development of micro-pattern gaseous detectors – gem - LMU

52 Chapter 4 Energy Resolution and Pulse Height Analysis
observes a greater pulse height for all three configuration at Edri ft = 1.25 kV/cm. This is followed by
a slight decrease going to higher drift fields. One assumes that this may be due to electric field lines
ending on the upper electrode of GEM3 preventing them from being amplified in the GEM foils.
pulse height [0.244 mV]
22
20
18
16
14
12
10
8
6
4
Cosmics Pulse Height vs E
drift
Δ UGEM3
= 330 V
Δ UGEM2
= 290 V
Δ UGEM1
= 320 V
Cu anode unseg
preamp: CATSA82
Ar:CO2
93:7 @ 1 bar
cm
kV
Etrans1,2
= Eind
= 1.30
cm
kV
Etrans1,2
= Eind
= 1.70
cm
kV
Etrans1,2
= Eind
= 2.00
0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Edrift[kV/cm]
Figure 4.19: MPV of the Landau spectra as a function of Edri ft at constant Etrans1,2 = Eind
This may be just a part of the answer since the physical processes in a triple GEM detector are very
complicated [Buzu 02]. Nevertheless, these effects result only in a small variation of 5% in the pulse
height concerning measurements at constant Eind = Etrans1,2 = 1.7 kV/cm.
4.5 Pulse Height Dependence on Strip Readout
The five-fold segmented anode was used to study the influence of strip size on the pulse height of the
anode signals. Five data points were taken when one, two, three, four and all five strips were readout
by one preamplifier. The dimension of a single readout strip is shown in Fig. 4.20. We used the
55 Fe source located at the central position and recorded data samples with all three preamplifiers. The
readout copper strips of the five-fold segmented anode were coupled together while open strips were
connected with a resistor of 100 Ω to ground as shown in Fig. 4.20. Fig. 4.21 repeats schematically
the capacities and charge flows necessary for the following calculations. Foremost, data samples with
40000 events taken with the CATSA82 preamplifier can bee seen in Fig. 4.22. For comparison, the
operation parameters were chosen to be identical to a setup with unsegmented anode (round data point
in Fig. 4.22). The pulse height is decreasing as more strips are readout being coupled together. This
results in a 50% smaller peak for five stripr being read out compared to only one strip-readout. The
unsegmented anode shows the lowest pulse height of the 55 Fe spectrum.
One observes for all preamplifiers a decreasing pulse height with increasing strip number, see
Fig. 4.23. This plot shows measurements at identical operation parameters recorded once with the
Canberra preamplifier and repeated with the ELab version.
The detected charge Qtot flows from the anode (Cdet) to the coupling capacitor Ccoup. At equilibrium
the voltage on Cdet and Ccoup is identical: