Development of the target at the external COSY experiment

The Liquid Hydrogen/Deuterium Target The design and operational characteristics of the Julich LH2/D2 target is described in detail in reference. Meanwhile it has been successfully used In the external experiments GEM, MOMO and TOF. Further improvements have been made especially in background reduction and simpler operation. The copper head, connected to a cold head (Leybold RD 210), can be cooled down to 14 K. The thin walled target tube is made from less then 0.4 g copper by a galvanisation technique. The cell windows for the beam entrance and exit are closed with Mylar foils glued by epoxy resin. Their thickness could by reduced to 0.9μ and the amount of glue is less than 6 mg. When the target is cooled down to 15 K the gas is liquefied in the copper head and the thin tube is filled with liquid target material. The tube is divided in an upper and a lower part by a 15 μm aluminium thin wall, A strong convection current is obtained that ensures perfect heat transfer between target and cooling regions and makes the target cell bubble free. The diameter of the target region is 6 mm, its length varies from 2 to 4 mm. Pressure stabilization system For the use of very thin target windows it is necessary to have a constant and stable low pressure difference between inside and outside of the target. This is obtained by using a purely mechanical stabilization system. The gas reservoir is a very flexible metal bellow with a weight on top installed in the same vacuum as the target cell. A narrow gas pipe connects the reservoir with the cooling section of the target appendix, establishing a closed volume with a fixed amount of target material. Since the total pressure in this system is the sum of the pressures in the vacuum chamber and the pressure resulting from the weight, the pressure difference stays constant under all working conditions. The minimum possible pressure difference is given by triple point pressure of the used target material. The reduction of the window thickness is limit ted by the mechanical force on the foils because of this pressure difference. In the first version the weight was chosen such that a fixed working pressure of 0.2 bar was obtained. This allows the liquefaction of several gases (e.g. H2, D2, N2, O2, He, and CH4) without modification of the setup