1. Mirco Christmann MAGIX Collaboration Meeting 2017
Construction and characterization of a GEM prototype detector for MAGIX
Mirco Christmann
MAGIX Collaboration Meeting 2017

2. Outline detector challenges construction of a GEM prototype detector
data acquisition
laboratory measurements
measurements in the electron beam of MAMI
summary and outlook
short review of detector challenges for MAGIX and working principle of GEM detectors
GEM detectors ideally suited for the MAGIX experiment
proceed
delivery  prototype detector
introduce the system for data acquisition
Iron source and cosmics
beamtime end of June
results relating to efficiencies

3. Detector challenges precise detector system
active area: 1.20 x 0.30 m2
spatial resolution < 50 μm
multiple points for track
reconstruction
high rate capability O(1 MHz)
alternative to drift chambers
Where we can not get the highest counting rates and the best spatial resolution at the same time
A possibility to full-fill this detector challenges are micro structured gas detectors

4. GEM detectors they satisfy this criteria
GEM foils have a low radiation length as well
high electric fields in the holes e- avalanche
high counting rates and spatial resolution reached routinely
working principle
pitch 140 micrometres
Amplification foil
double conical form
thin structure

5. Construction of a GEM detector
stretching and framing the foils
condition at delivery
active area: x 10 cm2
electrical contacts
leakage current < V
no frame yet
nitrogen  dry/ clean
electrical contacts on both sides

6. Construction of a GEM detector
stretching and framing the foils
coefficient of thermal expansion is different
cooled frame for stretching
laboratory oven
stretching
thermal
0.07 mm per m and °C  three times larger
Plexiglas
about one hour at 50-60°

7. Construction of a GEM detector
stretching and framing the foils
glue a fibreglass frame on the foil
cures in the laboratory oven
cut into shape
check leakage current again
quickly
12 hours at 50-60°

8. Construction of a GEM detector
stretching and framing the foils
glue a fibreglass frame on the foil
cures in the laboratory oven
cut into shape
check leakage current again
ready for assembling
do this for at least 3 gems and one cathode because we work with triple-gem-detectors

9. Construction of a GEM detector
profile measurements of the foils
evaluate the stretching procedure
laser sensor
plane table
curvature much lower than for pre-framed ones
how good our … is
foil fixed on Al-block
moved in a grid to measure the distance
raw data + calibration matrix from measurement without foil
distinguish between foil and rest  planar and quadratic fit
Good tool for larger foils in the future

10. Construction of a GEM detector
additional detector components needed
512-channel readout board
512 crossed copper stripes
how to read out the data  topic of next chapter

11. Construction of a GEM detector
additional detector components needed
512-channel readout board
chamber (frame and cap)
high voltage connection box

12. Construction of a GEM detector
additional detector components needed
512-channel readout board
chamber (frame and cap)
high voltage connection box
gas system
prevent to have air in the chamber
In this basic setup  system for data acquisition still missing

13. How to readout data? SRS (Scalable Readout System): data acquisition
APV cards (128 channels each)
Frontend chip
Need an external trigger signal
scalability

14. How to readout data? SRS (Scalable Readout System): data acquisition
APV cards (128 channels each)
scintillators for trigger signal
Frontend cards of the SRS
Max. current of PMTs 200 micrometres  relevant in later parts of this talk

15. How to readout data? SRS (Scalable Readout System): data acquisition
APV cards (128 channels each)
scintillators for trigger signal
APV signals
① synchronisation peaks
② separation of timing frames
③ signal in a timing frame
Take waveforms with a rate of 40 MHz
Time frame adjusted to the signal length
To make it more stable neighbouring channels

16. How to readout data? SRS (Scalable Readout System): data acquisition
APV cards (128 channels each)
scintillators for trigger signal
APV signals
1-d hit position
hit time
Now we have all tools to start measurements and analyse the results.

17. Lab measurements basic tests with more simple setups energy resolution
55Fe-Spectra
photo peak
5,89 keV
escape peak
2,9 keV
used only one strip  moved low energy iron 55 source over it  counted the pulses
one measured the deposited charge of a hit with a QDC
charge spectrum
electron capture
FWHM: 16%

18. Lab measurements basic tests with more simple setups energy resolution
55Fe-Spectra
gain curve (vary UGEM)
GEM voltages between 375 and 390 volts
!!!Expected this behaviour!!!
detailed study of the gain of each electronic part  real electron gain could be specified

19. Lab measurements basic tests with more simple setups energy resolution
55Fe-Spectra
gain curve (vary UGEM)
homogeneity test
cosmics
less events at the edges
geometry of the setup
First lab measurement with SRS
Cosmics suited perfect
Coincidence setup of two scintillators
Explained by exp. Setup:
-area of the scintillators covers only active GEM area
-distance between scintillators
-cosmics: angle not vertical
apart from that: results very homogenous
from electronic side: all channels work

20. Lab measurements basic tests with more simple setups energy resolution
55Fe-Spectra
gain curve (vary UGEM)
homogeneity test
cosmics
less events at the edges
geometry of the setup
16 by 16 channels taken together
find out if there are regions where the gas amplification of the GEM‘s is not so good
the validity of this measurement is not so high, because it included only about 30,000 events
in the electron beam of MAMI we get a much higher rate

21. MAMI beam time @X1 28/29th of June 2016 behind HV-MAPS
detector on xy-table, scintillators fixed
up to 350 fA
855 MeV
setup located
beamtime was no dedicated beamtime for MAGIX
High Voltage Monolithic Active Pixel Sensors
upcoming beamoptimization
Wehnelt voltage was varied
But before we come to the results I will show you the tools for data filtering

22. Filtering the raw data filter out the noise pulse width at 20% of Qmax
aim is to distinguish between signals, which belong to the ionisation… and noise signals
need suitable filter criteria for the raw data
therefore we have a look at the individual pulses again

23. Filtering the raw data filter out the noise pulse width at 20% of Qmax
distribution of pulse widths [25 ns]
done for all pulses
… is created
steps are 25ns
minimum after first 2 bars, could be a good cut level (pedestal peak?)

24. Filtering the raw data (A) width 1x25 ns (C) width 10x25 ns
confirmed if we have a closer look at typical waveforms for specific pulse widths
(A) No waveform  single short pulses are spread statistical  in average: a slight shift to the baseline
(C) Single pulses correlate with the trigger signal; light blue regions: two peaks in the average shape  probably belong to double hits
waveforms can be explained by two hits in a short time frame and they could cause a different number of electrons to drift in the direction of the readout board
(B) width 4x25 ns
(D) width 14x25 ns

25. Filtering the raw data filter out the noise pulse width at 20% of Qmax
distribution of pulse widths [25 ns]
B+D: mainly double hits
A: mainly noise
second filter: charge
(A) Part is mainly shock noise and should be filtered out
charge filter: in some cases necessary, but in most cases done by the SRS via a Pedestal File and a Zero Suppression Factor
Pedestal Run: charge sensibility of each channel
ZSF defines minimum charge of each channel
With this filter tools we can determine the efficiencies of the measurements taken at our beamtime

26. Results: High Rate Tests
6 series of measurement:
MHz
limited by trigger system
electron detection at 1 MHz with efficiency >99.9%
at maximum rate: efficiency >99.5%
requirement full-filled
look at the remaining events  efficiency
High rate capability order of 1 MHz

27. Results: XY-Scan 25 series of measurement:
11.4 kHz, raster pitch 2 cm
9 measurements in the middle:
99.85±0.04% efficiency
lower edges
efficiency is plotted as a colour scale
each square stands for one measurement
expected, because some electrons of the beam already hit the GEM frames
field distortions
but why is efficiency so much lower on the left and also in the lower part?

28. Results: XY-Scan 25 series of measurement:
11.4 kHz, raster pitch 2 cm
9 measurements in the middle:
99.85±0.04% efficiency
lower edges
alignment of detector
shift in beamspot positions
I had a look at the beam positions of each measurement
detector was not aligned perfectly  should be improved in following beam times

29. Summary & Outlook stretching and framing procedure
profile measurements uniform result
readout data: 512 crossed copper stripes SRS
filter data by pulse width
high rate tests efficiency 1 MHz
XY-Scan ±0.04% efficiency in the middle
physical introduction
proofed: stretching procedure is suited
assembling of the detector
first tests with iron source and cos. in lab
@ 20% of the charge maximum
able to calculate the efficiencies of some MAMI
and a reduced efficiency at the edges due to the GEM frames

30. Summary & Outlook go bigger go thinner 30 x 30 cm2 GEMs are ordered
same procedure (stretching, framing, …)
go thinner
thinner GEMs are ordered (10x10 cm²)
foil based readout (Master Thesis: Yasemin Schelhaas)
may improve it a little bit
reduce the material budget
in particular the 5 um copper coating
for the readout structure a foil based readout is also planned. And there Yasemin will continue with her talk.