed of light]]), and because particles of a particular energy have a higher speed the lighter they are, particle paths with many very strong signals can be identified as belonging to the lightest charged particles: [[electron]]s and their antiparticles, [[positron]]s. The TRT has about 298,000 straws in total.