development of micro-pattern gaseous detectors – gem - LMU

40 Chapter 4 Energy Resolution and Pulse Height Analysis
1
G(x, µ, σ) =
σ · √ 2π
· exp −(x − µ)2
2σ 2
where µ identifies the mean and σ the standard deviation of the distribution [Leo 94]. The total fit
function is the sum of the Landau function together with the two Gaussians for the 55 Fe spectrum. It
is indicated by the black envelope.
For calculating the energy resolution dE/E at full width of half maximum (FWHM), as it is commonly
used, one needs to transfer σ to full width at half maximum:
(4.1)
FWHM = 2 · σ · √ 2 ln 2 = 2.35 · σ (4.2)
Fig. 4.1 yields a dE/E = 22.5% for the first measurements. The K β - peak that underlies the dominant
Kα line can be seen on the right shoulder.
1000
900
800
700
dE
| = 0.2246
E Kα
Cu anode unseg
preamp: CATSA82
cm
Δ Ugem3=
330 V
Δ Ugem2=
290 V
Δ Ugem1=
320 V
data_gem_0089_pulseheight_fit
Entries 38413
Mean 115.4
RMS 42.7
χ 2 / ndf 1136 / 263
p0 1621 ± 37.6
p1 35.22 ± 0.13
p2 2.808 ± 0.070
p3 137 ± 2.5
600
p4
p5
70 ± 0.0
18.56 ± 0.32
500
K α norm
K α mean
729 ± 6.5
133.5 ± 0.1
K α σ 12.73 ± 0.10
400
p9
p10
55.07 ± 2.45
160 ± 0.1
p11 15 ± 0.0
kV
Edrift
= 1.25
cm
kV
Etrans1,2
= Eind
= 2.00
300
200
100
55Fe Pulse Height
muon
Mn K
Mn Kα
Mn K
α esc.
0
0 50 100 150 200 250 300
voltage [0.244mV]
Figure 4.1: Pulse spectrum of 55 Fe detected by the prototype 1.0 equipped with an unsegmented Cu anode. An
energy resolution of 22.5 % is observed for a centrally placed irradiation source. The escape peak lies around
ADC channel 70, the Kα peak around 133. The K β peak is shown on the right shoulder at channel 160.
4.1.2 Energy Resolution with a Five-Fold Segmented Anode
In order to compare results from the unsegmented anode with results from the five-fold segmented
anode, the latter is readout via one signal line, i.e. all five strips are shorted to one readout and
fed to the homemade preamplifier by the LMU ELab. Furthermore, the uppermost two GEM foils
were exchanged to new ones resulting in a higher applicable potential difference and gain. The same
electric fields were applied to the gaps to guarantee equal drift properties. The energy resolution of
the thus rebuilt detector shows a slightly better performance in terms of resolution (Fig. 4.2). The
spectrum is shifted by approximately a factor of 5 to higher values due to a greater amplification
β