Study of UV absorption and photoelectron emission in RPC (Resistive Plate Counters) detector with an UV source Carlo Gustavino (INFN-LNGS) RPC and their applications RPC working principle Afterpulses Gas mixture UV photoelectron emission from the cathode plate Experimental set-up Preliminary tests Conclusions

Adriano Di Giovanni RPCs are widely used as active elements in large experiments (CMS, ATLAS, BABAR, BELLE, ARGO, OPERA, etc) for their good resolution in both timing (~1 ns) and position (~1 cm) measurements over large areas (thousands of m 2 ). Example: THE OPERA EXPERIMENT (~3400 m 2 of RPCs)

3 RPC/row 7 rows Bakelite RPC (spectrometers)Float Glass RPC (Veto System) RPC sub-detectors under construction

RPC is composed of two plates with high volume resistivity (  cm), that generate a uniform electric field. When a crossing particle ionizes the gas between the electrodes, an avalanche process occurs, eventually developing into a streamer. The streamer discharges only a limited area, because of the high-volume resistivity of the electrodes. The time to recover the electric field is proportional to the electrode resistivity and must be long enough to allow the full ion recombination, to prevent self-sustaining discharges.

Secondary avalanche/discharge between the electrodes can be generated by the UV photons produced during the avalanche process. The gas mixture contains organic components to quench the UV photons. Tipical gas mixture: argon + IsoC 4 H 10 + C 2 H 2 F 4 + SF 6. The isobuthane is added to the noble gas to limit the amount of secondary streamers by absorbing UV photons which are radiated from the de- excitation of molecules. Freons have high electron affinity and are used to reduce the streamer size by capturing the outer electrons. The smaller streamer limits the number of UV photons produced. Probably, Freons are also effective in capturing UV photons. Electrodes (float glass or bakelite coated with linseed oil) Primary ionization Gas ionization HV Electron photoextraction Streamer Afterpulse UV photons

Single streamer afterpulses Multiple streamer (afterpulses) Best performance is obtained by minimizing the afterpulses. This is achieved by using a proper gas mixture. The RPC used in the experiments have elecrodes of Float Glass electrodes or bakelite.

Gas Mixture Big charge reduction with a small percentage of SF 6. The streamer size does not depends on the TFE/ IsoC 4 H 10 percentage TFE≥40 %, IsoC 4 H 10 ≥4 % to minimize afterpulses. N.B. High Voltage values are rescaled to superimpose the efficiency curves argon/TFE/IsoC 4 H 10 /SF 6 =48/47/4/1 % argon/TFE/IsoC 4 H 10 /SF 6 =49/47/4/0 % argon/TFE/IsoC 4 H 10 /SF 6 =50/47/2/1 % argon/TFE/IsoC 4 H 10 /SF 6 =79/16/4/1 % Full dots: streamer charge Empty dots: total charge

RPC electrodes Electrode properties: Planarity, smooth surface  =  cm High surface resistivity Low electron photoextraction efficiency N.B. RPC operation (ageing) can change the initial values. Float Glass: It is produced by putting the glass on a Tin bath. The glass float on Tin bath to obtain a perfect planarity. Therefore, the two sides of the glass are not equivalent. “Tin” side: surface resistivity:  /square Excellent planarity “Air” side: resistivity: ~10 12  /square. “good” planarity Tin/Air surfaces photoemission properties? In the RPC, the surface facing the gas is the “air” one. Bakelite: without linseed oil treatment the bakelite RPC is very noisy. It is commonly believed that the treatment improve the bakelite smoothness. Maybe the oil is effective because it prevents UV photoemission.

Electron photoemission The The absorption band of Buthane is limited to about 170 nm. Beyond this value UV are able to produce electrons from the cathode, that can generated a discharge. Above ~170 nm no buthane absorbtion

Preliminary measurements: Experimental set-up Very preliminary measurements have been done to measure the photoextraction of a float glass sample Filters band pass: from 160 to 260 nm (FWHM=10±4 nm) Poor Xenon lamp stability, unknown transmission (GaAs cathode as reference) Murphy’s law: Vacuum pump broken just after the preliminary tests…

NO FILTER 260 nm 230 nm 210 nm200 nm 185 nm 160 nm Photoemission Vs Wavelength (Float Glass) Signal is visible also above 170 nm (preliminary measurements).

Photoemission Vs sample material N.B. Preliminary! Strongly affected by the Xenon lamp instability

Conclusion and outlook In spite of their large use in High Emergy Physics, the RPC still need investigations. A better understanding of afterpulsing is needed to address the RPC thecnology and to improve its long term stability. A dedicated study of electrode properties have been started by using UV sources. = nm High stability, absolute calibration Pulsed (~ 1 Hz) Electrode material Surface treatment RPC Aging (deposits, gas polymerization) UV gas absorption coefficient