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

The first spectrometer in the Hall C

Environment parameters:• T=16 oC (at the base of the spectrometer)• P~900 mbar (few mbar higher wrt external lab)• Dry=50-60% (wetter wrt external lab)

RPC layers1 11

RPC layers12 22

Easternside

Row 1

Row 7

Hall C lateral wall

To compare experimental results with data from ext. lab, rescale applied voltages to fixed T0=293 K and P0=900 mbar:

V=Vapplied*(T/T0)*(P0/P)

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

N2 Test

Test with gas mixture

• We have the permission to use small quantities of the mixture Ar/C2H2F4/i-C4H10/SF6=75/20/4/1 in premixed bottles

•Test a RPC row (~9 m2) 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

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

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

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

Operating currents in Hall CLayer n.9

Layer n.18 Layer 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)

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/m2 (to be compared with 200-400 Hz/m2 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

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

Induced charge

The average induced charge at 5.5 kV is about 20 pC

Induced charge: layer 9

Entire row Middle + western RPCs

Induced charge: layer 18

Western RPC Middle RPC Eastern RPC

Conclusions

Up to now we have tested 9 installed RPCs with the gas mixture Ar/C2H2F4/i-C4H10/SF6=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)

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 side

Layer 9

Layer 18

Layer 19

RPC name di/dv (nA/kV)cm)V100 ; V500 (kV)Iop(nA); C.R. Q.R.

RPC name:Red=high temperature testGreen=low temperature test

Strip signals (I)Strip

Ground

1 M pick-up

Strip signals (II)

Adjacent strips

50 pick-up

Strip signals (III)Central strip1 M pick-up

Ground1 M pick-up

Central strip differential read-out

Far strips50 pick-up

Strip signals (IV)

Noise

Strip1 Mpick-up

Ground1 Mpick-up

Strip with differential read-out

Induced charge: layer 9

Total induced charge

Reconstructed hit position

Entire rowMiddle + western RPCs

Up Down

Induced charge: layer 18

UpDown

Western RPC Middle RPC Eastern RPC

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 oC

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

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