1. Gas pixel detector for x-ray observation
David Attié
P. Colas, E. Delagnes, Y. Giomataris,
M. Campbell, X. Llopart,
M. Chefdeville, H. van der Graaf, J. Timmermans, J. Visschers
I’m here to present a TPC review
NDIP08 – Aix-les-Bains – June 19th, 2008

2. NDIP08 – Aix-les-Bains – June 19th, 2008
Outline
Introduction: motivations for a gas pixel detector
The TimePix readout chip
Description
Architecture and schematic
Micro Pattern Gaseous Detector TPC
Description of Micromegas
Micro-TPC
Application for x-ray observations
Measurement of primary statistics in gas
TPC based polarimeter
Conclusion
NDIP08 – Aix-les-Bains – June 19th, 2008

3. Motivations for pixelized gaseous detector
Gaseous detector advantages:
2D/3D imaging
Low occupancy and low radiation length X0
 mean free path could be important
Spatial resolution:
σxy limited by the pad size (pitch/√12)
narrow charge distribution (RMS ~15 μm)
High granularity:
δ-ray recognition/suppression in TPC
possibility to count primary clusters & electrons
direction & energy of tracks:
low-energy e- for X-ray polarimetry
2 e- from double beta decay, nuclear recoils in WIMP
or neutrino interactions for dark matter search
ALICE TPC simulations
 Digital TPC as a tracking detector with very high spatial resolution for astrophysics & high energy physics experiments
NDIP08 – Aix-les-Bains – June 19th, 2008

4. Description of the TimePix chip
Chip (CMOS ASIC) upgraded in the EUDET framework from the Medipix2 chip developed first for medical applications
IBM technology 0.25 μm on 6 layers
Characteristics:
surface: 1.4 x 1.6 cm2
matrix of 256 x 256
pixel size: 55 x 55 μm2
For each pixel:
preamp/shaper
threshold discriminator
register for configuration
TimePix synchronization logic
14-bit counter
Noise: ~ 650 e-
70 e- per pixel, Cin ~ 15 fF
Pixel 1 2 3 4 5 55 μ m
14111 m
16120 m
14080 m (pixel array)
55 mm
Synchronization Logic
Interface 1 2 3 4 5
Preamp/shaper
Configuration latches
Counter
THL disc.
Llopart et al., NIMA 581 (2007) 361
NDIP08 – Aix-les-Bains – June 19th, 2008

5. TimePix Synchronization Logic Control
Each pixel can be configured independently in 5 different modes
Internal clock up to 100 MHz
Timepix Mode
TOT Mode
Medipix Mode
100 MHz
Internal Shutter
Shutter
DACs values
Mask P1 P0
Mode
Masked 1
Medipix
TOT
Timepix-1hit
Timepix
Internal Clock
10 ns
not detected
detected
Digital Signal
Analog Signal
Summed charge
NDIP08 – Aix-les-Bains – June 19th, 2008

6. Detectors using TimePix chip
Solid detector
Gas detector
x, y, F(x, y)  2D
x, y, z(t), E(x,y)  3D
Drift cathode grid
X-ray source + -
Ionizing
particle
Gas volume
Semiconductor
sensor
Flip-chip
bump bonding
connections + -
Amplification System (MPGD)
Medipix2/TimePix chip
TimePix chip
NDIP08 – Aix-les-Bains – June 19th, 2008

7. InGrid: Integrated Micromegas Grid
Micromegas is a Micro Pattern Gaseous Detector
formed by a metallic micromesh (hole pitch 70 μm)
sustained by 50 μm pillars above the anode
Multiplication between anode and mesh
Gain up to 105
~ 1 kV/cm
~ 80 kV/cm
pad
PCB
Integrate Micromegas detector directly on a CMOS chip by post-processing
Resistive layer for protection of a-Si:H
IMT Neuchatel
NIKHEF (MESA+, Univ. Twente)
NDIP08 – Aix-les-Bains – June 19th, 2008

8. InGrid: energy resolution
Energy resolution depends on the
grid geometry
Grids can be very flat
best energy resolution achieved:
 13.6 % with 55Fe source in P10
removal of Kβ 6.5 keV line:
 keV in P10
Hole pitch down to 14 μm
with various diameters
Different gaps (35-75 μm)
Until now: grid is 1 μm of Al
but can also be increased to 5 μm
by electrolysis to be more robust
Escape peak Kα
13.6 % FWHM
Escape peak Kβ
Gap: 50 μm; Hole picth: 32 μm,Ø: 14 μm
11.7% FWHM
Kβ-filtered spectrum with Cr foil
NDIP08 – Aix-les-Bains – June 19th, 2008

9. Micro-TPC using TimePix/Micromegas
Micro-TPC with a 6 cm height field cage
Size : 4 cm × 5 cm × 8 cm
Windows for
X-ray sources
Cover
Windows for
β sources
6 cm
Field cage
Micromegas
mesh
Gas mixture at atmospheric pressure
TimePix chip
NDIP08 – Aix-les-Bains – June 19th, 2008

10. Micro-TPC TimePix/Micromegas
TimePix chip
+ SiProt 20 μm
+ Micromegas
55Fe source
Ar/Iso (95:5)
Time mode
z = 25 mm
Vmesh = -340 V
tshutter = 283 μs
NDIP08 – Aix-les-Bains – June 19th, 2008

11. Measurements of primary statistics in gases
Diffusion σt should be big enough to separate electrons: e- per pixel ~ 1
Study of primary electrons and Fano factor F using RMS
Spectrum of number of electrons for 2000 events:
F: Fano factor
√b: single e- gain distribution rms (%)
ε: detection efficiency
N: number of primary e-
Sensitive to Kα & Kβ lines
FWHM = 9,5 %
5.9 keV line at ~ 226 e-
TimePix+Ingrid+ 15 μm SiProt
Argon + 5% Isobutane
NDIP08 – Aix-les-Bains – June 19th, 2008

12. Polarimetry using photoelectric absorption
Ideal polarimeter is a track imager with:
resolution elements < mean free path of photoelectron E
X-ray
Photoelectron
Differential photoelectron cross-section emitted
from the atomic s-orbital in non relativist limit:
θ polar angle, φ azimutal angle
Emission angles are modulated by the polarization P θ
Auger
electron φ
maximum in the plane γ direction
Nmax
Nmin
NDIP08 – Aix-les-Bains – June 19th, 2008

13. Prototype TPC polarimeter using TimePix/Micromegas
TimePix chip
+ SiProt 20 μm
+ Micromegas
55Fe source
Ne/Iso (90:10)
TOT mode
z < 5 mm
Vmesh = -450 V
tshutter = 0.2 s
NDIP08 – Aix-les-Bains – June 19th, 2008

14. Prototype TPC polarimeter using TimePix/Micromegas
Identify the cluster
Photoelection + eauger track in Neon+10 Isobutane
TimePix chip
+ SiProt 20 μm
+ Micromegas
55Fe source
Ne/Iso (90:10)
TOT mode
z < 5 mm
Vmesh = -450 V
tshutter = 0.2 s
Determination of the polarization
Barycentre
Principal axis
Reconstructed absorption point
Reconstructed photoemission
direction with identification
of the absorption point and the
removal of the final part
of the track φ
φ photoemission angle
Low Ek-edge of Neon  eauger are
isotropically emitted with a small
fraction of the photon energy
In low Z gas mixture tracks are
longer so angular reconstruction is easier
NDIP08 – Aix-les-Bains – June 19th, 2008

15. Example of TPC for x-ray polarimeter
Drift Electrode
GEM like
Readout Strips
130 μm pitch
Black et al. NIMA 581, 2007, 755
Gas mixture: Neon/DME 50:50 at 0,6 atm
Photoelectron y
e- Drift
x(t) x y z
Digitized Waveforms
Differentiated Waveforms
Image
X-ray
Trigger
Polarized 6.4 keV photons
2O mm
Uniform response
Modulation (P ~50 %)
No false modulation
An encouraging start
Counts
Unpolarized
5.9 keV photons 0o
45o
90o
Photoemission electron angle (degree)
NDIP08 – Aix-les-Bains – June 19th, 2008

16. NDIP08 – Aix-les-Bains – June 19th, 2008
Conclusions
TimePix chip/Micromegas + SiProt: demonstrator for the digital TPC
 useful tool for x-ray observations
Ultimate resolution for a TPC thanks to the single electron sensibility
Micro-TPC is an excellent tool to characterize photon absorption in gas mixtures
statistics of primary electrons and clusters
Fano factor (gain fluctuation)
Identification of the photoelectron angle by imaging the photoelectron track is very promising for soft x-ray polarimetry ( 2 keV < Eγ < 50 keV)
Still some technologic issues:
Self triggering capability
How to improve the readout of the chips (speed and larger surface) ?
- through Si connectivity: avoiding bonding wires
- fast readout technology (~5 Gb/s)
Sealed detector
NDIP08 – Aix-les-Bains – June 19th, 2008

17. The TimePix collaboration
NIKHEF Harry van der Graaf
Martin Fransen
Jan Timmermans
Jan Visschers
Sipho van der Putten
Arno Aarts
Saclay CEA/DAPNIA David Attié
Paul Colas
Esther Ferrer-Ribas
Arnaud Giganon
Yannis Giomataris
Marc Riallot
Univ. Twente/Mesa+ Jurriaan Schmitz
Victor Blanco Carballo
Cora Salm
Sander Smits
FREIBURG A. Bamberger
K. Desch
U. Renz
M. Titov
N. Vlasov
A. Zwerger
P. Wienemann
CERN Erik Heijne
Xavier Llopart
Medipix Consortium
domo arigato gazaimase = merci beaucoup
Ijo desus = c’est fini !
β- from 90Sr source in He/Isobutane 80:20
Thank you for your attention
NDIP08 – Aix-les-Bains – June 19th, 2008

18. NDIP08 – Aix-les-Bains – June 19th, 2008
Backup slides
NDIP08 – Aix-les-Bains – June 19th, 2008

19. Readout system for Medipix2/TimePix chip
MUROSv2.1:
Serial readout
VHDCI cable of length <3m
read 8 chips in mosaic
tunable clock [30-200MHz]
USB: ~5
Mosaic achitecture:
NDIP08 – Aix-les-Bains – June 19th, 2008

20. TimePix chip architecture
36×106 transistors on 6 layers
(~550 transistors/pixel  13.5 μW)
Reference clock per pixel up to 100 MHz
Characteristics:
analog power: 440 mW
digital power (Ref_Clk = 80 MHz): 450 mW
serial readout 100 MHz): 9.17 ms
parallel readout 100 MHz): 287 μs
Pixel modes:
masked
counting mode (Medipix, Timepix-1h)
Time-Over-Threshold  “charge” info
Common stop  “time” info
NDIP08 – Aix-les-Bains – June 19th, 2008

21. TimePix chip schematic
Previous Pixel
For each pixel
Ref_Clkb
Clk_Read
Mux
4 bits thr Adj
Mask
Mux
Input
Preamp
Disc
14 bits
Shift Register
THR
Shutter
Timepix Synchronization
Logic
Shutter_int
Ctest P0
Conf
Testbit
Polarity P1
8 bits
configuration
Test Input
Ovf Control
Ref_Clk
Clk_Read
Next Pixel
Analogic part
Digital part
NDIP08 – Aix-les-Bains – June 19th, 2008

22. First TimePix Quad 3 2 1 First Timepix quad + 300 μm Si crystal
Medipix mode counting
55Fe source
tshutter =40 s
Time mode
90Sr source
tshutter = 237 μs
Time-Over-Threshold mode
241Am source
tshutter = 5 s
Llopart & Campbell, CERN
NDIP08 – Aix-les-Bains – June 19th, 2008

23. NDIP08 – Aix-les-Bains – June 19th, 2008
TimePix & GEMs
Freiburg (+Bonn)
Ar CO2 70/30
Cartes de 181x181 en mode Time & et en TOT
Fournit les informations charge & temps en même temps
Fort potentiel pour la séparation de traces
He CO2 70/30
NDIP08 – Aix-les-Bains – June 19th, 2008

24. TimePix using Micromegas
Timepix chip + Micromegas on frame:
Moiré effects
+ pillars
Timepix chip + SiProt + Ingrid:
“Uniform”
MESA+
Resistive layer for protection
IMT Neuchatel
“counting” mode
NDIP08 – Aix-les-Bains – June 19th, 2008

25. Micro-TPC TimePix/Micromegas
TimePix chip
+ SiProt 20 μm
+ Micromegas
90Sr source
Ar  He
Time mode
z ~ 40 mm
Vmesh = -340 V
tshutter = 180 μs
spark-proof !
NDIP08 – Aix-les-Bains – June 19th, 2008

26. Micro-TPC TimePix/Micromegas
TimePix chip
+ SiProt 20 μm
+ Micromegas
90Sr source
Ar/Iso (95:5)
Time mode
z ~ 40 mm
Vmesh = -340 V
tshutter = 180 μs
NDIP08 – Aix-les-Bains – June 19th, 2008

27. Gas mixture containing Neon
NDIP08 – Aix-les-Bains – June 19th, 2008

28. Simulated quality factor
Bellazzini et al., NIMA 572 (2007) 167
NDIP08 – Aix-les-Bains – June 19th, 2008