Gas circulation and cleaning system

Figure 9: Scheme of gas circulation setup. 1,2 - absorbers-compressors; 3,4 - purification blocks; 5 - accumulation vessel; 6 - TPC chamber; 7,7$^\star$  - remotely controled valves; 8 - valves.

The requirement to keep the protium purity on the level $10^{-8}$ during the whole experiment imposes a serious problem. First it means that the outgassing rate inside the TPC volume should be as small as possible. Therefore, the final construction of the TPC will be bakeable and made of low outgassing materials. In addition, a circulating gas loop with purifying modules will be constructed. A schematic diagram of the gas circulation system is shown in Fig. 9. The basic elements of the system are the Zeolite absorbers used for continuous gas pumping. There will be two such absorbers. When one of them is cooled to the LN$_2$ temperature, the gas flows from the TPC volume through the purification block 3 and is accumulated in the absorber. When the absorber is heated, the gas returns back to the TPC through the purification block 4. The consequent operation of the two absorbers (with the intermediate accumulation vessel 5) allows to provide continuous gas circulation at constant gas pressure in the TPC volume. During 2000, a prototype of the circulation system was constructed and tested at PNPI that confirmed the main concept of the new system. A 3 l volume was used as analog of the TPC detector. The hydrogen pressure was 12 atm. In the first experiment a small 80 g Zeolite NaX absorber was used, and the hydrogen was purified in 12 cycles from a level of N$_2\sim$700 ppm to 8 ppm and from a level of O$_2\sim$1.5 ppm to 0.03 ppm. In the second experiment, a larger absorber with 570 g of Zeolite was prepared, and in this case a one step purification reduced the level of N$_2$ from $\sim$3.0 ppm to 0.3 ppm and the level of O$_2$ from 0.2 ppm to 0.09 ppm. The cycle time was about 20 min. These tests demonstrated that the circulation system operating with two 1000 g Zeolite absorbers could provide a complete gas cycling in the TPC (400 liters STP) every 1-2 hours. This is more than enough for the expected outgassing rate. The design of the final circulation system is now underway. The new system will be constructed with materials which previously had no contacts with natural hydrogen. The new system will be equipped with remote control electronics and valves.