1. CMS GEM Workshop III, April 20, 2012
A potential cost saver for the CMS GEM upgrade: GEM readout with zigzag strips Marcus Hohlmann Florida Institute of Technology
CMS GEM Workshop III, April 20, 2012

2. GEM Workshop III - M. Hohlmann
Motivation
Cost is one of the biggest current issues for the CMS GEM upgrade project
The single-largest budget item in the current GEM budget estimate is electronics:
~3M out of 6.4M total (TP CMS IN 2012/001)
Clearly, we should seriously look for ways to reduce the electronics cost if possible
One potential real cost saver would be the
significant reduction of readout channels
 Can a zigzag strip readout help with that?
4/20/2012
GEM Workshop III - M. Hohlmann

3. GEM Workshop III - M. Hohlmann
Previous BNL
Zigzag strips:
Previous exp. studies show <100 µm resolution with 2 mm strip pitch is possible: y
ava-lanche
BNL
x (measured coordinate)
Concept:
Charge sharing among adjacent strips allows quite sensitive position-interpolation in x-direction
We are sacrificing the measurement of the 2nd coord. (y) to gain precision in the 1st coord. (x)
CMS GE x/1 detectors are currently intended for 1D-coordinate measurements, so the zigzag approach is applicable to these detectors
4/20/2012
GEM Workshop III - M. Hohlmann

4. Zigzag strips vs. straight strips
Pitch [mm]
Typical Resolution
[µm]
Zigzag strips & analog r/o
2.0 80
Straight strips & VFAT
(current design, short end)
0.6
300
Improvement factor
w/ zigzag strips
3.33
3.75
Can reduce # of
readout channels
(and electronics
cost) by 70% of
current design
&
Improve resolution
by factor 3-4
A “figure of merit”: × 3.75 = 12.5
~ Potential for order of magnitude improvement over current design
 Well worth a try!
4/20/2012
GEM Workshop III - M. Hohlmann

5. GEM Workshop III - M. Hohlmann
Zigzag r/o PCB (10cm × 10cm)
2 sets of 10cm zigzag strips with different zigzag pitch (along strip)
PCB with
solder finish
24 fine
zigzag
strips
24 coarse
zigzag
strips
CAD Design by C. Pancake, Stony Brook
2 mm
0.5 mm
1 mm
LEMO footpads
APV25
Hybrid
(using only 38%
of 128 ch.)
+ SRS
Readout Board
for std. CERN
10cm×10cm
Triple-GEM Detector
BNL/FIT/Stony Brook Collaboration
Solid ground plane on the back side.
4/20/2012
GEM Workshop III - M. Hohlmann

6. 2nd batch: Gold plating finish
Ordered second batch with gold plating finish because hot-air solder leveling (HASL) finish shows non-uniform surfaces:
HASL
Much higher
surface
quality
Au plated
4/20/2012
GEM Workshop III - M. Hohlmann

7. GEM Workshop III - M. Hohlmann
What about noise?
 First commissioning step addressed noise question:
Measure pedestals and compare with regular straight strips
Does the noise increase substantially due to the
increased strip area and capacitance?
4/20/2012
GEM Workshop III - M. Hohlmann

8. Pedestal Distributions
Readout Board with 10cm long zigzag strips covered with one GEM foil placed 2 mm above induction plane:
# ev.
per
ch.
straight
strips
ADC channel
APV ch. 63
(example)
pedestal width: r.m.s 3 → 4 cts.
# ev.
per
ch.
zigzag
strips
APV25 Hybrid
(using only 38%
of channels)
+ SRS
ADC channel
4/20/2012
GEM Workshop III - M. Hohlmann

9. Pedestals for all channels
Readout Board with 10cm long zigzag strips covered with one GEM foil placed 2 mm above induction plane:
Pedestal width (rms)
[ADC counts]
48 channels connected
to zigzag strips (24+24)
Channels connected
to LEMO footpads (5)
channels not connected
(open APV inputs)
APV channel number
4/20/2012
GEM Workshop III - M. Hohlmann

10. First pulses with zigzag r/o
GEM stack being
assembled on
zigzag r/o board
Completed Triple-GEM with zigzag
r/o board under gas, HV, and x-rays (Cd109)
Strips ganged together using adapter
board plugged into a
Panasonic connector
Pulses recorded
with ORTEC
preamp and
(slow) lin. amp.
Currently setting
up for cosmics test
with APV + SRS…
4/20/2012
GEM Workshop III - M. Hohlmann

11. GEM Workshop III - M. Hohlmann
30cm × 30cm zigzag PCB
Plans for next design:
Design and build a zigzag readout pcb for the 30cm × 30cm self-stretch CMS GEM (on order w/ Rui)
128 zigzag strips of 10cm length covering the full 30cm width & read out by a single APV25 hybrid + SRS
Test also 30cm long strips on same board (given the good noise performance of the 10cm long strips)
Photo-montage
~ 300 mm
4/20/2012
GEM Workshop III - M. Hohlmann

12. Impact on electronics design
 Potential for removing the need for 2/3 of the electronics of current baseline design:
from Paul
Aspell’s
talk on Thu. 1
4/20/2012
GEM Workshop III - M. Hohlmann

13. GEM Workshop III - M. Hohlmann
Implications
Potential for saving 70% of readout channels and ~50% (?) of electronics cost, could mean potentially saving MCHF on the project
Simplifies also cooling, cabling, power, etc., which would lead to additional cost savings
Total project cost < 5 MCHF possible ?!
Analog pulse height measurement is mandatory for charge interpolation, so VFAT3 would not work → need a GdSP design
4/20/2012
GEM Workshop III - M. Hohlmann

14. GEM Workshop III - M. Hohlmann
Conclusion & Outlook
Noise increases by only 1.5 ADC counts (~ 0.1% of range) for zigzag strips compared w/ straight strips. Encouraging!
Can potentially read out one GE1/1 strip row with single 128-ch. front-end hybrid while improving spatial resolution; could lead to substantial channel number & cost savings.
Next:
Measure spatial resolution using tracked cosmics
Test zigzag prototype in 2012 test beam
Design, produce & test larger prototype boards
4/20/2012
GEM Workshop III - M. Hohlmann