Numerical simulations on single mask conical GEMs Tue 3rd March 2009, Bad Honnef Marco Villa – CERN

Micro Pattern Gas Detectors (MPGDs) My work @ CERN Large Area Micro Pattern Gas Detectors (MPGDs) Micromesh Gaseous Structure (MicromeGaS) Gas Electron Multiplier (GEM) R&D Data analysis R&D Simulations

The standard GEMs

Photolithographic technique used to prepare standard GEMs

Why simulations on conical GEMs? Why conical GEMs? Why simulations on conical GEMs? Large Area GEMs ↓ very difficult to align the two masks on the top and bottom side What are the properties of the GEM foils obtained with single mash lithographic technique? How do these properties depend on the geometry? spatial uniformity time stability discharge probability maximum achievable gain field shape electron transparency avalanche shape charging up properties

Simulations: basics Simulation ANSYS PACKAGE GARFIELD PACKAGE Ansys is used to define: the geometry; the material properties; the electrodes voltage; the e.m. boundary conditions; and to solve the e.m. equations with a finite elements analysis method GARFIELD PACKAGE Garfield is used to: read the Ansys fieldmaps; define the gas properties; simulate the behaviour of electrons in the gas

ANSYS(1): definition of the geometry Geometrical properties: kapton thickness = 50 µm copper thickness = 5 µm drift gap thickness = 8000 µm induction gap thickness = 8000 µm holes pitch = 140 µm hole smaller diameter = 55 µm hole larger diameter = 55 µm  95 µm In order to speed up the simulation, only the elementary cell has been considered the mesh density has been doubled on all the gas boundary surfaces

ANSYS(2): definition of the voltages At the beginning, test voltages have been used: drift field = 0.1 kV/cm induction field = 3 kV/cm GEM voltage = 20 V ↓ GEM field ≈ 3.6 kV/cm

GARFIELD(1): field and electrons drift

GARFIELD(2): field

ANSYS(3): re-definition of the voltages cathode 1MΩ GEM 1 top 0.5MΩ In the chamber: conversion gap = 3 mm transfer gap = 2 mm induction gap = 2 mm Therefore, the field values in the chamber are: drift field = 2.667 kV/cm induction field = 4 kV/cm GEM voltage = 366.7 V I ≈ 800 µA GEM 1 bottom 1MΩ GEM 2 top 0.5MΩ 5MΩ GEM 2 bottom 1MΩ GEM 3 top 0.5MΩ 2.5MΩ GEM 3 bottom 1MΩ anode to GND

GARFIELD(3): field and electrons drift

GARFIELD(4): field

GARFIELD(5): electron drift

GARFIELD(6): electron drift & diffusion Finally, the electron diffusion is implemented in the Garfield code a Monte-Carlo technique is used to simulate the longitudinal and transversal diffusion processes the Monte-Carlo integration step is set much smaller than the typical detector dimensions (good compromise = 250 nm)

GARFIELD(7): electron drift & diffusion

Preliminary conclusions Moving from the open-top geometry (95-55) to the cylindrical geometry (55-55) and then to the open-bottom geometry (55-95): the field peak @z=-25µm decreases, while the field peak @z=+25µm increases; if the thermal electron diffusion is switched off, the electron transparency decreases from 95-55 to 55-55, then it increases slightly; if the diffusion is off, all the stopped electrons end up on the upper copper surface  meshing problem? if the diffusion is off, the diameter of the outcoming electron beam decreases from 95-55 to 55-55, then is remains constant; the Monte-Carlo diffusion technique seems to work fine  is there any real need for a microscopic diffusion procedure?

implementation of the avalanche process (Monte-Carlo?) Next steps… check the beaviour of a microscopic diffusion process and compare the results with the Monte-Carlo method; electron transparency study using electrons from random coordinates on a far plane (100 µm over the upper copper foil); implementation of the avalanche process (Monte-Carlo?) charging up studies