Infrastructure on top of the tank
The clean room with its integrated lock system will provide the possibility to insert and withdraw detectors in a modular way while the vessel stays at cryogenic temperatures and operating pressure. It will also be used for the final preparation of the detectors and their integration into so called ``strings'', the loadable assemblies of up to five detectors. The design of the clean room is such as to avoid prolonged exposure of the crystals to the (clean room) atmosphere (humidity, aerosols, ²¹°Pb).
Radon is a major concern for the experiment. The interior of the cryostat has to be basically radon free (in total approximately 5000 ²²²Rn nuclei are allowed in the whole cryostat volume at one time). The inner lock is connected to the main gas volume of the cryostat. The radon concentration in the gas volume has to be lower by roughly eight orders of magnitudes compared to the tunnel air. Since the lock cannot be built completely without the use of non-metallic seals (which are subject to diffusion) it might therefore be necessary to suppress radon creeping into the lock through seals. Furthermore radon decay products might attach to aerosol particles that settle on the detector surfaces. Especially ²¹°Pb is a dangerous source on detector surfaces. The clean room will be of class 10000. The actual handling of detectors will be done in laminar flow-boxes which provide a class 100 environment. Assuming a radon level of 1 Bq in the clean room and reasonable particle deposition rates this leads to a contamination of less then 0.1 µBq/h of ²¹°Pb on the surface of the crystal.
The clean room air is filtered approximately 40 times per hour. About 120 m³/h of fresh (preferably radon-reduced) air are added continuously in the filtering process. The temperature will be regulated to 21°C, the relative humidity to about 30 %. The air conditioning and the radon reduction unit will require radon reduced water to regulate the humidity. Particle counters will be located at critical points within the clean room. Oxygen monitoring will be part of the safety system.
The main components of the clean room are:
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The lock system is the central device of the clean room. It encompasses an outer, an inner lock and two cable arms. The lock system is attached to the neck of the cryostat such that the two volumes are connected. A shutter can separate the inner lock from the cryostat. The outer lock can be separated from the inner lock and from the clean room by two rectangular shutters. The outer and inner locks consist of stainless steel cylinders with a diameter of 50 cm and 140 cm, and weights of 0.5 ton and 6 ton, respectively. The cable arms consist of steel cylinders with 630 mm diameter and a length of 3500mm with a weight of 1 ton. A sketch of the lock system is shown in Fig. 1.
The lock system is designed to house up to 13 Phase II and 3 Phase I strings as well as three service strings (calibration source or camera). The array is shown in Fig. 2.
An assembled string (which can either be a detector assembly or a calibration source or a camera) is transferred to the array within the inner lock by a rail system. Each string is attached to a sled, which can slide along the rails. The procedure to move the string from the final assembly station to its final array position within the inner lock can be separated into four movements (see Fig.3):
to the final array rail segment.
1: In the final assembly station a string is hanging on a rail segment. Once the shutter between outer lock and clean room is opened, a rail segment is placed to connect the rail within the outer lock to the rail of the string assembly station. The assembled string is consecutively carefully pushed into the outer lock. The connecting rail segment is removed before the shutter can be closed again.
2: After the shutter between outer and inner lock has been opened, a connecting rail segment is positioned between the rails in the outer and the inner lock. A magnetic transfer arm connected to the outer lock above the outer rectangular shutter is used to push the string to the inner lock.
3: The string is hanging on a rail segment that is attached to a wagon. A steel chain embedded into the circular rail is attached to the wagon. The steel chain and thus the wagon can be moved along the circular rail around the base plate by a crank lever. The wagon is positioned such that the wagon rail segment is connecting to the base plate rail which serves the destination array position.
4: Using a transfer arm the string is pushed from the wagon rail segment along the central rail to its final array rail segment. This rail segment can be lowered to the cryostat by means of the linear pulley system described below.
The array rail segments are attached to two tainless steel string-support wires. The signal- and HV-cables from the base plate to the lock feedthroughs are led between these two string support wires. The area between the wires available for the signal- and HV-cables is 20mm x 10mm.
The string-support wires and cables are installed quasi-permanently. In lifted up position they are housed in the two horizontal cable arms connected to the inner lock. The two string support wires per string and the detector cables run from a strain relief at the inner lock to a movable cable pulley at the outer end of the cable arm. The wires and the cables are led around the cable pulley backwards to the center of the inner lock. Here they go around a stationary cable pulley and are led downwards towards the cryostat. They are attached to the array rail segment that is fixed inside a bore of the base plate. The movable cable pulley inside the cable arm can be sled from the outer end of the cable arm towards the center by 3.5m with a crank lever. By sliding it from the outer to the inner end of the cable arm the array rail segment can thus be lowered from the base plate by 7m to the cryostat volume. A schematic view of the system can be seen in Fig. 4.
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