We would have preferred to use a new gas mixture with faster drift velocity to reduce the maximum drift time in each cell. Several candidates have been studied, but none has been found that is better than the Belle gas mixture of 50% helium – 50% ethane, so we will use this. Its performance has been established in the Belle CDC: low radiation length, good position resolution, good energy loss resolution, low cross section for synchrotron radiation X-rays, and little radiation damage.
The gas distribution system will be similar to that used in Belle. Separated pure-gas bottles are located in a gas stock room. Gas mixing is performed using two mass flow controllers in a room on the ground level. The mixed gas is fed into the detector through a gas system that is located on the roof of the electronics hut. The gas system consists of a circulation pump, flow controller, pressure controller, and oxygen filter (Fig. 6.6). A relatively modest flow rate (2 ℓ/min, which corresponds to one volume every two days) is necessary to remove oxygen from the CDC gas volume. Fresh gas is fed in at only 1/10 of the circulation flow rate to reduce cost. An oil free metal bellow pump maintains the gas circulation; the existing pump has been used for more than ten years without any problem. A small amount of hydrogen gas is mixed into the oxygen filter as a platinum catalyst.
