development of micro-pattern gaseous detectors – gem - LMU

54 Chapter 4 Energy Resolution and Pulse Height Analysis
Uin = Qdet
Cdet
= Qcoup
Ccoup
(4.11)
where Qdet is the part of the total charge Qtot that remains on the anode. Therefore the total charge
that was generated by primary ionizing particle and amplified by the three GEM foils, is divided in:
Qtot = Qdet + Qcoup
from these two equations one derives an expression for the charge on the coupling capacitor:
Qcoup = Qtot
Cdet
Ccoup
Uout
=
+ 1 FCV
(4.12)
(4.13)
The pulse height Uout is connected to the fraction of charges Qcoup via the charge-to-voltage conversion
factor FCV of the charge sensitive preamplifier. The Canberra has a conversion factor of
FCV = 1 V/pC and the conversion factor of the ELab preamp is 0.83 ± 0.04 V/pC (see Ch. 3.4.3).
The amplification factor of the CATSA82 preamplifier has not been determined since its coupling
capacity Ccoup = 50 nF results in less gain.
As the flash ADC records data in a 12 bit range with an acceptance of ±0.5 mV one can identify a
scaling factor mADC:
mADC = 0.244
mV
ADC channel
Using this we transform from the recorded pulse height to the detected charges Qcoup.
charge [pC]
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Δ Ugem1=
315 V
Δ Ugem2=
335 V
Δ Ugem3=
355 V
Qcoup
cm
kV
Edrift
= 1.25
cm
kV
Etran1,2
= 2.00
Eind
vs # Strips
cm
kV
= 1.67
χ 2 / ndf 2.842 / 3
p0 0.4629 ± 0.06472
p1 0.5055 ± 0.1228
Ar:CO2
93:7 @ 1 bar
Canberra 2004
p0/(p1*x+1)
0
0 1 2 3
# strips
4 5
p0/(0.0876*x+1.124)
(4.14)
Figure 4.24: Amount of collected charge that reaches the preamplifier as a function of the number of readout
strips being coupled together. The decreasing pulseheight is due to increasing detector capacity as a function
of readout strips.
In order to proof equation 4.13 the data taken with the Canberra2004 preamplifier is transformed as
it is shown in Fig. 4.24. A fit function with two free parameters is applied to the data (red line in
Fig. 4.24) and yields a strip capacity C PCB
strip of: