Dectris EIGER 2 - General Usage; Second Threshold; Gating Mode Double

9.7.1. General Usage
To use the external gating feature, the trigger_mode ”extg” needs to be set. If this mode is set, the behavior
of the detector is controlled by a signal applied to the trigger input. In this mode, the enable out signal only
mirrors the trigger input signal and does not give feedback on the internal status of the detector.
For the best results, the count rate correction and retrigger should be turned o in gating mode:
detector | cong | countrate_correction_applied {value: ”false”}
detector | cong | counting_mode {value: ”normal”}
9.7.2. Second threshold
Both thresholds can be used simultaneously if required. The second threshold needs to be activated by setting
the parameter threshold/2/mode to enabled.
9.7.3. Gating Mode "double"
In gating mode ”double”, the EIGER2 system will write the counts detected during the rst gated window into
counter A and the value of the second gated window into counter B (see gure 9.5). The next gated window
will be added again to counter A and so on. The minimal time between two gate signals is 100 ns. After the
number of gate windows reaches the congured number of images per exposure nexpi for both counters A
and B, a readout will happen (e.g. if nexpi=4, the readout will happen after 8 gated windows have passed,
meaning 4 gated windows will have been summed up in each counter). The readout takes less than 2 ms after
which the exposure of the next image can be started. This will be repeated until the set number of nimages
is reached, where every measurement produces 2 images (nimages=6 would correspond to 3 measurements).
Due to this, the congured number of images nimages has to be an even number.
Until the actual readout happens, the input line will continue to alternate the counters for every window re-
ceived (typically derived from a continuous overall timing clock, such as e.g. the synchrotron ring clock). This
way, the synchronization of the gates is maintained and the counters A and B will always be associated with
the same gate delay in every readout.
In this mode, it is very important to avoid missed trigger signals. Check the quality of the trigger signal and
avoid any ringing of the signal or other unwanted interference. Refer to the Technical Specications for details
on the trigger input signal level and impedance.
The images of the two counters can be read out using the stream2, lewriter2 or the monitor interfaces. The
counter corresponding to each image can be gured out using the image_id, passed as stream2 header or ti
metadata. Counter A will always have an even image_id, while counter B will always be uneven.
This feature can be used for example in a pump-and-probe experiment to capture the state before and after
the pump simultaneously in a single measurement.
DECTRIS EIGER
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