development of micro-pattern gaseous detectors – gem - LMU

4.3. Cosmic Muon Spectrum 45
4.3 Cosmic Muon Spectrum
The 55 Fe source served well for determination of the energy resolution since it provides a discrete
line spectrum for calibration. However, X-rays do not provide an external trigger and therefore can
not be used to evaluate, for example, the efficiency (see Ch. 5) of the triple GEM detector.
Looking for sources with higher energies, the omnipresent cosmic ray muons are a favorable choice
for further detector tests. Their energies are high enough to trigger the scintillation counters while
still penetrating the detector. This section illustrates the characteristics of cosmic muons as minimum
ionizing particle (MIPs) and reports on measurements of their spectrum with the prototype 1.0.
4.3.1 Pulse Height Spectrum of Cosmics
4000
Cu anode unseg
Δ Ugem3=
360 V
data_gem_0062_pulseheight_fit
3500
preamp: CATSA82
Δ Ugem2=
340 V
Δ Ugem1=
320 V
Entries
Mean
36697
27.85
3000
2500
2000
1500
1000
500
Cosmics Pulse Height
cm
kV
Edrift
= 1.25
cm
kV
Eind
= Etrans1,2
= 2.00
RMS 20.42
χ 2 / ndf
132.3 / 92
Prob 0.003768
Constant 1.732e+04 ± 1.491e+02
MPV 14.79 ± 0.12
Sigma 4.535 ± 0.037
Landau
0 20 40 60 80 100 120
voltage [0.244mV]
Figure 4.8: Pulse height spectrum of cosmic muons recorded with prototype 1.0 and single plane readout.The
spectrum is cut off due to preamplifier performance.
Muons account for 80% of all charged cosmic ray particle at ground level [Grup 08]. A minimum
ionizing muon, that is a particle of βγ ≈ 4, carries an energy of Eµ ≈ 420 MeV. In a gaseous environment
of Ar/CO2 at a fraction of 93/7 this muon creates approximately 97 primary electron-ion-pairs
per cm due to inonization [PDG 10]. According to this and considering the dimension of the detector,
about 39 primary electrons are realeased by a cosmic ray muon that penetrates the drift region
perpendicular to the active area. Since the angular distribution of cosmic ray muons is proportional to
cos 2 θ, with θ being the zenith angle, this is the most probable value to assume [PDG 10]. For a given
maximal incident angle of θ = 63 ◦ , that is defined as the zenith angle of a muon hitting the inner
edges of both scintillators (see Ch. 5.1), this number increases to approximately 86 charges in the
drift gap. As discussed in Ch. 1.1.1 this calculation holds for the average energy loss
dE
dx which is
denoted by the mean in the presented histogram. As the prototype 1.0 contains a drift gap of 4 mm, it
can be treated as a thin absorber and a Landau distribution is used to describe the muon’s energy distribution
(see Ch. 1.1.1). Fig. 4.8 illustrates such a distribution as it is taken with the first detector setup.
The data taking of cosmics with an equivalent sample size compared to 55 Fe X-rays is achieved during