13.01.2005 A. Bamberger 1

Experimental study of double GEM readout using MediPix2 chip

A. Bamberger, M. Debatin, J. Ludwig, M. Titov, N. Vlasov

13.01.2005 A. Bamberger 2

Typical geometry:5 µm Cu on 50 µm Kapton

70 µm holes at 140 mm pitch

Thin metal-coated polymer foil

chemically pierced by a high density of holes (technology

developed at CERN)

F. Sauli, Nucl. Instrum. Methods A386(1997)531F. Sauli, Nucl. Instrum. Methods A386(1997)531F. Sauli, http://www.cern.ch/GDDF. Sauli, http://www.cern.ch/GDD

13.01.2005 A. Bamberger 3

Advantages of GEM

DRIFT

TRANSFER

• positive ion feed back minimized

• high rates

• small rate of discharges for highly ionising particles...see later

F. Sauli, 2002 IEEE Proceedings

13.01.2005 A. Bamberger 4

Properties

• gas amplification up to 6000 easily achievable with Ar/CO2

• sufficient for min. ionizing particles in gas thicknesses of few mm

S1 S2 S3 S4

Induction gapInduction gap

e-

e-

I+

13.01.2005 A. Bamberger 5

Comparison µMEGAS and GEM

replottet from

NIM A 424 (1999) 321, NIM A 479 (2002) 294

Discharge probability with α-particlesSpatial resolution in laboratory tests

µMEGAS: 15 µm

GEM: 40 µm

BUT:

man

y or

ders

of

mag

nitu

de

NIM A 477 (2002) 23

NIM A 425 (1999) 262

13.01.2005 A. Bamberger 6

Double GEM 10·10 cm²

28 channels readout electronics

resistive chain for HV <4000 V

semitransparent drift electrode

13.01.2005 A. Bamberger 7

Some features of the apparatus

• all essential elements within the gas tight box: compact, easy handling i.e. tilting is possible

• Noise reduction due to short leads• breaking gas volume/flow for changes turned out

to be an affordable disadvantage (recovery within a few hours)

• Multi electrode analog readout (L3 muon amplifiers 7x4): important for checking gas gain

13.01.2005 A. Bamberger 8

Overall view

•

pocket for MediPix2 board and cable

13.01.2005 A. Bamberger 9

Double GEM

drif t elektrode

readout elektrode

GEM 1GEM2

GEM 1

GEM 2

ED

ET

EI

DRIFT

READOUT

DRIFT

TRANSFER

INDUCTION

• thickness of drift field 6 mm

• transfer gap 2 mm

• induction gap 2 mm resistors for protection

ΔVGEM= 350 – 400 V, ED , ET , EI~ 2.5 kV/cm

subject to further optimisation

Gas: Ar/CO2

13.01.2005 A. Bamberger 10

Homogeneity and energy resolution for 55Fe photons

• homogeneity < ±5%

• energy resolution of photo- electrons of 5.9 keV:

FWHM 28%

Homogenität der Fe55 Amplitude

0

50

100

150

200

250

300

350

1 3 5 7 9 11 13 15 17 19 21 23 25 27

Auslesestreifen

Pea

kpo

siti

on

(A

DC

K

anäl

e)

strip readout

13.01.2005 A. Bamberger 11

New readout electrode configuration with 2x2 cm2

before after

movie shows 4x4 matrix with source

HC

AL

rea

dou

t !

13.01.2005 A. Bamberger 12

Inserting MediPix2 into the GEM stack

gap for separation of electrodes crosses MediPix2

13.01.2005 A. Bamberger 13

Close-up of the arrangement

• surface of MediPix2 level with readout plane

• „ring“-like electrode helps to detect possible discharges near MediPix2 due to cross talk,

• „dummy“ MediPix2 with bonds showed no obvious discharging up to 4000 V, Eind= 3.5 kV/cm

• readout of MediPix2 normal functioning over many hours

13.01.2005 A. Bamberger 14

Readout with MUROS2

• the parameters: lower threshold between 2000 - 3000 e-

• upper threshold ~ 10 times higher

• HV 3900 V, 410 V across GEM

• Collimated 55Fe source used:

4 mm opening at a

distance of 35 mm

source

colli-mator

MediPix2

13.01.2005 A. Bamberger 15

Short term shot of 55Fe photons

14 mm

14 m

m

estimates blob size: 10 x10 pixles = 550x550 µm2

at gain ~ 3 103

13.01.2005 A. Bamberger 16

180 s exposed sample and displacement of colim. source

structure of joint between GEM electrodes seen

source with collimator moved by 2 mm

13.01.2005 A. Bamberger 17

MediPix2 exposed 30 min to source w/o collimator: Boundary of GEM

electrodes

steep slopes

13.01.2005 A. Bamberger 18

Some considerations for the resolution

Basics:1. transverse diffusion of Ar/CO2: 150 - 200 μm/cm2. size of energy deposition of 5.9 keV photon 300 – 500

μm 3. defocussing effect GEMs

Comments:• drift space is 6 mm cone like, slanted tracks reveal 1.)• dispersion of edge due to electrode boundary reveals 3.)

(two bounderies of the doube GEM setup involved !)• the „hit over threshold“ feature complicates the

disentangling

7 mm

no source

13.01.2005 A. Bamberger 19

Further investigations

• oberservation of min. ion. tracks

• quantify broadening due to drift volume

• use 50µm pitched GEMs

• reduce transfer and induction gap (1 mm), (are bonding loops above the chip a problem?)

• use gas mixtures with other nobel gasses

13.01.2005 A. Bamberger 20

Consideration for low photon energy spectroscopy: Conversion in gas or in Si

• at low energies (few keV) signal/noise dominated either by statistical fluctuations of primary clusters (GEM/μMEGAS) or by the „baseline“ fluctuation (Si converter with coupled electronics like MediPix)

• σ/N = 0.13 (5.9/E)0.5 for double GEM• σ/N = 200/1639 (5.9/E) for Si (σ = 200 e-)• break-even-energy at 5.3 keV

Therefore it is favorable to use gas based amplifiers below a few keV

13.01.2005 A. Bamberger 21

Summary• extremely robust operation of GEMs (no

faulty GEM, or visible change of hole during operation during 2 month observed)

• HV-stable condition for operation of a „naked“ MediPix2 (~week) with a double GEM. No broken MediPix sofar!

• acurate position resolution seems to be achieveable

13.01.2005 A. Bamberger 22

The effort would be in vain without the help of

• F. Sauli• M. Campbell• E. Heijne• X. Llopart• A. Zwerger

MANY THANKS !