Post on 19-Jan-2016
description
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 !