development of micro-pattern gaseous detectors – gem - LMU

3.5. Methods of Signal Analysis 37
3.5 Methods of Signal Analysis
Signals by the triple GEM detector and recorded with the flash ADC are processed for further studies
with the following software algorithm : an inverse Fermi-function is fitted to the rising edge:
f (x) =
par0
exp par1−x
par2
+ par3 (3.6)
which can be seen in Fig. 3.12. Due to the characteristics of the flash ADC, namely its 12bit range and
1 GHz sampling frequency, the axes scaled with a factor mADC = 0.244 mV and 1 ns, respectively.
The behavior of the fit function is determined by four parameters, starting with par0 that reflects the
maximum of the signal, i.e. the pulseheight.
par 3
par 0
par 2
par 1
> 3 σ max
mean 1
mean 2
Figure 3.12: A typical signal generated by 55 Fe in the GEM detector and recorded with the FADC. The inverse
Fermi-function is shown in red, fitted to the rising edge.
The second parameter par1 provides the inflection point of the pulse and therefore a possibility for
calculating the exact event time. In the measurements presented in this work, the signal time should
stay constant, as for cosmics the implemented trigger scintillator deliver a fast signal. This holds also
for the 55 Fe source measurements since in this case the signal is triggering its acquisition.
In the inverse Fermi-function the par2 represents the rise time of the signal. Solving equation 3.6
for given values of 10% and 90% of the maximum pulseheight, one gets a factor of 4.4 which has to
multiplied with par2 for receiving the signal’s risetime in this range. The added forth parameter par3
corresponds the offset of the preamplifier. In order to record a complete pulse spectrum for measurements
of cosmic muons including also very small signals, every pulse passing the discriminator in the
hardware chain was recorded without any further software discrimination since every signal triggered
by the scintillators can be considered a real event.
This is not necessarily true for X-ray measurement, where no scintillators are triggering but the first