1. MICROMEGAS per l’upgrade delle Muon Chambers di ATLAS per SLHC
Arizona, Athens (U, NTU, Demokritos), Brookhaven, CERN, Harvard, Istanbul (Bogaziçi, Doğuş), Naples, Seattle, USTC Hefei, South Carolina, St. Petersburg, Shandong, Thessaloniki
M.Alviggi, GR1-Napoli, 16 dicembre 2008
Thanks to P.Iengo per la maggior parte dei plot

3. Micromegas as candidate technology
Combine triggering and tracking functions
Matches required performances:
Spatial resolution ~ 100 m (track< 45°)
Good double track resolution
Time resolution ~ few ns
Efficiency > 98%
Rate capability > 5 kHz/cm2
Potential for going to large areas ~1m x 2m with industrial processes

4. Prototype P1 Standard bulk micromegas fabricated at CERN-TS/DEM
Homogeneous stainless steel mesh
325 line/inch = 78 m pitch
Wire diameter ~25 m
Amplification gap = 128 m
450mm x 350mm active area
Different strip patterns (250, 500, 1000, 2000 µm pitch; 450mm and 225 mm long)
Drift gap: 2-5 mm

5. Test beam set up
2007 Test beam set up
2008 Test beam set up
P1 CERN H6 beam line in November 2007 & June to August 2008
120 GeV pion beam
Scintillator trigger
External tracking with three Si detector modules (Bonn Univ.); independent DAQ
Three non-flammable gas mixtures with small isobutane admixture used in 2008: Ar:CO2:iC4H10 (88:10:2), Ar:CF4:iC4H10 (88:10:2), Ar:CF4:iC4H10(95:3:2)
Data acquired for 4 different strip patterns and 5 impact angles (0 to 40 degrees)

6. Readout DAQ based on ALTRO CHIP FEC DAQ PC (ALICE DATE) 32 channels
trigger
DAQ PC (ALICE DATE)
FEC
32 channels
200 ns integration time
64 charge samples/ch
100 ns/sample
15 pre-samples
1 ADC count ~ 1000 e-
32 channels
Micro
Megas
Two inverted diodes for spark protection
Zero channels died
Typical ADC spectra
Noise subtraction (from 12 pre-samples)
Cluster position from center of gravity 6

7. Cluster charge distribution
Gas mixture: Ar:CF4:iC4H10 (88:10:2)
Drift gap 5 mm; drift field = 200 V/cm
Strip pitch = 250 µm
Horizontal axes: ADC count
ADC count = 1000 electrons
Gain measurement from HVmesh scan
Exponential gain increase
Stable operation (small spark rate) for gain of 3–5 · 103
Cluster charge (ADC counts)
HVmesh = 460 V

9. Spatial resolution Si tracker σextr≈ 50 µm MM cluster position
Convoluted
Strip pitch: 250 µm
Gas: Ar:CF4:iC4H10 (88:10:2)
Track impact angle: 90°
Convoluted resolution of Si tracker + extrapolation
σ(Si+MM) = 63 µm
MM intrinsic resolution
σ(MM) ≤ 40 µm

10. Spatial resolution … more
‘x-raying’ the micromegas
Look for areas where micromegas is inefficient, i.e. tracks in Si tracker with no hits in the micromegas  pattern of pillars
Si tracks
Equipped micromegas
area (8 mm)
2.54 mm
Pillar distance

11. Micromegas as a TPC…. Track inclination: 40° Ar:CF4:iC4H10 (95:3:2)
Drift field: 360V/cm
Cluster
First strip
Last strip
Even with non-optimal r/o electr.
measuring the arrival time on each strip it is possible to measure the drift velocity or, with known drift velocity, the drift distance
Rob Veenhof
vd = 8.4 cm/us
Drift path (mm)
8 cm/µm
Drift time (ns)

12. …Micromegas as a TPC
A time resolution of a nanosecond results in space points with a resolution along the drift direction of 100 µm
Each micromegas gap delivers a set of space points, the more the track is inclined the more space points are available
Solves the problem of spatial resolution for large track inclination

13. Therefore… Robust detector Works with non-flammable gases
Spatial resolution is excellent with small strip pitch
MM as TPC will give track segments & excellent space resolution; needs time measurement (ns)
Electronics should measure time, may relax on charge
Trigger capability to be proven with faster electronics

14. What next ?
Test beam stopped ; setting up cosmics test stand in and in few other labs (Naples,Demokritos)  optimize gas mixtures wrt drift velocity, diffusion, primary ionization, sparks, ageing…
Analysis of test beam data taken in 2008 with goals:
Definition of readout segmentation
Definition of requirements for r/o electronics
Construction and test of 1300 x 400 mm2 prototype (Rui de Oliveira)

15. The 50% prototype Active area: 1.3 x 0.4
Segmented mesh (cut) to reduce mesh capacity
250 and 400 µm strip pitches
Long and short strips
Construction has started in CERN/TS-DEM
Expect MM board to be ready by early 2009
Chamber to be completed spring 2009
Test beam in May 2009
The stretched micromegas mesh on its frame

16. Backup
Richieste CSN1
RD51

17. σ = 30 ns
σm ≈ 1.2 ns
ns*100

18. Inclined tracks
Cluster
First strip
Last strip
Impact angle: 50°

19. Micromegas as TPC (II)
Track under 50° with relative time info

20. Software tool
Software tool* for quasi online and off-line reconstruction (based on ROOT)
Permits alignment of Si tracker modules with MM chamber
Combines data from Si tracker and MM
‘online’ resolution
Also: simple event display
*) Thanks to Woo Chun Park (U. South Carolina)

21. Simple event display
Si module1
Si module3
Si module6
Micromegas

22. Spatial resolution – ‘online’
Si tracker
Micromegas
Beam
Residuals of MM cluster position and extrapolated track from Si
Convolution of:
Intrinsic MM resolution
Tracker resolution (extrapolation)
Multiple scattering
⎬ ~60 µm
Gas: Ar:CF4:iC4H10 (88:10:2)
Drift field: 200 V/cm
Strip pitch: 500 um
Strip width: 400 um
Strip pitch: 1000 um
Strip width: 900 um
Strip pitch: 250 um
Strip width: 150 um
: 85 um
MM: 60 um
: 102 um
MM: 82 um
: 212 um
MM: 203 um mm mm mm