PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
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If an external signal prohibits acquisition using the GATE or VETO signals, the channel is also
dead (though on purpose). As further described in section 7.4, the use of these signals may
depend on the application:
a)
On one hand they may be used to reject an individual pulse (e.g. externally summing
multiplicities from several channels and issuing a short validation pulse at the right
time in the validation process). In this case the actual length of the pulse does not
correspond to a dead time. The VETO input is set up for this purpose and we call this
mode of operation GFLT (global first level trigger for validation).
b)
On the other hand GATE or VETO may block validation of events for certain amounts
of time (e.g. changing sources or activating beams). In this case they should be counted
clock cycle by clock cycle as dead time. Both the VETO and the GATE inputs are
available for this purpose, VETO as a global signal for the whole system, GATE as a
dedicated signal for each channel. VETO acts at the time of pulse validation, GATE
acts at the time of the rising edge of the pulse. However, the VETO input can be routed
to replace the GATE input with a user option.
c)
In a third class of application, the acquisition may only be of interest when GATE or
VETO are off. The pulse rejection logic would be similar to b), but livetimes and count
rates should only be counted when GATE/Veto are off as count rates would be different
in on and off times. (In b) one would be more interested in an overall livetime and
average count rate and additionally the time inhibited by GATE or VETO to make
corrections.)
The appropriate way to count GATE or VETO dead time may thus depend on the experiment.
See below (GDT) for the current methods implemented in the firmware.
6.6.1.3 Dead time associated with host readout
Fig. 6.11 After a run start command arrives from the host at T0, the Pixie-4 module acquires
several buffers of data (T1-T2, T3-T4) until the external memory (EM) is full and is read out by
the host at T5. (For single buffer mode not using external memory, the transfer is replaced by a
host readout, which however takes much longer.)
The final type of dead time comes from the readout of data from Pixie-4 memory to the host
PC. In MCA mode, this is limited to the access arbitration for the spectrum memory. The
memory has only a single port for both the increments according to the pulse height computed