1. Test on installed RPCs with the gas mixture. Preliminary results. Alessandro Paoloni on behalf of the OPERA RPC Group (Bologna, LNF, LNGS, Napoli, Padova, Zagreb) OPERA collaboration meeting Hamburg 3-5 June 2004

2. The first spectrometer in the Hall C Environment parameters: T=16 o C (at the base of the spectrometer) P~900 mbar (few mbar higher wrt external lab) Dry=50-60% (wetter wrt external lab) RPC layers 1  11 RPC layers 12  22 Eastern side Row 1 Row 7 Hall C lateral wall To compare experimental results with data from ext. lab, rescale applied voltages to fixed T 0 =293 K and P 0 =900 mbar: V=V applied *(T/T 0 )*(P 0 /P)

3. Tests with nitrogen For the installed RPCs validation, we kept the detector at 6.5 kV, below the discharge region, but above the operating voltage with the gas mixture: Test the HV system  found that an additional PET layer is needed for the insulation of HV electrodes from the strips Test mechanical stresses by comparing the currents before and after the iron slab installation  in case reduction of the filling material thickness N 2 Test

4. Test with gas mixture We have the permission to use small quantities of the mixture Ar/C 2 H 2 F 4 /i-C 4 H 10 /SF 6 =75/20/4/1 in premixed bottles Test a RPC row (~9 m 2 ) at a time, flushed at 5 refills/day The row is covered by a panel of 32 horizontal strips, served by 2 IC boards Up to now tested the row n.1 of layers n.9,18 and 19

5. Test set-up The strip panel is self-triggered using two Timing boards installed on the InterConnection boards Each timing board is endowed with a double threshold discriminator acting on the analog sum of 16 strips Threshold low = ~20 mV Threshold high = 25 mV (40 mV in the first measurements) 32 ADC channels used for measuring the charge induced in each strip Also counting rates and currents are measured

6. Test of the set-up in Frascati Used a small size old RPC prototype Higher Pressure wrt GS lab  expected operating voltages below 6 kV in Hall C Counting rate Induced charge

7. Reference plots at the external lab RPC BES1451 under test at Ciocco facility for ‘‘ageing’’ studies at cosmic rays rate MACRO FE boards used Effect due to fast increase of the current with high resistivity ? Chosen operating point: 5.5 kV

8. Operating currents in Hall C Layer n.9 Layer n.18Layer n.19 The currents at the operating point are below 350 nA, lower wrt to QC tests at external GS lab Their increase is faster for the eastern RPC (near the lateral wall of Hall C)

9. Counting rates in Hall C Layer n.9 Layer n.19Layer n.18 The counting rates at the operating point are of the order of few Hz/m 2 (to be compared with 200-400 Hz/m 2 at cosmic rays rate) The counting rates are higher for the eastern RPC (near the lateral wall of Hall C) There is not a plateau, but an exponential dependence on the operating voltage

10. Threshold studies The rates have been measured at 5.5 kV with the lower threshold at 20 mV Good higher threshold stability till 60 mV

11. Induced charge The average induced charge at 5.5 kV is about 20 pC

12. Induced charge: layer 9 Entire row Middle + western RPCs

13. Induced charge: layer 18 Western RPC Middle RPC Eastern RPC

14. Conclusions Up to now we have tested 9 installed RPCs with the gas mixture Ar/C 2 H 2 F 4 /i-C 4 H 10 /SF 6 =75/20/4/1, observing: 1.Very low operating currents and counting rates 2.No plateau for the counting rate vs the voltage (reference to external lab. measurements essential for determining the operating point) 3.Current and rate higher for RPCs near the lateral side of Hall C These results are a first indication of the goodness of the RPC installation procedure More statistics needed (first installed rows, rows with filling material substituted)

15. What from QC tests ? BE1074 7.9 ± 0.3 2.8 ± 0.4 2.021 ; 2.099 386 ; II BIM1331 2.5 ± 0.6 0.92 ± 0.01 1.911 ; 1.988 464 ; II BE1136 8.7 ± 0.2 0.45 ± 0.01 1.917 ; 1.973 568 ; OK BES1710 1.7 ± 0.1 0.70 ± 0.01 1.965 ; 2.043 472 ; II BI1653 3.8 ± 0.3 1.34 ± 0.04 1.864 ; 1.903 388 ; II BES1698 5.7 ± 0.2 1.27 ± 0.02 1.950 ; 1.999 377 ; II BES1551 2.7 ± 0.3 4.0 ± 0.3 2.004 ; 2.047 372 ; II BI1220 14.1 ± 0.4 0.90 ± 0.02 2.031 ; 2.070 450 ; OK BES1680 19.7 ± 0.2 0.94 ± 0.01 1.914 ; 1.977 530 ; II Eastern sideEastern side Layer 9 Layer 18 Layer 19 RPC name di/dv (nA/kV)    cm) V 100 ; V 500 (kV) I op (nA); C.R. Q.R. RPC name: Red=high temperature test Green=low temperature test

16. Strip signals (I) Strip Ground 1 M  pick-up

17. Strip signals (II) Adjacent strips 50  pick-up

18. Strip signals (III) Central strip 1 M  pick-up Ground 1 M  pick-up Central strip differential read-out Far strips 50  pick-up

19. Strip signals (IV) Noise Strip 1 M  pick-up Ground 1 M  pick-up Strip with differential read-out

20. Induced charge: layer 9 Total induced charge Reconstructed hit position Entire row Middle + western RPCs UpDown

21. Induced charge: layer 18 UpDown Western RPC Middle RPCEastern RPC

22. P = 1000 mbar P = 900 mbar Lower operating voltages expected at GS because of the lower pressure Frascati test of the set-up 6.25 kV 5.65 kV T = 20 o C

23. Strip panel not covering the entire RPC and picking up signals outside its area (more signals at the edge of the panel) Frascati test of the set-up

24. Found a miscalibration of the nanoamperometers used in hall C (not OPERA standard): the currents are lower by a factor of about 2 Current on the negative electrode from nanoamperometers Current on the positive electrode from power supply Further investigations needed to understand and solve the problem