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  m 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: Established technologies New RPC frontend permits high rate with the present RPC technology No performance compromise Points of weakness: 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 after- pulse 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: Working voltage/current  efficiency CO2 fraction  suppress after-pulses Front end threshold/dynamic range  negligible e.m. noise Source intensity vs. counting rate 14 strips on 20x20 cm^2 gap build according to ATLAS standard 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

More counting tests Try to separate the limit of the chamber from the limits of our setup 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)

A photon counting test 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 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

Efficiency and charge per count Estimation of average charge per count from differential ratios: 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 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