PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
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channel header (CHAN_TRIGTIME). As the rising edge occurs a filter time before the event
is recognized, the channel time is typically a few microseconds ahead of the event time.
Using C-parlance and enumerating beginning at 0, we would reconstruct the true time of arrival
for any event as:
EventTime = EVT_TIMELO;
Eve= EVT_TIMEHI*pow(2,16);
Eve= BUF_TIMEHI*pow(2,32);
EventTime *= 1e-6/75s;
This EventTime can be used to match events between modules and to compute time differences
between events.
ChannelTime serves as a refinement to compute the time of arrival differences between
channels of the same event. For most purposes, it is sufficient (and more convenient) to simply
compute a relative time difference, e.g.
ChannelTimeDiff
0,1
= (CHAN_TRIGTIME
0
- CHAN_TRIGTIME
1
)*1e-6/75s
However, it has to be kept in mind that the 16-bit word may overflow between being recorded
in channel 0 and channel 1. For example, channel 0 may trigger first at CHAN_TRIGTIME =
65500 and channel 1 may trigger later at CHAN_TRIGTIME = 105 and thus
ChannelTimeDiff
0,1
would be 65395 * 1e-6/75s even though the correct time difference would
only be 141 * 1e-6/75s. If such large time differences can be known to be impossible from the
physics of the experiment (e.g. max. time of flight delay) or from the acquisition setup (e.g.
max. coincidence window), such overflows can be handled by simply adding 65536 to the
smaller CHAN_TRIGTIME if the time difference is greater than the known maximum (e.g.
63000
*
1e-6/75s).
If an absolute ChannelTime is required, it can be computed as follows:
ChannelTime = CHAN_TRIGTIME;
Chann= EVT_TIMEHI*pow(2,16);
Chann= BUF_TIMEHI*pow(2,32);
ChannelTime *= 1e-6/75s;
For a small fraction of events, the EventTime or the absolute ChannelTime will experience an
overflow in a lower word in one header which is not reflected in the next higher word recorded
in a different header. These can typically be easily recognized and corrected for.
1)
The channel time is recorded before the event time. Hence, the time counter may have
seen a 16 bit roll over (change from high to low values of the low word) between the
recording of the channel time stamp and the recording of the event time stamps. For
example, a channel may trigger at CHAN_TRIGTIME = 65100 and the event may
be recognized after that at (EVT_TIMEHI, EVT_TIMELO) = (5467, 123). The correct
absolute ChannelTime would be reconstructed from <BUF_TIMEHI>, 5466, and