development of micro-pattern gaseous detectors – gem - LMU

Chapter 6
Rise Time Studies
For a better understanding of the signal creation in the GEM detector the rise time was studied
in dependence on the operational parameters. This chapter starts with the time resolution of the
prototype 1.0 and shows afterwards measurements done for electric field variations in the gap regions.
6.1 Time Resolution
The rise time can be determined via the inverse Fermi fit to the rising edge of the signal (10% − 90%
of the pulse maximum), as presented in Ch. 3.5. For investigation of the time resolution of the
detector a localized primary charge distribution in the drift region is necessary. This includes
analization of the electronics response using a temporarly defined pulse. For this analysis the charges
generated by X-rays from the 55 Fe source can be used. Since the primary produced photoelectron
loses its energy due to ionization of further atoms in a small range we can approximate the pulse as
punctual in time and location. Fig. 6.1 and Fig. 6.3 show that such a point-like distributed charge
generates a rise time of approximately 166 ns and 102 ns for signals recorded via the preamplifiers
CATSA82 and ELab, respectively. The Gaussian fits to the histogram of 55 Fe measurements present
therefore good resolution, in addition as the detector is self-triggering. A resolution of these rise
times as sharp as 7% and 2% for the CATSA82 and ELab preamplifier, respectively, is thus given by
default.
The cosmic ray muons, instead, produce electron-ion-pairs along their track while traversing the
detector. Thus the charge produced by muons that is responsible for the signal creation is distributed
over the whole drift region. The time between the first charges reaching the anode and the last depends
on the drift velocity and the size of the drift gap.
Fig. 6.2 and 6.4 show the time resolution of exemplary data samples of cosmics recorded with the
CATSA82 as well as the ELab preamplifier. The rise time resolution is 26% and 20% for the individual
preamplifiers.
The drift velocity of the electrons was determined by a GARFIELD/MAGBOLTZ simulation. In
this simulation a parallel plate field is assumed ignoring the field configurations near the GEM
foils (see Fig. 6.5). The program calculates the drift velocity in a given gas mixture, pressure and
temperature for the indicated geometries. This results, for instance, in a drift time for electrons from
the upper end of the drift region to the uppermost GEM foil of tdri ft = 99.5 ns for a electric field of
Edri ft = 1.25 kV/cm, as it is drawn in Fig. 6.6.
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