THE TARGET AREA OF THE EXTERNAL COSY EXPERIMENTS

For hadron physics experiments the combination of a well defined coole d particle beam and a small target volume provides a well defined inte raction vertex. With a thin target a precise energy definition is obta ined and systematic errors due to Coulomb scattering and secondary int eractions with the target material are reduced. A liquid hydrogen/deut erium target of a few millimeter length has been constructed and opera ted since three years in the external experiments at COSY Julich. A sm all target together with a wide acceptance of the detectors behind req uire the diminution of background reactions. Therefore, very thin targ et windows of 0.9 mu m Mylar foils are realized with the use of a pure ly mechanical pressure stabilization system. With a fast heating syste m one can overcome the problem of background reactions with rest gas c ondensing on the foils. For the precise definition of the incoming bea m, veto systems consisting of two annular plastic scintillators are in stalled before the target in order to detect all particles not passing through the target cell and their reaction products. The improvements of the liquid hydrogen/deuterium target and the design of veto system s are described.