DIRAC: A High Resolution Spectrometer for Pionium Detection

Abstract The DIRAC spectrometer has been commissioned at CERN with the aim of detecting π + π − atoms produced by a 24 GeV /c high intensity proton beam in thin foil targets. A challenging apparatus is required to cope with the high interaction rates involved, the triggering of pion pairs with very low relative momentum and the measurement of the latter with resolution around 0.6 MeV /c . The general characteristics of the apparatus are explained and each part is described in some detail. The main features of the trigger system, data-acquisition, monitoring and set-up performances are also given.