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

2. 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

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

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

5. 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

6. Electrode : 2 mm thick glass Gap: 0.3 mm Gas:
Detector configuration used for MC study :
Electrode : 2 mm thick glass
Gap: 0.3 mm
Gas:
C2F4H2/i-C4H10/SF6 = 85/5/10
2/11/2010
Tapasi Ghosh RPC 2010, GSI

7. 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

8. For Timing RPC operating at E =100 kV/cm, = 123/mm, = 10.5/mm,
Avalanche growth :
For Timing RPC operating at
E =100 kV/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

9. 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

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

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

12. 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 : A. Blanco NIM A 535 (2004) 272
D. Gonza’lez-Di’az et al. NPB (Proc. Suppl. ) 158 (2006) 111.
2/11/2 010
Tapasi Ghosh RPC 2010, GSI

13. 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

14. 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

15. Variation of  and  with electric field (parametrised)
C2F4H2/i-C4H10/SF6 = 85/5/10
Ref: Werner Riegler et al. , NIM A 500 (2003) 144.
2/11/2010
Tapasi Ghosh RPC 2010, GSI

16. 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

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

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

19. 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

20. Thank You ….
2/11/2010
Tapasi Ghosh RPC 2010, GSI

21. 2/11/2010
Tapasi Ghosh RPC 2010, GSI

22. Variation of  and  with electric field :
Townsend and attachment coefficients from ref…
[Ref . S. Biabi, Imonte, program to compute gas properties, Version 4.5 ].
2/11/2010
Tapasi Ghosh RPC 2010, GSI

23. 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

24. 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