development of micro-pattern gaseous detectors – gem - LMU

Chapter 7
Gas Gain Studies
This chapter describes the gas gain of prototype 1.0 filled with a gas mixture of Ar/CO2 at the ratio
93/7.
7.1 Gain Studies for Ar/CO2 at 93/7
The gas gain G is defined as the ratio of total created charge over primary charge in the detector.
Due to recombination and electron transparency it is possible that not all liberated electrons in the
triple GEM detector reach the anode. Furthermore, not all charges that reach the anode contribute
to the recorded pulse due to voltage equalization on Cdet and Ccoup (see Ch. 3.4.3). Thus it is more
reasonable to use the expression effective gas gain Ge f f [Bach 99]:
G := Ntotal
Nprimary
= e αx
chargelosses
−→ Ge f f = Nsignal
Nprimary
where Nsignal corresponds to the fraction of created charge Qcoup reaching the coupling capacitor Ccoup
of the preamplifier:
Nsignal · e =: Qcoup
with the elementary charge e = 1.6 × 10 −19 C. The gain depends on various parameters as the
gas pressure and temperature, the gas itself and on the applied voltages as well as the geometry of
the GEM holes. As stated in Ch. 1.2.4 the gain is exponentially depending on the thickness of the
amplification gap x in Eq 7.1. The first Townsend coefficient α is directly proportional to the applied
fields [Leo 94]. To determine the effective gain we used the Kα line of the 55 Fe source. The amount
of detected charges Nsignal is taken from the mean value of the Gaussian fit to the histogram (cf. Ch.
4.1):
Nsignal = Uout
FCV · e = Kαmean · mADC
FCV · e
where FCV is the charge-to-voltage conversion factor, i.e. the internal amplification of the preamplifier
and mADC the scaling factor of the flash ADC, 0.244 mV per ADC channel. The estimated error
∆Nsignal corresponds to the standard deviation σ of the fitted Kα peak in the same way.
Data were taken with a centrally placed irradiation source and readout via the Canberra2004 preamplifier
with the tabulated conversion factor FCV = 1 V/pC. The temperature and pressure were held
constant at T = 296 ± 1 K and p = 1020 ± 10 mbar, respectively. It can be seen that the effective
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(7.1)
(7.2)
(7.3)