1. CBM Muon Detection Workshop GSI Darmstadt, October 16 to 18, 2006
Micro Pattern Detectors and a First Sketch of a High Granularity Muon Chamber for CBM
Christian J. Schmidt
GSI
CBM Muon Detection Workshop GSI Darmstadt, October 16 to 18, 2006

2. Outline History of micro pattern detectors The GEM in particular
Sketch of a GEM-based Muon chamber

3. Micro-Pattern Gas Detectors: many similar concepts
Very creative phase in the ninties:
seeking advances beyond the MWPC
for targeted HEP-experiments
MSGC by Anton Oed (first µ-detector concept originated in neutron physics)
GEM by Fabio Sauli
MICROMEGAS by Y. Giomataris and G. Charpak et al.
Micro-DOT by Biagi
µCAT (Compteur a trous) by Lemonnier
... Micro Wire, Micro-Pin Array (MIPA), Micro-Tube ...
... Micro-Well, Micro-Trench, Micro-Groove ...
most importantly: micro = fast due to short ion drift
Overview: F. Sauli and A. Sharma: Micropattern Gaseous Detectors,
Ann. Rev. Nucl. Part. Sci. 49(1999)341

4. The Gas Electron Multiplier (GEM)
Ions
Electrons
Amplifier Mode
In hole high fields allow
Gas amplification
pictures from Sauli
Transparent Mode
At gain 1, electric fields transport charges through the holes

5. GEM- History since invention 1997 by F. Sauli
HERA-B: First HEP-Experiment to push from kHz to MHz event-rate
First HEP-Experiment to employ MSGCs on large scale
- Detect minimaly ionizing particles (tracks) in inner tracker
detector, depositing approx. 1 to 10 keV
- But get highly ionizing events up to times stronger also
rapid ageing
direct discharge damage to microstructures
- GEM was to introduce two step charge amplification,
leaving MSGC-amplification at a moderate gain of 50 to 300.
damage at MSGC
HERA-B inner tracker module,
an MSGC-GEM combination
200 built at Heidelberg
50 my development effort
by Eisele et al.

6. GEM- History COMPASS @ CERN: Large scale employment
of tripple GEM tracking detectors 31cm x 31cm
(operative!)
Today, HEP have largely decided for Si-Strip-Detectors:
CMS
LHCB
ALICE ...
GEMs not robust enough for signal spectrum with MIPs and HIPs
Silicon strip detectors are industrially availability today (Hamamatsu)
HEP application left: TPC – at Linerar Colliders etc. (TESLA)

7. Other current and some exotic applications for GEMs
Single photon detection employing mutiple, cascaded GEMs
Breskin et al., C. Richter (Diss.)
Gain and readout for TPCs in HEP (M. Killenberg et al.)
Coherent Neutrino Detection (J. Collar et al. 2003)
low background materials
sensitivity to sub keV recoil energies
feasibility of very high gain (~105) at very high pressure (20 atm)
Solid converter neutron detection (CASCADE-Detector)
Optical readout of scintillation after
neutron conversion through 3He
Fraga et al., G. Manzin (ILL)
GEM-preamp for Ultra Cold Neutron detection (Heidelberg)
X-Ray Polarimeter (E. Costa et al., R. Bellazini et al.)

8. Particular, unique properties of GEMs
allow multi-stage amplification
amplification decoupled from readout
avoid high gain photon positive feedback
single photon detection with suppression of
ion- as well as photon-feedback
inherent high rates capability through micro scale
minimal magnetic distortion through micro scale
robust technology
“simple” lithographic production process industrially available
production by CERN and in the future by Polish CERN licencee ... ???
3M has also developed a large scale production process for 12´´ x 12´´

9. Multiple GEM structures
high gain -
- high breakdown limit
S. Bachmann et al,
Nucl. Instr. and Meth. A479 (2002) 294

10. GEM high rates capability
FAST ELECTRON SIGNAL (NO ION TAIL)
The total length of the detected signal corresponds to the electron drift time in the induction gap:
Full Width 20 ns
(for 2 mm gap)
taken from Sauli et al.:

11. GEM high rates capability > 105 Hz/mm2
taken from Sauli et al.:

12. Towards a Micropatterned Muon Chamber
Model a Readout System
Is it feasible to step from Silicon strips in the STS
to a micro patterned gas detector as first Muon chamber...
... employing the same readout front-end ?
In contrast to Silicon, gas detectors allow for gain (fudge factor)
As learned yesterday, we expect to get at maximum (close to the axis) 1 Track/cm2/event with a 1 MHz event rate.
 too much for wire chambers but comfortable for micro pattern detectors such as GEMs (limit beyond 10 MHz/ cm2)
Mostly MIPs, but should expect all kinds of garbage (e.g. neutrons)!
First Muon chamber measures about 2m in diameter, A~3m2

13. Have a look at a Silicon Strip Detector
Sensor thickness ~ 250 to 300 µ
Sensor pitch 50.7 µ
Gap 32.7 µ
Manufacturer quotes
1.5 pF/cm strip length
e = 11.8
The average strip length will be about 10 to 15 cm,
giving 15 to 22.5 pF strip capacitance
- central sensors have shorter strips, longer cables
- outer sensors have longer strips, shorter cables
- strip area about 5 mm2

14. Model Sensor to Scale Capacitance
Purely 2D electrostatic problem, 2D Poisson solvers available, also some analytical solutions
Capacitance scales with ratios of characteristic geometric lengths: strip-gap/strip-width, strip-width/sensor-thickness
Model Silicon strip as a strip-line: get C = 1.23 pF/cm
sensor thickness non essential, lower electrode may be abolished
Strip impedance 69 Ohm, signal phase velocity 0.4 c, effective e = 6.5
Capacitance scales with effective e as 1/e e

15. Gas Detector Scaled from Silicon
With identical, scalable geometry, e = 1 or 1.1 get C = 190 fF/cm
Scale readout by a factor of 10 and get the same capacitance:
Strip pitch 0.5 mm, sensor thickness mm
Gems at 2 to 3 mm distance have no influence on capacitance
Strip impedance 175 to 200 Ohm
Strip width of 180 µ comfortably realizable
 Feasible strip length for silicon readout electronics: 65 cm e

16. Corresponding Detector Strip Area
65 cm of 0.5 mm strip pitch corresponds to 3.25 cm2
Central region needs higher granularity, up by a factor of about 10 in order to avoid hit pile-up.
Cap. = 1.2 pF, so 10 pF may be spend on cabling, A = 0.33 cm2
Lateral regions have little rate, so may be operated with higher strip areas.
What is the necessary resolution? Here we need some input from the tracking side!

17. Proposal for a CBM First Muon Chamber
MIPs: dE/dx for Argon at normal pressure 2.7 keV/cm, giving e-ion pairs per cm.
For Silicon sensors, a MIP gives about e-hole pairs.
Parallax and local tacklet tracking may demand for higher granularity in drift-time detection, resulting in 1 mm drift resolution.  Q = 9 e-/mm
need quite some gain for detection  two GEM layers to guarantee gain of 1000 to
Drift
Drift Region
GEM 2
GEM 1
RO Strips
Signal channeling electrode

18. Conclusion
A GEM gas chamber appears to be a suited solution for the first Muon chamber. GEMs nicely match the specified rate capability of Hz/cm2.
Detector capacitance may be chosen to perfectly adapt to Silicon optimized readout electronics.
The Gas detector wins with a factor of 6.5 effective e over Silicon, allowing for much longer structures (longer by the same factor of 6.5).
Lack of signal intensity needs to be compensated for by gain
The double GEM layer allows for gain up to with limited sparking probability.