Preliminary results on optimization of gas flow rate for RPCs Kalmani Suresh TIFR, Mumbai, INDIA

Requirement of gases in ICAL Total number of RPCs in ICAL = 3 (modules) 150(layers)  64(RPC’s) = 28,800 Total gas volume = 28,800  184cm  184cm  0.2cm = 195,010 litres For example: One volume change/day with 10% gas top-up in a re-circulating scheme will be expensive. Gas Avalanche (%) Streamer Maximum Volume (L) Density (g/L) Weight (Kg) R134a 95.5 62.0 186,234.6 4.25 791.5 Isobutane 4.3 8.0 15,600.8 2.51 39.16 SF6 0.2 0.0 390 6.164 2.40

Motivation Recycling of gas mixture for reduction of gas cost and complying with environmental regulations. Further reduction in the operating cost of ICAL by optimizing the flow of gas, without compromising on the detector efficiency and performance in the long run. Study contamination and formation of radicals in the gas mixture flowing through the RPCs.

The First study One 1m×1m RPC (called AB13) was sealed on February 5, 2010. Its operating parameters such as strip rate and current were monitored 24×7, along with laboratory temperature and relative humidity. After 130 days of uninterrupted operation, the gas inside RPC was analyzed using a Residual Gas Analyzer (RGA)

Schematics of gas flow and sealing

Lab temperature and Noise rate

Lab temperature and Current

RGA spectra of sealed Gas

Summary of RGA RGA spectra contains nearly 50% air and 3% water vapour. RPC AB13 was sealed for 130 days. Amount of air accumulated ~1000ml. Therefore, the leak rate ~8ml/day. Removing air and moisture, major fragments are of R134a Isobutane fraction has gone down drastically Study is extend to the prototype stack to verify the consistency of the results by isolating 9 layers out of the 12 layers in the stack. The remaining 3 layers were used as a reference.

Sealed gas test for C217 stack Stack of 12 1m×1m RPCs L0, L4 and L11 were used as reference Other RPCs sealed on April 27, 2010 Summary of the study

Strip Rates Day of sealing Flow restarted after 32 days

Tracking Efficiency Day of sealing Flow restarted

Current Reference Day of sealing Flow restarted after 32 days

Summary: Current

Leak rate of all RPCs

Leak Rate

Current, Strip Rate and Leak Rate

Current, Strip Rate and Leak Rate

Implementation: Existing system A capillary (300 micron and 5 meters long wound on pulley), which gives a flow of about 1 volume change in about 10 days is connected to 4 RPC where the gas flow is in open loop. Current, Temperature, Humidity and strip rate are being monitored to study the performance. – functioning normal Finally a Close Loop System is being fabricated to study in detail the necessary flow rate for the final ICAL.

Summary Variation in Noise rate and Dark current attributed to different leak rates If proper sealing, as part of QC is done, then the detectors can operate satisfactorily for more than a month with single gas fill and hence reduce the cost of replenishing gas!