Adriano Di Giovanni

1. 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

2. RPCs are widely used as active elements in largeexperiments (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 m2). Adriano Di Giovanni Example: THE OPERA EXPERIMENT (~3400 m2 of RPCs)

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

4. RPC is composedof two plates with high volume resistivity (1010-1012 cm), that generate a uniform electric field.When a crossing particle ionizes the gas between the electrodes,an avalanche process occurs, eventually developing into astreamer.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.

5. Secondary avalanche/discharge between the electrodes can be generated by the UV photonsproduced during the avalanche process. The gas mixture contains organic components to quench the UV photons. Tipical gas mixture: argon + IsoC4H10 + C2H2F4 + SF6. 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) HV Primary ionization Gas ionization Electron photoextraction Streamer Afterpulse HV UV photons

6. 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. Single streamer afterpulses Multiple streamer (afterpulses)

7. Gas Mixture argon/TFE/IsoC4H10/SF6=48/47/4/1 % argon/TFE/IsoC4H10/SF6=49/47/4/0 % argon/TFE/IsoC4H10/SF6=50/47/2/1 % argon/TFE/IsoC4H10/SF6=79/16/4/1 % • Big charge reduction with a small percentage of SF6. • The streamer size does not depends on the TFE/ IsoC4H10 percentage • TFE≥40 %, IsoC4H10≥4 % to minimize afterpulses. • N.B. High Voltage values are rescaled to superimpose the efficiency curves Full dots: streamer charge Empty dots: total charge

8. RPC electrodes • Electrode properties: • Planarity, smooth surface • =1010-1012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: 109-1011/square Excellent planarity “Air” side: resistivity: ~1012/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.

9. 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

10. 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…

11. 185 nm 160 nm 230 nm 200 nm 210 nm 260 nm NO FILTER Photoemission Vs Wavelength (Float Glass) Signal is visible also above 170 nm (preliminary measurements).

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

13. 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. • =160-260 nm • High stability, absolute calibration • Pulsed (~ 1 Hz) • Electrode material • Surface treatment • RPC Aging (deposits, gas polymerization) • UV gas absorption coefficient