performance, can only be improved be increasing MCP gain (which may require reducing amp gain for avoiding saturation
effects and non-linear amplification).
If the analogue outputs are satisfactorily, one can check the corresponding timing (CFD) outputs on the sockets “NIM” or
“ECL”. If your module is set to NIM-output levels you can directly verify the signals on an oscilloscope (coax input, 50Ω
terminated). For the ECL output setting the presence of signals can be probed likewise on the NIM output with an oscilloscope
(but with at least 1MΩ input impedance). Note that this may disturb the signal from the ECL output). Now the thresholds on
all channels can be adjusted, ideally so that even the smallest pulse heights from particle/photon triggered MCP charge cloud
are above the threshold but noise is still fully discriminated (typical threshold level 1.5x - 2x higher than the continuous noise
level).
Figure 3.12 shows such a typical case. It should be noted that it may be beneficial to allow occasional noise triggers in order to
safely detect also the smallest real signals and not to “lose” counts. This will not lead to false data because if they do not appear
on all signal chains or if such random counts can be dismissed in coincidence-triggered measurements. Such signals will either
not be processed by the data acquisition or can easily be sorted out later during data analysis (see below).
Figure 3.12: Typical (analogue) oscilloscope screen output showing delay-line signals from a DLATR+ board:
analogue signals (monitor output) on upper trace and the correlated CFD outputs (NIM) on the lower trace. Both
traces are triggered by the NIM signal. The pulse height distribution of the analogue signals can be seen and also
the effect of the threshold setting on the registered events (cut-off of smaller signals not being registered).
However, one should avoid a too-low threshold setting which may cause a so-called “pre-trigger” operation mode of the CFD
circuit. In this mode the CFD threshold will not block off signals that have been slightly distorted by noise in a way that the
CFD circuit can function normally, see Figure 3.13. If the pre-trigger signal is registered a false time will be measured. For
delay-line signals this can be recognized in a false time sum for this detected particle / photon on the respective layer and the
event can be dismissed by software. However this may lead to non-linear imaging and timing response on the detector.
Figure 3.13: Signal traces as in Figure 3.12, but with low threshold setting very close to the noise level (left). Pre-
triggered signals are present. The right picture shows the CFD output of an erroneous pre-triggered event.
Page 44 of 83 MCP Delay Line Detector Manual (11.0.1304.1)
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