570 Nuclear Instruments and Methods in Physics Research A252 (1986) 570-572 North-Holland, Amsterdam DEVELOPMENTS FOR DOUBLE-WIRE DRIFT CHAMBERS USED FOR AN IN-BEAM EXPERIMENT AT LEAR W. DUTTY, J. FRANZ, N. HAMANN, R. PESCHINA, E. RC)SSLE, H. SCHLEDERMANN, H. SCHMITT and H.G. ULMER Fakultiit fiir Physik der Universitiit, D-7800 Freiburg, FRG The signal wires in double-wire drift chambers need to be mechanically linked together. We describe the construction and test of links, which decrease the overall chamber efficiency by only 10 3. Where the primary particle beam passes through the drift chambers, certain regions must be desensitized. The construction and test of these dead spots is described. The detection of particles in the desensitized regions is suppressed to the level 2 × 10-3, while the neighbouring signal wire remains at least 70% efficient. 1. Introduction With the CERN experiment PS185 at LEAR we study [1] the reactions ~p ~AA, A,~° + c.c., and ~,,~ in the threshold region (~ momenta up to 2 GeV/c). The experimental setup is shown in fig. 1. The antipro- ton beam from LEAR passes through a thin scintillator ($1) and is incident on a target system which allows for $1 521, , . S2s b am i mr ,, ,,~1 T1 T2 T3 S31 $3s Icm 1 I Target region ~" beam.~- " 1/2~- t I i 0 10 50cm ! 3 I I I I k~ ~÷ I 5 D. Fig. 1. Track-imaging forward detector of PS185. A ~p ~ .~A -,~-+ p~r- event is indicated. 1= target system (see mag- nified view), 2 = MWPC stack, 3 = drift chamber stack, 4 = scintillator hodoscope, and 5 = magnetic solenoid with drift chambers inside. the selection of neutral final states. It consits of five 2.5 mm thick CH 2 cells (T), which are sandwiched between scintillator sheets ($3) and surrounded by cylindrical veto counters ($2). In the case of A A pairs the pro- duced hyperons travel - according to their decay length - a mean distance of 3 to 10 cm into the forward stack of chamber planes which closely follows the target. 41% of these pairs are subject to the hadronic decay mode AA ~ ~r+plr -. The four charged particles leave tracks in a 10 plane multiwire proportional chamber stack (20 x 20 cm 2) and a 13 plane drift chamber stack (56 x 60 cm 2). Most of the momentum is carried forward by the decay baryons, which are detected in a scintillator trigger hodoscope and finally deflected in a 0.1 T mag- netic solenoid with three drift chambers inside (up to 116 x 116 cm 2) for charge identification. The analysis of hyperon-antihyperon production events requires a precise track image of the measured particles. In particular, at incident ~ momenta close to the kinematic threshold all outgoing baryons are con- fined to a narrow forward cone. In order to resolve the left-right ambiguity in the drift chambers, and to achieve an overall high spatial resolution, we use doub- lets of sense wires, which have to be mechanically linked together because of the electrostatic repulsion. A technique developed for this, which leads to only small local inefficiencies, is described in the next section. The last section deals with a solution for desensitizing those regions in the chambers, where the primary antiproton beam from LEAR passes through the experiment (in- tensity of the order 106 p's per second, spot size < 1 mm 2 at the target). 2. Construction of links between sense wires The basic geometry of our graded-field drift cham- bers is shown in fig. 2. The distance between the sense 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)