1. The dynamic behaviour of Resistive Plate Chambers
Presented by
Marcello Abbrescia
University and INFN - Bari - Italy
VII Workshop on
Resistive Plate Chambers and related detectors
Clermont-Ferrand FRANCE
October 20-22, 2003

2. Marcello Abbrescia - University of Bari
The story up to now
Exponential growth
Drift
xsat
Saturation
“Drift” 
Many quantities in the formula above are themselves stochastic variables.
The induced charge is the “convolution” of all these variables.
A Monte Carlo rather elaborated
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

3. When saturation does not play
Charge Spectra shape:
these are the most fundamental information you can get
Freon rich mixture
Gap: 9 mm
Comparison between Monte-Carlo predictions and experimental data
Gap: 2 mm
Argon rich mixture
: primary cluster density (from 3 to 5 cl/mm)
: 1st Townsend “effective” coefficient
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

4. ...and when saturation becomes important
This is not a fit!
Simulation
Experiment
HV=9.2 kV
Gas mixture:
C2H2F4/C4H10 97/3 + SF6 2%
Input for simulation: Colucci et al., NIM A 425 (1999) 84-91
Experimental data from Camarri et al., NIM A 414 (1998)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

5. ...and when saturation becomes important
This is not a fit!
HV=9.4 kV
Simulation
Experiment
Gas mixture:
C2H2F4/C4H10 97/3 + SF6 2%
Input for simulation: Colucci et al., NIM A 425 (1999) 84-91
Experimental data from Camarri et al., NIM A 414 (1998)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

6. ...and when saturation becomes important
This is not a fit!
HV=9.5 kV
Simulation
Experiment
Inefficiency peak
Saturation broad peak
Gas mixture:
C2H2F4/C4H10 97/3 + SF6 2%
Input for simulation: Colucci et al., NIM A 425 (1999) 84-91
Experimental data from Camarri et al., NIM A 414 (1998)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

7. ...and when saturation becomes important
This is not a fit!
HV=9.7 kV
Simulation
Experiment
Saturation broad peak
Inefficiency peak
Gas mixture:
C2H2F4/C4H10 97/3 + SF6 2%
Input for simulation: Colucci et al., NIM A 425 (1999) 84-91
Experimental data from Camarri et al., NIM A 414 (1998)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

8. ...and when saturation becomes important
This is not a fit!
HV=9.9 kV
Simulation
Experiment
Saturation broad peak
Inefficiency peak
Gas mixture:
C2H2F4/C4H10 97/3 + SF6 2%
Input for simulation: Colucci et al., NIM A 425 (1999) 84-91
Experimental data from Camarri et al., NIM A 414 (1998)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

9. ...and when saturation becomes important
This is not a fit!
HV=10.1 kV
Simulation
Experiment
Saturation broad peak
Inefficiency peak
Gas mixture:
C2H2F4/C4H10 97/3 + SF6 2%
Input for simulation: Colucci et al., NIM A 425 (1999) 84-91
Experimental data from Camarri et al., NIM A 414 (1998)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

10. The “single cell” model
None of the simulations up to now take into account the
role of the bakelite resistivity ... we could be simulating metal or insulating electrodes  Cg Rb Cb 
Recovery time independent of the cell dimension ...
typical avalanche radius: 100 m
typical avalanche charge: 1 pC
typical external charge contained in 100 m: 10 pC
A few numbers:
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

11. What happens in the “single cell”
=20 Hz
Applied HV
=1500 ms
Area of the cell = 1 mm2
There is a sort of feedback ...
High HV “at start”
Big pulses
  5  cm
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

12. Marcello Abbrescia - University of Bari
Effective HV vs. rate
Applied HV
10 Hz
The effective HV diminishes and its distribution is broader.
13 Hz
20 Hz
(...until HVeff is too low)
Two consequences:
lower HV at high rate
greater HV variations at high rate
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

13. Marcello Abbrescia - University of Bari
Efficiency vs. rate
Experiment
 4  1010 cm
Simulation
 8  1010 cm
From C. Bacci et al., NIM A 352(1995)
Cut-off rate of the experimental efficiency well simulated: slightly higher value of resistivity needed (significant?)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

14. Rate capability dependances
 8  1010 cm
 1011 cm
HV=10100 V
HV=9800 V
 4  1011 cm
HV=9600 V
 1012 cm
HV=9200 V
 4  1011 cm
HV=10100 V
independent of applied voltage
Cut-off rate
True in the “single cell” model only
strongly dependent on resistivity
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

15. Rate capability in streamer mode
 8  1010 cm
Streamer charge distribution has been simulated also by “artificially” multiplying by 10 both avalanche charge, and area on the electrodes.
Cut off rate reproduced also in the streamer case.
Simulation
streamer
Exp. avalanche
Efficiency rate dependance different.
Exp. streamer
From R. Arnaldi et al., NIM A 456(2000) 73-76
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

16. Efficiency vs. HV and rate
Very good agreement
~ 2 Hz/cm2
At high rate the shape of the simulated efficiency curve seem to change and differ from the experimental one.
~ 1.5 kHz/cm2
Experimental
Simulation
Data from G. Aielli et al., NIM A 478(2002)
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

17. Time resolution Simulated Experimental
Data from C. Bacci et al., NIM A 352(1995)
Simulated
Experimental
0.2 kHz/cm2
=0.9 ns
1 kHz/cm2
=1.2 ns
4 kHz/cm2
=1.6 ns
General behaviour well reproduced ...
Simulated time resolutions slightly less than experimental.
1. Systematics on drift velocity
2. “Instrumental” effects not reproduced
Possible Reasons
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

18. Time delay and resolution vs. rate
Time resolution
4 ns
3.8 ns
Experimental (arbitary zero)
Experimental
Simulation (absolute scale)
Simulation
The absolute scale of the simulation refers to the passage of the ionising particle.
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

19. The future: The multi cell model
The area of the inefficiency cell increases as the bakelite surface resistivity decreases.
“large”
“small”
avalanche
The net results is that the current flows not only in the “central” cell, but also in the neighbouring ones. Cg Rb Cb
Rs,b 
“Surface” coupling resistors
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari

20. Marcello Abbrescia - University of Bari
Conclusions
It is the first time the role of resistivity is taken into account in simulation of Resistive Plate Chambers ... The model of the dry water becomes wet.
Even the single cell model reproduces well the efficiency and time resolution vs. rate.
The role of resistivity and collected charge seems to be clear.
Charge spectra are well understood.
Slight differences remain, both for the “static” and the “dynamic” case ... could be due to
uncertainty in gas (and other) parameters.
possible refinements of the model.
RPC 2003 Clermont-Ferrand October 20th, 2003
Marcello Abbrescia - University of Bari