The effect of surface roughness on RPC performance

The effect of surface roughness on RPC performance Tapasi Ghosh Variable Energy Cyclotron centre Kolkata, India 2/11/2010 Tapasi Ghosh RPC 2010, GSI 1

Motivations : Non-uniformity on the surface profile of electrodes in RPC create fluctuations in the field and therefore on avalanche. This perturbation in field has bigger influence on the intrinsic timing resolution and efficiency. Present study develops a MC method to simulate the physics processes inside the detector and effect of surface roughness on its’ response. Finally simulated results compared with analytically obtained values. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 2

Case study: Bakelite based RPC in India : 2/11/2010 Tapasi Ghosh RPC 2010, GSI 3

Surface profile for the case study: Ref : S. Biswas et al. NIMA 602 (2009) 749. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 4

Distribution of roughness over whole detector area: The roughness has a Gaussian distribution. For current simulation, a Gaussian distribution is assumed for roughness. Sigma of ∆d/d = 1 -10 % are generated. (localized roughness profile is not implemented ) Efficiency improves after application of silicone: Surface gets smoother. . Ref : S. Biswas et al. NIMA 602 (2009) 749. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 5

Detector configuration used for MC study : 1. Electrode : 2 mm thick glass 2. Gap: 0. 3 mm 3. Gas: 2/11/2010 C 2 F 4 H 2/i-C 4 H 10/SF 6 = 85/5/10 Tapasi Ghosh RPC 2010, GSI 6

MC simulation to get single-gap timing RPC response : Gas gap of size d (= 0. 3 mm) is divided into several steps of size x = d/N corresponding to time t = x/v, where v = drift velocity. In each step primary ionization, avalanche multiplication processes are simulated. Avalanche development probability is governed by Space charge effect also considered. Ref: Werner Riegler et al. , NIM A 500 (2003) 144. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 7

Avalanche growth : For Timing RPC operating at E =100 k. V/cm, = 123/mm, = 10. 5/mm, v = 210 μm/ns, λ = 0. 1 Primary ionization: 10. clusters/mm Due to space charge avalanche saturates at Nsat = 1. 6 × 107 electrons. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 8

Induced signal : Finally, movement of the electrons in the electric field induces a current signal. Current signal induced on an electrode is for single-gap parallel plate RPC. N(t) = No of electrons at time t which is calculated by simulating the avalanches of individual primary electrons. r = Relative permittivity of glass. b = Thickness of glass and d = gas gap. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 9

Charge spectra : Including all detector parameters and considering the space charge effect the average charge is calculated. Threshold applied: 20 f. C. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 10

Time Spectrum : RMS of the time distribution at 20 f. C threshold provides the time resolution. Time resolution = 80 p. S. Shape has a tail at higher threshold-crossing time. 2/11/2010 Tapasi Ghosh RPC 2010, GSI of the threshold. Shape of the distribution is independent 11

Effect of non-uniform surface on RPC performance (analytical calculation) : The influence of small fluctuations on the variables can be estimated by where S = ( - ) v and K = effect of intrinsic fluctuations in the signal generation process. It is assumed that the origin of rms∆E/E is only the gap-thickness non-uniformity and so, It is assumed that sudden variation of the field is small. Ref : 2/11/2 010 A. Blanco NIM A 535 (2004) 272 Tapasi Ghosh. NPB RPC 2010, Suppl. GSI D. Gonza’lez-Di’az et al. (Proc. ) 158 (2006) 111. 12

Influence of roughness on time resolution: A 4% roughness, time resolution worsens by 20% Ref : D. Gonza’lez-Di’az et al. NPB (Proc. Suppl. ) 158 (2006) 111. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 13

Implementation of surface roughness in MC : A Gaussian distribution is assumed for the surface roughness. Gap thickness is assumed to be varied according to this distribution. Sigma of the distribution varied to simulate different roughness effect As, Electric field is assumed to vary similar to gap width. , and drift velocity are obtained at changed fields. Avalanche process considers changing , and drift velocity. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 14

Variation of and with electric field (parametrised) C 2 F 4 H 2/i-C 4 H 10/SF 6 = 85/5/10 Ref: Werner Riegler et al. , NIM A 500 (2003) 144. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 15

Variation of drift velocity with electric field (parametrized) : Ref: Werner Riegler et al. , NIM A 500 (2003) 144. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 16

∆rms. T/rms. T (%) Comparison of variation of time resolution with surface roughness : rms∆E/E (%) 2/11/2010 Monte Carlo shows larger effect at higher roughness (>5%) Tapasi Ghosh RPC 2010, GSI 17

Variation of efficiency with surface roughness : 2/11/2010 Tapasi Ghosh RPC 2010, GSI 18

Conclusions : A Monte-Carlo code developed for simulating response of timing RPC. Standard procedures applied for primary ionization, avalanche, signal generation. Space-charge effect is simulated by saturation of avalanche. Results are presented for a single-gap timing RPC. Surface roughness is considered to be gaussian distributed. Variation in electric field is similar to the variation in gap. Final effect of time resolution compared with analytical results. MC shows more worsening in time resolution compared to the analytical results. Efficiency get reduced with increasing roughness. 2/11/2010 Tapasi Ghosh RPC 2010, GSI 19

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Variation of and with electric field : Townsend attachment coefficients from ref… [Ref. S. Biabi, Imonte, program Tapasi to compute gas properties, Version 4. 5 ]. 2/11/2010 Ghosh RPC 2010, GSI 22

Variation drift velocity with electric field : (may be not needed ) Drift velocity used For 100 KV/cm field is v = 210 µm/ns. The lines show the drift velocity for different gases as predicted by Magboltz. Ref : 2/11/2010 Tapasi Ghosh RPC 2010, GSI 23

Effect of non-uniform surface on RPC performance : Variation in thickness over the surface causes variation in field and thereby changes , and drift velocity. Intrinsic time resolution can be expressed analytically The influence of small fluctuations on the variables can be estimated by It is assumed that the origin of rms∆E/E is only the gap-thickness non-uniformity and so, 2/11/2010 Tapasi Ghosh RPC 2010, GSI 24