development of micro-pattern gaseous detectors – gem - LMU

Chapter 4
Energy Resolution and Pulse Height
Analysis
This chapter addresses the analysis of measurements of cosmic muons as well as X-rays from a 55 Fe
source. At first the resolution of pulse spectra resulting from a 55 Fe source is documented as it is
obtained by prototype 1.0 for different anode geometries recorded by various preamplifiers. As Ch.
4.2 illuminates, the resolution was found to depend on the source position. The following section
about the energy distribution for cosmic ray muons completes the topic of energy resolution of the
prototype 1.0. Ch. 4.4 treats the charge transfer processes in a GEM and the impact of the electric
fields on it. Detailed measurements regarding the pulse height dependence on the electric field of the
induction gap, Eind, can be found subsequently. After the installation of a five-fold segmented anode,
we describe in Ch. 4.5 changes of the pulse height as a function of segmentation.
4.1 55 Fe Pulse Spectra and Energy Resolution
4.1.1 Spectrum Recorded with Unsegmented Anode Readout via CATSA82 Preamplifier
As in Ch. 1.1.2 already covered, the photons generated by the 55 Fe decay carry mainly an energy of
Eγ = 6.50 keV, 5.90 keV and 2.68 keV. They interact with gas molecules due to the photoelectric
effect. Recording these peaks in the spectrum of 55 Fe is the motivation of the following measurement
in order to state the energy resolution of the triple GEM.
The source is placed centrally above the active area as described in Ch. 3.1 and readout via the first
trigger line with self-triggering detector. The lower scintillator (Sc1, cf. Ch. 3.4.1) is used as a
veto-trigger for discriminating accidental recording of cosmic muons. In Fig. 4.1 the pulse height
spectrum of 55 Fe is plotted as taken with the first setup of prototype 1.0, that is including an unsegmented
Cu-anode and readout via the CATSA82 preamplifier. The peak on the left is due to remaining
cosmics which traverse the detector under a large zenith angle without hitting the bottom scintillator.
Their spectrum is fitted with a Landau distribution and will be reviewed in upcoming sections.
The peaks of interest are the central and the right ones which represents the characteristic escape
peak, the Kα line and the K β peak, respectively. For stating quantitatively the energy resolution, three
Gaussian functions are fitted to the histogram:
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