RPCs with Ar-CO2
mix
G. Aielli; R.Cardarelli; A. Zerbini
For the ATLAS ROMA2 group
RPC prototypes for ATLAS upgrade



Need for CSC replacement to
work at LHC nominal luminosity
Demanding detector:
 10 kHz/cm^2 counting rate
 < 5 ns time resolution
 < 100 mm space resolution in
eta
 Second coordinate available
 Trigger capability desired
3 proposal:
 TGC with fine segmentation
 Hybrid Small tubes + RPC
 Micromegas
CSC replacement
The Hybrid MDT RPC
solution

Points of strength:




Points of weakness:




Established
technologies
New RPC frontend
permits high rate with
the present RPC
technology
No performance
compromise
Complex integration
Two separated gas
systems
Lack of space
Combined test at GIF
see Roberto presentation
Use of the Ar – CO2 mix for
the RPCs



MDT and CSC detector run with Ar-CO2 mix, respectively with 7% and
20% of CO2
The ability of RPCs of running with either of the mix would represent a
huge advance in general and in particular tor the ATLAS upgrade
hybrid solution
This type of mixture was always considered as critical for several good
reasons:





Absence of electronegative component  quick increase of discharge
size with electric fields
Photon quenching by CO2 insufficient at high fields and high afterpulse probability
Low electronic density target  smaller primary ionization
A possible working point would than be located at very low field
intensity  extremely small signals  need for a very performing front
end electronics
A first attempt is presently running at GIF parasiting the MDT setup
gas. A second run is foreseen in January with 20% of CO2.
Ar-CO2 first results
The test is very preliminary and was aimed to find out if a comfortable
working region exist in the parameter space given by:





14 strips on 20x20 cm^2 gap build according to ATLAS standard
0.8
Counting rate and currents for pure
argon and Ar-CO2 (7%)



The current slope reflects the
electrode resistance (rho ~ 10^11
ohm cm) and is not much related
with the presence of photons.
The counting rate is much lower
than expected and shows a strange
decreasing trend after a given
maximum. The OR logic can be
biased by long return to 0 due to
the high charge tail of the sample
Here Vth=45 and V=4.5
counting rate - Argon
12
counting rate - Argon-CO2
0.7
Current - Argon
10
Current - Argon-CO2
0.6
Counting rate (kHz/cm^2)

Working voltage/current  efficiency
CO2 fraction  suppress after-pulses
Front end threshold/dynamic range  negligible e.m. noise
Source intensity vs. counting rate
8
0.5
0.4
6
0.3
4
0.2
2
0.1
0
2000
2500
3000
3500
Applied Voltage
4000
4500
0
5000
Current (mA)

More counting tests
Single strip to total OR counting
ratio used to estimate the
multiplicity vs. applied voltage.
Source filter factor 50



The multiplicity grows slowly to
about 2. At lower voltages larger
discharges are prevalent
The counting rate is biased by
after-pulses above 3700V. The
estimated working point is around
3500V
This is consistent with a rate of
10kHz/cm^2 with filter=1 (effective
factor of 30 more photons)
3
250
2.5
200
2
150
Multiplicity
1.5
Total OR
100
1
50
0.5
0
2700
0
3200
3700
4200
Total OR (kHz)

Try to separate the limit of the chamber from the
limits of our setup
average multiplicity

A photon counting test
3300 V
3200 V
3100 V
3400 V
3500 V
3600 V
3600 V - Vth 66 mV LV=6V; 5 missing strips
1.4



The counts grow
“linearly” with the incident
photons up to a filter
factor between 20 and 10
Above factor 10 the
counting does not reflect
the incident photons (to
be understood)
There is some limited
improvement with applied
voltage
The flattening does not
depend on the current
neither on the counting
rate
1.2
1
Rate (kHz/cm^2)

0.8
0.6
0.4
0.2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
inverse of filter factor


All but the topmost curve are done
with lower amplification: LV=4.5 V
Vth=66 mV. This last is done with
higher amplification, removing 5
strips with excessive noise
The linear part improves but
qualitatively the curve is the same
1
Efficiency and charge per
count
Efficiency * acceptance (7 strips removed)
10
0.14
efficiency * acceptance
Average charge per count (pC)
0.12
1
Vth=66 mV
LV=6 V
0.1
0.08
Filter=1
Filter=0
0.06
0.04
0.02
Vth=15 mV
LV=3.5 V
0.1
3000
3100
3200
3300
3400
3500
3600
3700
0
2500
Applied voltage




Estimation of average charge per count
from differential ratios:
 I ( ) 


 N ( )  V V0
Calculated at constant field and
variable photon fluence
The yellow point is taken by lowering
the threshold as much as possible
Full curve to be taken again with
precise current measurement
2700
2900
3100
3300
3500
3700
3900
Applied voltage




Measurement of efficiency *
acceptance with Vth=15 mV LV=3.5V
5fC on the prompt signal.
The current difference corresponds to
about 0.2 mA at the end of plateau
7 strip excluded to reach this operating
Vth apply a geometrical factor 2
The working conditions are 960mBar
and 14 °C
Conclusions





First test of RPCs working in Ar-CO2 (93%-7%) mix
in proportional mode was performed at CERN GIF
High photon counting rate to be further investigated
Efficiency plateau and clean working point detected
 3600V (limit of after-pulses). Corresponding
charge per count estimate  0.2 pC
For a more comfortable working point a 20% CO2
mix will be tested in January with a refurbished
chamber
A combined test with MDTs is also planned to study
in details the detector performances under high
irradiation