1 Test on RPC Veto Detector Model —— Anticoincidence Detector for Daya Bay Neutrino Exp. Speaker: Jiawen Zhang 5 June 2006

2 Outline  RPC Introduction  RPC R&D in IHEP Of CAS  RPC Performance Study  RPC Mass Production for BESIII  RPC for Daya bay Neutrino Exp.  Precondition and requirement  Test Design  Test Result  Summary

3 RPC Introduction What is RPC? RPC is composed of two resistive plates with gas flowing between them. High voltage is applied on the plates to produce a strong electric field in the gas. When a cosmic-ray passes through the gas between the two plates, a signal will produce, which is then picked up by the pickup strip and sent to the DAQ system. Advantages:  Simple structure  Cheap to make a larger area detector

4 RPC R&D in IHEP Of CAS The RPCs for the BESIII Muon tracker detector were constructed by using a new type of phenolic rosin laminates developed in IHEP of CAS. The methods of improving surface quality is similar to other bakelite plates, and have been used to construct RPCs elsewhere. Oscilloscope traces of 100 triggered cosmic ray registered in a RPC prototype at 8 kV. The average signal amplitude from a pickup electrode is about 400 mV with a 50 Ω termination. No secondary streamers were recorded

5 Bakelite surface

6 Currently, our bakelite plates are produced with the old molds of the factory, which are old with detects. If large quantity of bakelite plates are needed, new molds can be ordered, therefore the bakelite surface may be more smooth, and the single counting rate may be more decreased. RPC production in the clean room

7 R&D —— prototype performance %

8 R&D ---- long-term stability neutron irradiation other experiments Beam test neutron irradiation Max: 98.8% Min: 95.3% Average: 97.2%

9 R&D ---- Humidity effect During the R&D test, we added water vapor into the gas for about one month. No effects were observed. All the HV connectors to the RPC are covered with insulation glue. Last summer, the humidity was about 80-90% lasting about 2 week in Beijing. We tested the RPC bare chambers and the assembled modules, no problem was found, and we will do more test soon.

10 R&D ---Flammability gas The flammability Iso-butane of the mixture gas maybe catch fire, if its proportion is more than 15%. We used the gas mixture of argon:F134a: Iso-butane = 50:42:8, Therefore the problem won ’ t be avoided with the ventilation requirement for Radon removal.

11 RPC mass production for BESIII Single layer RPC (bare chamber) efficiency ε>95% Barrel 7.5Kv Min. 85.6% Max % Aver.95.39% Barrel 8.0Kv Min % Max. 99.2% Aver.96.4%

12 A new bare chamber single counting rate is below 1000Hz/m 2 ( only training 1-3days), if training for a long time, the single counting rate will be below 500Hz/m 2 Barrel 7.5Kv Min Max Aver Barrel 8.0Kv Min Max Aver RPC mass production for BESIII

13 2 layers of RPCs form a super layer ， ε >98% Mean 0.98 Average eff: 0.99 RPC mass production for BESIII

14 The requirements of Daya bay Neutrino Exp.  The anticoincidence detector is used to decrease the cosmic ray background to improve measurement precision Near SiteFar Site Neutrino rate (/day)56080 Muon Flux (Hz/m 2 )~ Table 1. Neutrino instance and Cosmic ray flux According to the design, we know the radius of central detector is 1.6m. And by calculation, the cosmic ray through the detector is 8Hz near site, and 0.36Hz far site. Taking example of far site, the cosmic ray through 4 modules every day is 4×0.36Hz×3600s×24h= >>80

15 Anticoincidence detector ε~99.9%, 125(>80) cosmic ray won ’ t be removed Anticoincidence detector ε~99.99%, 12(~15%) cosmic ray won ’ t be removed Most of cosmic ray can be removed by spectrum & time relation of later neutron signal So the result is acceptable!

16 The design of detector  Requirement  Higher efficiency  Less noise But these two factors are contradictions, so the key problem is how to balance the two factors. A fact is that a water Cherenkov may be used. (efficiency is ~ 95% [1],noise <0.1Hz. [2] )

17 The design of detector(2)  Outer detector scheme Adopt 2 dimension readout RPC operate in streamer mode The gas mixture used as Ar:C 2 H 2 F 4 :C 4 H 10 =50:42:8 HV:+4000V ， -4000V Single gap, 3 layers in one module Each layer overlapping assembly, no dead space. Each module overlapping assembly too, so between modules no dead space.

18 The design of detector(3)  Efficiency and noise Efficiency of each layer is ε ~95%, and adopt choose 2 out of 3 as a hit, their coincidence efficiency is ε eff =ε 3 +C 3 2 ε 2 (1- ε)= × ×(1-0.95)=99.3% The efficiency of the module with the water Cherenkov is 1-(1- ε eff )(1- ε)=1-(1-0.99)(1-0.95)=99.95% The RPC bare chamber noise rate ~800Hz/ m 2, the shaped signal width is τ=100ns=10 -7 s, so the module noise rate is 3C 3 2 r 2 τ=3 × 3 X(800) 2 ×10 -7 =0.576Hz/m 2 since the module has 3 layers RPC, so we can do the track for itself. By using the track information, the noise can be reduced to <0.05Hz/m 2. If we reduce RPC noise rate to 300Hz/m 2, the noise can be reduce to more lower. In addition, Because the noise of the water Cherenkov is very small, the total noise is not more than 1Hz !

19 Electronics and readout The design of detector(4) the same as BESIII Muon detector. Each FEC can handle 16 channels ， and a total of 16 FEC composed a data chain. All the data chains are connected to the VME readout system. A fast-OR signal from each FEC is sent to the trigger system. The primary bitmap signal are transferred from parallel to serial, hence reduce significantly the cables. The width of the shaped signal is 100ns.

20 Test Result(1) Group 1 (have been used as a telescope sys.) 99.5±0.25%

21 Test Result(2) Group 2(haven ’ t used) 99.3±0.4%

22 Test Result(3) The result analysis The reasons as follows:  Many child cosmic rays produced by high energy cosmic ray traverse at the same time.  Spatial electromagnetic interfere Both two can arose signals at the same time in two RPC layers! So this result contains its contribution! At the HV 8000V, The 2 out of 3 coincidence noise is about 10Hz/m 2 Which is far more than 0.578Hz/m2. Hz/cm 2 V

23 Summary  RPC is economical for a larger area detector  The RPC performance developed by IHEP is excellent, single gap RPC efficiency>95%, noise rate <800Hz/m 2, dark current <2μA/m 2  Adopt 3 layers, 2 dimension readout, RPC and module overlapping assembly, no dead space. Adopt choose 2 out of 3 as a hit, their coincidence efficiency >99%, the noises <0.05Hz/m 2  Electronics and readout are same to BESIII Muon detector.

24 Reference 1.Determination of Neutrino Mixing-Angle θ13 Using the Daya Bay Nuclear Power Facilities ， version Preliminary study of Daya Bay reactor neutrino experiment, Yaxuan Sun, Ph.D thesis The end Thanks!