Dipartimento di Fisica, Università del Salento Studies of Electron Drift Velocity and Charge Spectra in RPC by a UV laser source G. Chiodini, M.R. Coluccia, E. Gorini, F. Grancagnolo, M. Primavera and S. Stella INFN Lecce & Dipartimento di Fisica, Università del Salento Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Summary Goals Experimental Set-Up Experimental Results Conclusions Laser Source Characterization Measurement technique Experimental Results Electron Drift Velocity Charge Spectra Townsend Coefficient Conclusions Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Goals Measurement of electron transport and amplification properties of the gas mixture used for Resistive Plate Chambers (RPCs) in the ATLAS experiment: 94.7% Tetrafluoroethane (C2H2F4) as main component 5% isobutane (C4H10) as photons quencher 0.3% SF6 to inhibit streamers development Two parallel electrode plates ~1010 Wcm plastic laminate 2 mm gas gap filled with gas mixture at atmospheric pressure Plates externally coated with graphite To apply uniformly high voltage Plates internally treated with linseed-oil two readout strip panels strips perpendicular each other Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Experimental Set-Up Two lasers: N2 (ionization) and He-Ne (alignment) Phototube (trigger signal) Metal box housing the RPC prototype Optical devices (lens, beam splitter, optical filters, mirrors) to align and focus the laser beam on RPC Two RPC prototypes RPC I with resistivity of ~1.4 x 1011 Wcm RPC II with resistivity of ~1.71 x 1010 Wcm Dimension 10x20 cm2 2 mm gas gap All the following results are referred mostly to RPC II prototype. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Measurement Technique A N2 (l = 337 nm) pulsed laser is used to induce ionization in the gas gap Laser is focused with a 10 cm focal lens Ionization localized in a small area around the focus (< 20 mm) Ionization cluster of one up to hundreds of electrons depending on the laser intensity z A charge moving in the gas volume under the influence of an electric field induce a signal on readout strips Typical signal waveform corresponding to an avalanche initiated by one electron Single ionization condition is reached by finely reducing the beam intensity by optical filters. From each waveform we get the maximum: avalanche drift time + pulse height. For a fixed electric field, 5 measurements have been performed varying the ionization position along z axis inside the gap. Spanned ionization region was 400 mm with 100 mm step. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Laser Source Characterization Laser beam dimension at the focus lens: ~ 20 mm Pulse duration: ≤ 600 ps Working conditions: To ensure RPC rate capability Laser repetition rate = 1 Hz Tuning laser light intensity by mean of optical filters Single ionization mode z = 1.5 mm HV = 10300 V T = 0.1 z = 1 mm Fraction of events with an avalanche signal for two different optical filters ( Transmittance T=10% and 3.5%) as a function of high voltage. Plateau for large enough voltage values Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Optical Filter Calibration To optimize the laser intensity we have to estimate the number of ionization produced by the laser spot Measured performed at HV=10500 V and RR=1 Hz changing optical filters T. Pm(n) to have ionization per laser pulse is Poisson distributed: m is the average number of ionization ionization probability scale quadratically with light intensity-> m=m0T2 Fit performed with f=1-Pm(0)=1-e-m Average ionization rate of the source m0 = 115 ± 20 e- For T= 0.1 we have one ionization in average. HV = 10500 V z = 1.5 mm Measurements performed with optical filter with 10% transmittance (one ionization in average) and RR = 1 Hz (no electric field reduction). Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Electron Drift Time Distribution HV=10300 V Z = 1500 mm from anode HV=10300 V Z = 1300 mm from anode HV=10700 V Z = 1300 mm from anode Drift velocity measurement is based on the determination of the average electron drift time <t> = T1-T0 where T1 is the time of the pick-up strip signal maximum and T0 is the phototube trigger time. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Data Trend for RPC II Drift time and signal amplitude during a typical run at 9900 V. Values appear stable during all the data taken and uniformly spread around a constant value. This is a further confirmation that no long term systematic effects due to electric field reduction, related to the ionization rate, were present. Not true for RPC I old measurements (see next). HV corrected online by P and T environmental changes. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Drift Velocity Measurements Systematic errors included in quadrature Resulting drift velocity values as a function of the applied electric field. For two high voltage values, measurements repeated after several months (Result II) -> consistency with previous measurements. Comparison with MAGBOLTZ* calculations -> Good agreement between simulation and experimental results. * S. Biagi, “Montecarlo simulation of electron drift and diffusion in counting gas under the influence of electric and magnetic field”, Nucl. Instr. Meth. A421 (1999) 234-240. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

RPC I vs RPC II (a good lesson) Similar set of measurements performed with RPC I prototype Results not in good agreements with the MAGBOLTZ calculations. Two explanations: Large volume resistivity (one order of magnitude bigger than the RPC II case) Low content of humidity in the Bakelite electrodes The time needed to the electrode to get charged again (strongly dependent by Bakelite resistivity) was so long with respect to the laser ionization rate. Even for very low laser RR the electric field get locally reduced. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Charge Spectra Exponential Growth: TOWNSEND AVALANCHE MODE <q> = q0 exp(h z) with h effective Townsend coefficient h = a - b a first Townsend coefficient b attachment coefficient Avalanche Fluctuation. Charge distribution can be: exponential-like or Polya-like The parameter r= lmult/xion distinguishes the 2 cases. r = E /(aUion) In our case r ~ 40: an exponential decreasing spectra is expected. TOWNSEND AVALANCHE MODE Exponential Growth Saturation Constant “Drift” SATURATED AVALANCHE MODE ??? Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Charge Spectra: Single Avalanche vs HV All the following results are referred to RPC II prototype. z = 1.5 mm T = 0.0315 Software threshold = 3 sigma of noise Studying the charge spectra of a single avalanche as a function of the electric field allows to understand if some non linear phenomena take place. The single avalanche spectrum changes gradually from an exponential decreasing shape ( 9900 V), to a Polya distribution (10100 V) and, finally, to a distribution presenting a “ flat region” (10300 V). Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Charge Spectra: Single Avalanche vs path The single avalanche charge spectrum changes also moving inside the gas gap . The single avalanche spectrum changes gradually from an exponential decreasing shape ( x=0.9 mm), to a Polya distribution (x=1.1 mm) and, finally, to a distribution with a “flat region” (x=1.3 mm) . In fact, the spectra obtained in high ionization and multiplication condition, after reducing the drift space (-> decreasing the total charge) shows that the “flat-region” spectrum shape following gradually an exponential decreasing shape. HV = 10700 V T = 0.0315 Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Charge Spectra: Several Avalanches vs CS Charge spectra measured by changing the optical filter in such a way to compare multiple avalanches spectra with single electron avalanche spectra. Increasing the optical filter transmittance the spectrum apparently changes when more avalanche overlap. The charge spectrum of avalanches generated by several electrons presents an evident and broad maximum. The average ionization is 7 with an RPC efficiency near to 1. HV = 10300 V z = 1.5 mm Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Charge Spectra: Summary High statistics study to see spectra details The distortion of the charge spectra is induced by space charge effects (electric field generated by the avalanche is comparable with the external electric field). Similar distortions are exhibited by the total avalanche development by increasing: Electric field Multiplication path Number of initiated avalanche T = 0.1 @ z = 1.5 mm Flat ® secondary phenomena Polya ® saturation Exp ® saturation Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Effective Townsend Coefficient: Method I Effective Townsend Coefficient: h = a – b . Difference between the first Townsend and the attachment coefficient. Method I: can be rigorously applied only in a proportional regime. This is connected with the exponentially increase of the mean of the pulse height distribution as a function of the distance from the anode <V>=Vth+V0ehz Fitting with this curves we get h for each high voltage value. Method II gas gap center Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Effective Townsend Coefficient :Method II Method II: consisted into fit the pulse height distributions where avalanche saturation doesn’t occur. This correspond to use only distribution “EXP”-like. For each high voltage 2 z points -> 2 h values to compare. HV=10700 z=1 mm a) b) c) Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Effective Townsend Coefficient : Results Comparison of experimental results with the MAGBOLTZ calculation: Results obtained with Method I seems to be not sensitive to the electric field. Values about half the one evaluated by MAGBOLTZ. Of course we are in presence of saturation effect related to the space charge. The coefficient h extracted with Method II is consistent with the value calculated by MAGBOLTZ. Systematic errors non included yet Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno

Conclusions An extensive study of electron transport and amplification properties in the gas mixture filling RPC ATLAS-like have been performed as a function of the electric field in saturated avalanche mode. Small size ATLAS-like RPC prototype has been used. Results of drift velocity and gas amplification, for different value of the electric field have been obtained. Experimental Set-up have been designed to perform these measurements together with DAQ and DCS systems. Drift velocity experimental results have been compared to the values calculated by the program MAGBOLTZ and satisfying agreement was found. The evaluation of gas amplification was found to be complicated by space charge effects typical of the RPC operating regime of saturated avalanche Charge spectra distribution follow an exponential decreasing curve for electron avalanche charge less than ~ 106 e- For large amount of charge start to saturate assuming a Polya-like curve shape Increase of the avalanche charge distorts the spectra that exhibit a flat region The effective Townsend coefficient evaluated at distances small enough from the anode for which an exponential behavior of the charge spectra was still reliable, agree with the calculation from MAGBOLTZ. Feb 14, 2008 G. Chiodini et al. - RPC2007 Fine III Anno