The cooling system for the silicon detector is located in and should therefore be compatible with the ultra-high vacuum of the accelerator. It is capable of removing up to 100 W of heat from the detector at a temperature of -35..-40 °C. The system consists of two loops. A closed internal loop which cools the detectors and frontend electronics. This loop crosses the vacuum envelope, and is cooled by an open external loop which consists of the HERMES cooling water. The heat pump between the internal loop operating at low temperature, and the external which is just below 10° C, consists of a set of Peltier modules. The cooling power of these modules is controlled by the current passed through them, which gives us the possibility to stabilize the temperature of the detector. The Peltier modules are housed in a secondary vacuum to reduce the contact with the environment on room temperature. A schematic diagram of the system is presented in the figure.
General layout of the cooling system.
Peltier elements consist of two different materials through which an electrical current is passed. In this circuit heat is absorbed or generated at the contacts of the unlike materials, depending on the direction of the electrical current, the Peltier effect. The second effect is present when an electrical current and a heat flow occur simultaneously in a material. Depending on the relative directions a small amount of heat is absorbed (electrical current parallel to heat flow) or generated (antiparallel), the Thomson effect. These two effects are proportional to the current. Finally, the resistance of the materials causes the generation of Joule heat, which is proportional to the square of the current.
The Peltier effect removes heat from one end of the module, which is used to cool the internal loop. The same amount of heat is produced at the other side which is cooled by the cooling water from the HERMES experiment. The amount of heat removed is 11.5 WA-1 for a single module. Part of this cooling is used to compensate the Joule heating, which is divided almost equally between the hot and cold ends of the module, and the heat conducted through the module when a temperature difference between the two sides is maintained. The conductance is about 0.42 WK-1, and the resistance of the module about 2.6 Ohm. In the operating conditions about 25 W of cooling power (at -10° C) per module is available while 160 W must in that case be removed by the cooling water. This requires a flow of about 0.35 m3hr-1 for a temperature rise of 4 K (for a total of ten modules).