development of micro-pattern gaseous detectors – gem - LMU

46 Chapter 4 Energy Resolution and Pulse Height Analysis
overnight measurements. This is due to the fact that the muon flux rate of 1cm −2 · min −1 [Grup 08]
is rather small and results in a detection frequency of approximately 0.7 Hz.
From Fig. 4.8 it is obvious that the Landau function fitted to the histogram is in good agreement
with the data. However, one can observe that the spectrum is cut off at low energies. This is an
artefact of the preamplifier CATSA82 which amplifies the signal not high enough. The discriminator
in the hardware readout chain had to be set at a threshold that prevented signals corresponding to a
low energy loss of the muon from being detected. Therefore a complete analysis of the energy loss
could not be accomplished. With the implementation of another preamplifier (ELab) this problem
was solved and allows for further studies in the next section.
4.3.2 Complete Pulse Height Analysis of Cosmic Signals
By exchanging the preamplifier in the signal readout a full spectrum of energy loss distribution
of cosmic ray muons is detectable. During the implementation of the ELab preamplifier also the
five-fold segmented anode (cf. Ch. 3.1.1) was embedded. Fig. 4.9 shows an exemplary data set of a
full energy loss spectrum.
100
90
80
Cu anode seg
preamp: ELab
Δ Ugem3=
360 V
Δ Ugem2=
340 V
Δ Ugem1=
320 V
cm
data_gem_0102_pulseheight_fit
Entries 10000
Mean 118.2
RMS 76.18
χ 2 / ndf 503.9 / 335
70
Prob
Constant
6.199e-09
361.1 ± 5.9
MPV 70.22 ± 0.69
Sigma 26.65 ± 0.39
kV
Edrift
= 1.25
cm
kV
Eind
= Etrans1,2
= 2.00
60
50
40
30
20
10
Cosmics Pulse Height
Landau
0
0 50 100 150 200 250 300 350
voltage [0.244mV]
Figure 4.9: Pulse height spectrum of cosmic muons. Recording via ELab preamp. Full spectrum detectable.
The green line corresponds to a Landau fit [Kolb 84]. Its quality is indicated by the χ 2 /nd f - value
which should be close to 1 for a meaningful fit. Recent studies provide a different parametrization
for the energy loss distribution for thin absorbers which differs from the commonly applied Landau
distribution. The differences between the original Landau function and the fluctuations including
“straggling function” (Landau-Bichsel-distribution) [Bich 06] can be seen in Fig 4.10. The solid line
shows the corrected energy loss for minimum ionizing particle (βγ = 3.6) penetrating through a
1.2 cm thin layer of Argon; whereas the dotted line indicates the original Landau function for such
particle. The most probable value (MPV) is slightly shifted to lower energies and the distribution
extents to zero. In comparison to that the classical Landau distribution provides a smaller range of
energy loss and a shorter tail. Comparing this with the experimental results it can be stated that the
Landau-Bichsel function would describe the measurements better since the distribution also extends