The ionisation hodoscope of the DIRAC experiment

The ionisation hodoscope (IH) of the DIRAC (PS 212) experiment at CERN is optimised to separate ionisation signals produced by otherwise unresolved double tracks from ionisation signals produced by a single particle. While its total thickness is only 1.5% of a radiation length, it consists of four planes and provides two measurements of ionisation loss in each of the two projections. It suppresses the background from single-track events by a factor 50 while keeping more than 80% of double-track events. The IH design and performance during data taking in 2001–2003 are described.