DIRAC NOTE 2002 – 09 New Ionization Hodoscope : design and characteristics

Pions, originated from pionium decay, are characterised by low relative momentum and cross upstream detectors at rather small relative distance. These atomic pairs cause double energy release by ionisation in a sensitive volume of the detector with respect to the ionisation loss by a single pion. Therefore, the uncertainties resulting from the inefficiency in detecting two tracks with relative distance approaching to zero can be significantly reduced. The previous dE/dx detector consisted of 2 planes by 16 vertically oriented 2 mm thick scintillator slabs [1]. Therefore it could resolve close tracks only in one projection. It had geometrical inefficiency due to approximately 0.5 mm wide gaps between slabs. New Ionisation Hodoscope consists of 4 planes: two planes with vertically and two planes with horizontally oriented slabs. Adjacent slabs are separated by thinner gaps (about 60 μm). So it allows to distinguish close tracks in both projections with better acceptance. We will demonstrate the feasibility of separation of double ionisation events from single ones with the help of the new dE/dx detector taking into account a system of corrections, which is different from [2].