Gain Energy resolution

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Contact : Stéphan AUNE CEA/DSM/DAPNIA/SEDI (saune@cea.fr)

MICROMEGAS BULK

DAPNIA

Saclay

Micromegas in a bulkGaz

(ex: Ar + 5% isobutane)

Drift (HV 2)

Pad readoutIonizingparticle

Mesh (HV 1)

e-Con

vers

ion

gap:

1 m

m t

o m

Am

plifi

catio

n ga

p~

100

m

E ~ 50 kV/cm

E ~ 1kV/cm

Strips or pads

In a Micromegas detector (micromesh gaseous detector) the mesh (microgrid) is held parallel to the readout plane at a distance about 100 microns, at a potential of several 100 V. This creates the electric field responsible for gas amplification.The planarity and the parallelism of the mesh influence the gain homogeneity of the device. Downward excursions of the gap would limit the gas gain by breakdown.In a standard Micromegas, the gap was obtained by an array of pillars on the PCB and the mesh had to be stretched on a frame. The pillars are made out of photoimageable solder mask (Photoresist) available in films of different thicknesses (38 to 100 microns). The bulk concept is to use two Photoresist films to permanently hold the mesh between two arrays of pillars. The whole detector (pad array and mesh) is in one piece, a bulk.

development

Le pain

La tartine

The sandwich

The pizza

Le bulk !!!

UV

Bare PCB with strips or pads

PCB with a photoresist layer

A part of the photoresist is insolated (exposed to UVs)

PCB with a mesh between two photoresist layers

The bulk is ready for cabling.

The mesh is 100 m from the PCB.

Lamination• High temperature deposition (100°C) and under pressure of the photoresist film to glue it to the PCB.• Cut the edges, remove the protection film of the photoresist. • Deposition of the mesh stretched on a frame.• Lamination of a second layer of photoresist on the mesh.

PCB in

Wrapped PCB outThe laminator

Development• Dissolution of the non insolated photoresist using a Sodium carbonate jet at 40°C. • Deep rinsing with de-ionised water.• Oven and UV hardening of the bulk.

DevelopmentBulk before

development

Insolation

PCB with mask

• Positionning of the mask defining the pattern to polymerise by UVs (pillars, edges,...).• Insolation of the photoresist.( = 350 to 450 nm).• Hardening in the oven

UV insolator

Insolation

Lamination 2

Lamination 1

Manufacturing process

4

3

2

1

Advantages of the bulk Robustness and simplicity of operation Suppression of the dead area possibility of segmenting the mesh

Further possibilities• Multi-stage Micromegas• Micromegas mosaïc• Segmented mesh

Upgrade of the process• Laser insolation• Use of de fine meshes• Resistive liquid photoresist

1,E+02

1,E+03

1,E+04

1,E+05

1,E+06

300 350 400 450V mesh (V)

Gai

n

Ar + 5 % Iso

Ar + 10 % Iso

Ar + 15 % Iso

Please touch Stainless steel woven mesh(19 microns, 500 LPI)

Principe Micromegas

Mesurements with a Fe55 source Detector gain as a function of the mesh voltage

120 x 140 mm bulk

Mesh prisonned between 2 pillars

Results

CEA Saclay / DAPNIA

S. Aune, G. Charpak, P. Colas, R. De Oliveira, A. Giganon, I. Giomataris, Ph. Rebourgeard(Saclay and CERN)

Laser cut mesh (70wide)

Application: T2K test at CERN