“Simulation of 10 Gap” “ALICE-TOF MRPC”

“Simulation of 10 Gap” “ALICE-TOF MRPC”

Presented by:Katayoun Doroud

World Laboratory fellow under Supervision of:

Crispin Williams

ALICE TOF General meeting, CERN – Build 29, 9 December 2009

Outline

MRPC Simulation Procedure considering space charge effects.

Simulation results for ALICE TOF 10 gap MRPC

Conclusion & outlook


MRPC Simulation Procedure

• Simulation of a single gap RPC.

• “n” events randomly chosen and summed up together.

• This will provide a single event for the MRPC.

Event 1

Event 2

Event n


Single gap simulation procedure

1- The gap has been divided into small steps (500)

E0

2- The through-going charged particles passes perpendicularly through the gas gap3- HEED++ program used to simulate the creation of the primary ionization (electrons and positive ions) 4- Each primary electron-ion pair has then been placed at the correct position inside the gas gap

5- The actual electric field strength calculated dynamically at each step

(actual electric field = applied electric field + electric field of space charge)


MAGBOLTZ program results

6- Recalculation of avalanche parameters (v, α, η), the new value of the avalanche parameters in each slice will be used during the avalanche multiplication calculation

Avalanche parameters simulation

Townsend & attachment coefficients vs. electric field

for the gas mixture of:

TFE/SF6 : 93/7


Electron multiplication simulation procedure Two different approaches are used to simulate

the avalanche multiplication:

W. Riegler NIMA 500 (2003) 144

1- Small number of electrons:

Each electron is considered in turn and its fate is decided: either (a) it is captured (b) it does nothing (c) it has an ionising collision.

Looping over each electron in a particular position step, we can generate the number of electrons for the next step.

Advantage:Avalanche fluctuations will be

properly implemented. Disadvantage:Time consuming especially when the

number of electrons becomes large.


Procedure 2 for Electron multiplication

2- Large number of electrons:

This provides the increased number of electrons for the next step.

Advantage: The simulation of avalanche growth will be very fast.Disadvantage: The fluctuation is not considered but these number alters with a

Gaussian distribution to include random nature of the electron avalanche.

dnp (x+dx) = ne. eα.dx

dnn(x+dx) = ne. eη.dx

dne(x) = np(x) – nn(x)


Diffusion & Induced signal simulation

Electrons will be redistributed (Gaussian distribution) using longitudinal diffiusion coefficient in both these approaches.

Weighting field concept is used to correctly calculate the induced signals from the movement of these charges (I ind = Ewnev).

The induced fast signal is simulated by summation of the induced signal due to the movement of electrons in each time step from the beginning of the multiplication process until all the electrons reach to the anode.

The induced total charge is calculated by summing up all the positives ions that have been created during the multiplication process and remain in the gap after all the electrons have arrived at the anode.

For the n gap RPC, we have simply sum up the amount of the fast induced charges and the total induced charge created in each n gaps.


Positive ions Electrons

negative ions

Electric field

Simulation of the development of the avalanches in the 0.25 mm gas gap


A typical event has been chosen and 6 snapshots are shown here :

Simulated Fast/Total in SRPC Gas mixture: 93% C2F4H2 & 7% Sf6

Electric field: 100 kV/cmQF/QT = 1/αd = 1/(126.9 × 0.025) = 0.031


Simulation of ALICE-TOF 10 gap MRPC

designed in two stacks of five gap;

A: connector for signal; B: pins for signal from cathode pads to be brought to central pcb;C: honeycomb panel; D: PCB (0.8 mm thick);E: gas gaps (0.25 mm thick); F: central PCB (1.6 mm thick);G: internal glass plates (0.4 mm thick); H: external glass plates (0.55 mm thick); I: cathode pickup pads; J: anode pickup pads.


Charge spectrumAdvantages of MRPC comparing to SRPC:

• The most probable value has shifted away from zero • There is a well defined peak


Ratio of Fast to Total Charge for the 10 gap MRPC

“Recombination an important effect in MRPC”

Clearly induced total charge obtained from ‘No recombination’ does not describe the data.


Recombination

Different percentages of the recombination between positive and negative ions, 100 %, 97 % and 94 %, considered.

This recombination has not been known until we have simulated the MRPC.


Average induced Total signal

Comparison between the average simulated total and the total charge obtained from the test of ALICE 10 gap MRPC ( Akindinov, NIMA 532 (2004) 562-565).


Conclusion A simulation procedure for the development of avalanches, considering space

charge in RPCs has been described and has been extended to simulate 10 gap MRPC.

This simulation procedure has been encoded into a program written in FORTRAN. The experimentally observed efficiencies, time resolution and the average

avalanche charges can only be explained if we consider the phenomena of recombination( almost 100%).

This recombination has not been known until we have simulated the MRPC. Recombination reduce the total amount of charges in the gap and this reduction

is very important for the rate capability of the MRPCs.

Precise study of recombination of charge carriers is going to be performed. Fast algorithm for radial electric field calculation in order to investigate the effect of

this field and variation of transversal diffusion coefficient with it. Remeasuring the value of the fast/total charge for ALICE 10 gap MRPC with different

percentage of SF6 and comparison with the simulated values.

Outlook


Documents

“Simulation of 10 Gap” “ALICE-TOF MRPC”