Adriano Di Giovanni

Study of UV absorption and photoelectron emission in RPC (Resistive Plate Counters) detector with

an UV source

Carlo Gustavino (INFN-LNGS)

RPC and their applicationsRPC working principleAfterpulsesGas mixtureUV photoelectron emission from the cathode plateExperimental set-upPreliminary testsConclusions

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 m2).

Example: THE OPERA EXPERIMENT (~3400 m2 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 (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 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 + 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)

Primary ionization

Gas ionization

HV

HV

Electron photoextractionStreamer

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 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

argon/TFE/IsoC4H10/SF6=48/47/4/1 %




Full dots: streamer chargeEmpty dots: total charge

RPC electrodes

Electrode properties:•Planarity, smooth surface=1010-1012 cm•High surface resistivity•Low electron photoextraction efficiencyN.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” planarityTin/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

MirrorFiber

HVOscilloscope

Trigger

RC Filter

Vacuum chamber

Compound (cathode)

Anode

Xenon Lamp

Signal

Optical Filter

Preliminary measurements: Experimental set-up

Very preliminary measurements have been done to measure the photoextraction of a float glass sampleFilters 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 FILTER260 nm230 nm210 nm200 nm185 nm160 nm

Photoemission Vs Wavelength (Float Glass)

Signal is visible also above 170 nm (preliminary measurements).

Photoextraction efficiency Vs Wavelength

1

10

100

1000

GaAs Bakelite Oiled Bakelite Float Glass

Cathode material

Amplitude (mV)

GaAs

Bakelite

Oiled Bakelite

Float Glass

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.

=160-260 nmHigh stability, absolute calibrationPulsed (~ 1 Hz)

• Electrode material• Surface treatment• RPC Aging (deposits, gas polymerization)• UV gas absorption coefficient• • •

Adriano Di Giovanni

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