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

2. 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

3. 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.

4. The First study
One 1m×1m RPC (called AB13) was sealed on February 5, 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)

5. Schematics of gas flow and sealing

6. Lab temperature and Noise rate

7. Lab temperature and Current

8. RGA spectra of sealed Gas

9. 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.

10. 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

11. Strip Rates Day of sealing Flow restarted after 32 days

12. Tracking Efficiency
Day of sealing
Flow restarted

13. Current
Reference
Day of sealing
Flow restarted after 32 days

14. Summary: Current

15. Leak rate of all RPCs

16. Leak Rate

17. Current, Strip Rate and Leak Rate

18. Current, Strip Rate and Leak Rate

19. 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.

20. 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!