development of micro-pattern gaseous detectors – gem - LMU

68 Chapter 6 Rise Time Studies
2 ns
> 10 ns
Channel 1) Preamplifier response to 2ns pulse
Channel 4) Scintillator pulse exiting differentiating circuit
Figure 6.7: Response of the CASTA82
preamplifier to a δ - shaped pulse of 2 ns
width.
for the CATSA82 preamplifier and
2 ns
10 ns
Channel 1) Preamplifier response to 2ns pulse
Channel 4) Scintillator pulse exiting differentiating circuit
Figure 6.8: Response of the ELab preamplifier
to a δ - shaped pulse of 2 ns width.
τ = R · Ccoupl = 2.5Ω · 1.5 nF = 3.75 ns (6.3)
regarding the ELab preamplifier. This estimation shows that the ELab preamplifiers rise time is
negligible whereas the big coupling capacity of the CATSA82 preamplifier affects the rise time
distribution significantly.
In order to analyze the internal response time of the preamplifiers a δ-like pulse of 2 ns width and 10
mV maximum pulse height is fed to the input. Since the trigger scintillators provide a frequent signal
we can use this as a pulse generator. Combined with a discriminator and dual timer as already used
in the conventional readout, see Ch. 3.4.1, the resulting box-shaped pulse is fed to a differentiating
circuit. This setup supplies the narrow pulse which can be seen in Fig. 6.7 and Fig. 6.8 together with
the response of the two preamplifiers. In this manner the internal rise time of the preamplifiers is
determined to approximately 10 ns for both (see channels no. 1 in the corresponding Fig. 6.7 and
6.8). The differentiating circuit transfers the scintillator pulse to a sharp negative pulse followed by a
smaller overshoot that is repeated around 14 ns after the main pulse. The overshoot with 2 ns delay
after the main pulse is not resolved by the preamplifiers. A plateau of the rise time is reached in 10 ns
after the main pulse. The second small positive pulse of the test signal gives a second rise to the
preamplifier response. The CATSA82 preamplifier shows a slightly faster response to the δ - shaped
pulse than the ELab preamplifier whose amplitude is 50% higher, in contrast.
This rise time is negligible in comparison to the 100 ns electron drift time and the 55 Fe induced rise
times around 106 or 166 ns.
6.2 Rise Time Dependence on Induction - Field
In order to understand the signal rise time formation, data were taken in dependence of the E-field
on the induction gap with the X-ray source as well as with cosmic ray muons. This is motivated by
the estimate that the detectable pulse begins with the presence of the amplified charges at the lower