1. Engineering Design Review
MWPC requirements
Engineering Design Review
April 16th 2003
Werner Riegler
CERN
Delia is a technical student (engineer), Nicolas is a project associate (engineer)
Werner Riegler CERN, April 16th, 2003

2. Werner Riegler CERN, April 16th, 2003
LHCb Muon System
5 muon stations, one in front (M1), 4 behind (M2-M5) the calorimeter
A muon trigger requires the coincidence of hits in all 5 stations within the
bunchcrossing time of 25ns in a certain spatial window that selects the
muon momentum.
Granularity 1x2 cm to 10x20cm.
Werner Riegler CERN, April 16th, 2003

3. Werner Riegler CERN, April 16th, 2003
PNPI proposal:
1.5/2mm pitch,
5mm full gap,
30m wire, Ar/CO2/CF4 gas mixture,
double gap, wire pads, cathode pads
Wire pad chamber for LHCb muon system,
B. Botchine, A.Kashchuk, V. Lazarev, N. Sagidova, A. Vorobiev, A. Vorobyov,
LHCb
Detector physics and performance simulations of the MWPCs for the LHCb muon system,
W. Riegler, LHCb
Crosstalk, cathode structure and electrical parameters and of the
MWPCsfor the LHCb muon system,
W. Riegler, LHCb
Werner Riegler CERN, April 16th, 2003

4. Werner Riegler CERN, April 16th, 2003
Detector
2 chamber layers are connected into one frontend.
4 layers of MWPCs are combined into one station.
Werner Riegler CERN, April 16th, 2003

5. Difference to ATLAS, CMS
The ATLAS Cathode Strip Chambers are intended for position resolution. Amplifier peaking time 80ns, bipolar shaping, ‘crosstalk intended’ on cathode strips for center of gravity.
The CMS Cathode Strip Chambers are intended for position resolution (cathodes strips) and timing (wires). Cathode amplifier peaking time 100ns, wire amplifier peaking time 30ns.
The LHCb MWPCs are intended for highly efficient timing within a certain spatial granularity at the LVL0 trigger. Amplifier peaking time of 10ns, pulse width<50ns, unipolar shaping, low crosstalk.
Since crosstalk is  RinCpp and since  (20 MHz) is high we have to minimize the pad-pad capacitance Cpp and amplifier input impedance Rin. Because we want unipolar shaping we need a baseline restorer in the front end.
Werner Riegler CERN, April 16th, 2003

6. Werner Riegler CERN, April 16th, 2003
Geometry and Fields
Volt /cm
Pitch 2 mm
Gap 5 mm
Wire 30 m
HV V
Cathode field 6.2 kV/cm
Wire field 262 kV/cm cm
Werner Riegler CERN, April 16th, 2003

7. Werner Riegler CERN, April 16th, 2003
Gas Properties
For 10 GeV muons we expect about
 42/45 clusters/cm    0.24/0.22 mm
 2.35/2.38 e-/cluster,
 99/107 e-/cm
Drift velocity of m/ns is ‘saturated’
meaning that a small change in electric
field does not affect the time resolution.
Werner Riegler CERN, April 16th, 2003

8. Electron Drift Isochrones (ns)
2mm pitch
1.5mm pitch 5 5 10 10 15 15 20 20 25 30 25 30
Average arrival of electrons is  30ns.
It seems that time resolution can always be improved by reducing the wire pitch, BUT
Werner Riegler CERN, April 16th, 2003

9. Ionization Statistics
Because the primary ionization statistics I.e. Poisson distributed cluster number
=exponentially distributed distance between clusters around average of 0.23mm the
Time resolution doesn’t improve for pitch <1.5mm.
This is a fundamental limit of time resolution for an MWPC at 1bar.
Werner Riegler CERN, April 16th, 2003

10. Choice of Parameters and Specifications
Wire Pitch
Gas Gap
Wire Diameter
Werner Riegler CERN, April 16th, 2003

11. Werner Riegler CERN, April 16th, 2003
ra: anode wire radius
Va: anode wire voltage
: ion mobility
q: charge per hit
R: hits/cm2
1) rc=(s/2) Exp(h/s) Equivalent cathode radius
2) Ea=Va/(ra ln(rc/ra)) Anode wire surface field
3) Ec= Va/(s ln(rc/ra)) Cathode surface field
4) t0= ra2 ln(rc/ra)/(2Va) Signal tail
5) I(t)= q/2ln(rc/ra) 1/(t+t0) Induced Signal
6) Q(t)=q/(2ln(rc/ra))*ln(1+t/t0) Induced charge (ions) after time t
7) V=Rsh2q ln(rc/ra)/(4Va0) Voltage drop due to space charge effect
8) Ec/Ea= ra/s Ratio of wire to surface field (3)/(4)
9) Va= Eara (h/s+ ln(s/2  ra)) Voltage for a given surface field (1),(2)
10) G = [V/(ra ln(rc/ra)Emin)]^[Vln2/ (ln(rc/ra)dV)] Gas Gain, Emin, dV Diethorn param.
Werner Riegler CERN, April 16th, 2003

12. Werner Riegler CERN, April 16th, 2003
Choice of Wire Pitch
At fixed gap 5mm, gas gain 0.75*105 and wire diameter 30m:
Increasing the pitch mm is
HV kV/cm  good
Cathode surface field kV/cm  good
Wire surface field kV/cm 
Sensitivity to imperfections  good
Signal tail t 
Signal integrated after 10ns %  bad
Pulse width, last electron: ns 
Time Resolution  bad
1MHz (300e-/cm) V  bad
Cathode charge distribution 
Werner Riegler CERN, April 16th, 2003

13. Werner Riegler CERN, April 16th, 2003
Choice of Gas Gap
At fixed pitch 2mm, gas gain 0.75*105 and wire diameter 30m:
Increasing the Gap mm is
HV kV/cm  bad
Cathode surface field kV/cm 
Wire surface field kV/cm 
Sensitivity to imperfections  good
Signal tail t 
Signal integrated after 10ns %  bad
Pulse width, last electron: ns  bad
1MHz (300e-/cm) V  bad
Cathode charge distribution  bad
Werner Riegler CERN, April 16th, 2003

14. Choice of Wire Diameter
At fixed pitch 2mm, gas gain 0.75*105 and gap 5mm
Increasing Wire Diameter m is
HV kV  bad
Cathode Surface Field kV/cm  bad
Wire Surface Field kV/cm  good
Sensitivity to imperfections 
Signal tail t ns  bad
Signal integrated after 10ns %  bad
Pulse width, last electron: ns 
1MHz (300e-/cm) V  good
Wire Stability  good
Cathode charge distribution  -----
Werner Riegler CERN, April 16th, 2003

15. Sensitivity of the performance on chamber imperfections
The drift velocity is saturated I.e. it has a very weak dependence on the electric field. Therefore we mainly worry about gas gain variations that can move the working point within the plateau.
If G0 is the nominal gas gain, we want the gas gain in 95% of the area of a single gap to be within G0/1.25 and G0*1.25 I. e between 0.8G0 and 1.25G0.
The remaining 5% of the area should have a gain within a factor 1.5 I.e.
between 0.67G0 and 1.5G0
A gain change of a factor 1.25(1.50) corresponds to Voltage change of 34(62)V on top of the 2750V corresponding to 1.25(2.25)%.
The gas gain changes by a factor 1.25(1.5) if the wire surface field changes by 1.25(2.25)%.
 What chamber imperfections are allowed in order to keep the wire surface field within 1.25(2.25)% ?
Werner Riegler CERN, April 16th, 2003

16. Measured Efficiencies
40V
40V
60V
60V
Wires
Cathodes
Werner Riegler CERN, April 16th, 2003

17. Werner Riegler CERN, April 16th, 2003
The entire wire plane has to be
within 370(490)m in y direction.
The cathode - cathode has to be
within 113(202)m.
Werner Riegler CERN, April 16th, 2003

18. Werner Riegler CERN, April 16th, 2003
 y
 x
A single wire has to be within 350(450)m in y direction.
A single wire has to be within 163(293)m in x direction.
Werner Riegler CERN, April 16th, 2003

19. Werner Riegler CERN, April 16th, 2003
Specifications
Keeping everything else in perfect position, for a single gap chamber with 5mm gap, 30m wire and 2mm pitch, the allowed offset in order to find a wire surface field difference of 1.25(2.25)% is
gap: (202) m
single wire X: 163 (293) m
single wire Y: 350 (450) m
wire plane Y: (490) m
Which we translate into specifications of
gap: 95% in 90m 1% 5% in 180m 2%
pitch: 95% in 50m 0.35% 5% in 100m 0.7%
wire y-offset: 95% in 100m 0.1% 5% in 200m 0.4%
wire plane y-offset: % in 100m 0.1% 5% in 200m 0.42%
Added in squares: % 2.2%
Werner Riegler CERN, April 16th, 2003

20. Werner Riegler CERN, April 16th, 2003
Specifications
Moving everything in the worst direction:
gap: -90m %
wire plane: +100m %
single wire Y: +100m 1.14% 1.38% 1.14%
single wire X: -50m/0/+50m 1.18% 2.10% 1.18%
Werner Riegler CERN, April 16th, 2003

21. Werner Riegler CERN, April 16th, 2003
Guard Wire
Without a guard wire the wire surface fields and gas gains on the edge wires would be
… kV/cm
… x 105
Using a 200m diameter guard wire at the same pitch of 2mm gives
… kV/cm
… x 105
Werner Riegler CERN, April 16th, 2003

22. Wire Tension, Instability
We use a 30m wire with 60g tension (half of the elastic limit).
The maximum allowed wire length for a gas gain of 106 would be 65cm.
We use a maximum wire length of 30cm i.e. we are safe.
Wire sag is not an issue since the wires are vertical.
We specify that no wire should have a tension of less than 50g.
Werner Riegler CERN, April 16th, 2003

23. Werner Riegler CERN, April 16th, 2003
Many ‘different’ chamber types with different capacitance, grounding, signal flow …
Werner Riegler CERN, April 16th, 2003

24. Crosstalk Specification
Double gap chamber
95% efficiency if the threshold is set to 30%
of the average signal.
99% efficiency if the threshold is set to 20% of
the average signal.
In order to have a double gap chamber well
within the plateau we want to be able to use
a threshold of 15% of the average signal.
Werner Riegler CERN, April 16th, 2003

25. Specification of Capacitive Crosstalk
Crosstalk due to ‘direct induction’ is ‘irreducible’ and given by our choice of 2.5mm cathode-wire distance.
In case of a hit on a given pad, the probability that a neighboring pad fires should be < 5%.
We want to be able to operate the chamber at a threshold of 15% of the average muon signal.
This specifies the ‘crosstalk ratio’ I.e. the fraction of signal allowed signal on a neighbor pad:
Assuming 95% of the signals within 3x average the crosstalk ratio has to be < 15%/3 = 5%.
Werner Riegler CERN, April 16th, 2003

26. Werner Riegler CERN, April 16th, 2003
Pads, Readout Traces
0.5mm guard trace between pads
0.4mm gap between pad and guard trace
1.6mm boards in case of traces
0.8mm board otherwise
0.25mm readout traces
0.25mm guard traces at 0.25mm pitch
We carefully studied readout and guard
trace geometries to minimize crosstalk.
Calculations were done with MAXWELL.
Werner Riegler CERN, April 16th, 2003

27. Werner Riegler CERN, April 16th, 2003
Crosstalk Simulation
We modeled each chamber
including all pads,
mutual capacitances,
amplifiers …
Werner Riegler CERN, April 16th, 2003

28. Werner Riegler CERN, April 16th, 2003
Crosstalk Simulation
pF Cathode Pad Capacitance
pF Wire Pad Capacitance
maximum pad-pad capacitance
3.95 pF
maximum crosstalk
2.27 %
maximum opposite sign crosstalk %
Werner Riegler CERN, April 16th, 2003

29. Crosstalk Measurement
Crosstalk on M2M3R1 prototype at very high gas gain (1.8 x 105) is still <5%.
Werner Riegler CERN, April 16th, 2003

30. Werner Riegler CERN, April 16th, 2003
Crosstalk, Stability
We have a very good clue and understanding of the ‘smooth RC’ behavior of the chambers.
Since we have large capacitances (up to 220pF), even very small voltage drops due to parasitic inductances and ‘bad’ grounding can cause problems.
E.g. a 20V signal on the ground together with a 200pF capacitance fires the 5fC threshold.
Although we had every chamber type in stable conditions in a testbeam the frontend-board to chamber connection has still to be properly engineered.
Werner Riegler CERN, April 16th, 2003