New Technology of Making Resistive Plate Counters Without Linseed Oil Treatment

1. New Technology of Making Resistive Plate Counters Without Linseed Oil Treatment Jiawen ZHANG 8/18/2004

2. Introduction • Resistive Plate Chambers (RPCs) have excellent time resolution. Currently, more large experiments are building RPC based detector systems. • The surface quality is the most parameter of bakelite plates, it affects the performance of RPC. In order to threat the inner surfaces, one method is treated with linseed oil, another is use glass as resistive plate material. • Based on the operational experiences of the Babar RPC systems and the problems associated with linseed oil coating, we decided to investigate new approaches for constructing RPCs when we started to consider the design of the muon identifier for the BESIII.

3. New Resistive Plate • We have made some key advances since we started this project two years ago.The most important innovation is a method to improve the surface smoothness(~200nm) and control resistivity(109 -1014Ω·cm) when the resistive plates are produced in the factory. • The surface of our resistive plates is covered by a layer of specially formulated plastic film. The prefabricated film laminated onto the surface of the phenolic paper plate during the high pressure lamination process can reduce the surface defects caused by the steel plates used in the lamination process. The thickness of the film is 50 m and the resistivity of the resistive plates can be adjusted to optimize the performance of RPCs.

4. RPCprototypes • The resistive plates produced by a local lamination plant were assembled into RPCs. The bulk resistivity of the resistive plates used for constructing this prototype RPC was about 91011Ω·cm measured at 20 oC. The thickness of the two resistive plates was 2 mm and the gap size was also 2 mm.Spacers and edge frames were glued between the two electrode surfaces.

5. Test system • we use 3 scintillator detectors as RPC trigger signal.A computer controls the system, therefore the tests are completely automatic. Electronic system is partially NIM, while CAMAC and personal computer control system are used for the data collection and the high voltage control.

6. Performance of prototypes • The test results described in this section lasted several days from the end of May to the beginning of June 2004. The chamber gas mixture was an Ar/C2F4H2/C4H10, The temperature of our lab was 20 oC to 28 oC, the RPC efficiency is not significantly affected by the temperature. But changes in lab temperature can cause some minor irregularities in our dark current and singles rate data. the signals amplitudes increased almost linearly with the high voltage from about 200 mV at 7 kV to about 840 mV at 9.4k V. • For RPCs working in a low counting rate environment, the efficiency level on the efficiency plateau, the length of the efficiency plateau, the dark current and the noise rate are the most important parameters to judge their performance.

8. Long term behavior • The prototype RPCs were monitored from June 2003 to July 2004, and were placed near a target in the electron test beam at IHEP from October 2003 to March 2004 and were exposed to scattered electrons. Dark currents and counting rates were monitored during this radiation test. The efficiencies of the RPC prototype reached 96 to 98% at 8 kV from the beginning of the test and remain high thereafter. The dark current was less than 10 μA/m2 at the start of the test and dropped to less than 1 μA/m2 in about 4 weeks, the singles counting rates at 100 mV threshold reduced from approximately 0.2 Hz/cm2 to 0.05 - 0.06 Hz/cm2 after the initial training period.

9. The dark currents and counting rates of the prototype RPCs increased during the neutron radiation test and recovered to the level before the neutron radiation in about 10 days after the radiation exposure.

10. Single Counting rate (Hz/cm^2) Before Rad After Rad Efficiency Before Radiation After Radiation High Voltage(V) High Voltage(V) Can be seen in this Fig. the behavior of the prototype RPC was not significantly changed after the electron radiation exposure.

11. Product 400m2 RPCs have been manufactured. The throughput is 10m2 per day. The eligibility rate of products is more than 95%. Before training The efficiency plateaus started from approximately 7.0 kV, When HV= 7.5kV more than 90% RPCs’ efficiencies are more than 95% More than 92% RPCs’counting rate are less than 0.6Hz/cm2 . More than 80% RPCs’ dark current are less than 25μA/m2.

12. Counting Rate at 7.5kV (Hz/cm2) Dark current at 7.5kV (μA/m2) Efficiency at 7.5kV Statistic result of the RPC without training

13. Conclusion • The prototype RPCs for the BESIII spectrometer manufactured by using a new type of resistive electrodes that we developed showed very promising performance without the conventional linseed oil treatment. The BESIII RPC prototypes had high efficiency, long plateau, low singles counting rate and dark current. • After a fairly long training period, the dark current dropped to the level of less than 1 A/ m2 with our operating gas in the streamer mode and the singles counting rate reached the level below 0.1 Hz/cm2. The performance of the RPC prototypes has been quite stable over a period of one year.

14. Thanks!