PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
lxiv
In any given event, a single Pixie-4 module will have up to four channels with a “hit”, i.e. a
valid local pulse without pileup. The four channels thus form one of 16 possible Hit Patterns,
stored in the lower 4 bits of a DSP parameter. In this representation, the Hit Pattern ranges from
“no channel hit” [0000] over “only channel 1 hit” [0010] to “all four channels hit” [1111]. For
each event, the Hit Pattern is checked against the user defined “Coincidence Pattern” to
determine if it is acceptable (Local Test). If acceptable, the event is recorded and processed, if
not, the event is rejected and data acquisition continues.
The user can define the Coincidence Pattern to accept one or more hit patterns. For example,
in the
Coincidence
Tab of the P
ARAMETER
S
ETUP
Panel, there are 16 checkboxes for the 16
possible hit patterns, and selecting one sets the corresponding bit in the coincidence pattern. In
the example shown in Figure 7.3, accepting only Hit Pattern [0001] makes the Coincidence
Pattern 0x0002. Several of the check boxes can be set at the same time, for instance to accept
any pattern with two or more channels. If all checkboxes are set, any possible Hit Pattern is
acceptable and the Coincidence Pattern is 0xFFFF.
Each channel with a pulse above threshold, whether trigger enabled of not, contributes to the
hit pattern the moment the pulse is validated as not piled up (i.e. a filter time after the rising
edge of the pulse). The hit pattern is read for comparison with the coincidence pattern about
66 ns after the first pulse is validated. If several channels contribute to an event, the minimum
coincidence window -- the time period in which (delayed) channels can contribute to the hit
pattern -- is thus ~66 +13 ns. If longer delays between channels are expected from the physics
of the experiment, this added width can be increased up to a value of ~870 microseconds (16
bit counter).
A difference in filter times between channels will cause even channels with simultaneous
pulses to contribute to the hit pattern at different times. The Pixie Viewer thus calculates the
required minimum window width to compensate for any such difference and displays it. The
application of this minimum value depends on the System global KEEP_CW, controlled by the
popup menu below the
Window Width
. The values of the popup menu have the following
meaning:
Increase for E filter, never decrease
At each change of
Window width
or the energy filter times, ensures that the
Window width
is at least the required minimum. If a larger number is entered in
Window width
or if filter
times are changed which would lower the required minimum, keep the larger value.
When using this mode to modify the coincidence window as required by the experiment,
one should remember to verify its value after changes in the energy filter times. In
particular, when energy filter times are increased in one channel after the other, the increase
in the first channel will increase the
Window width
but the increase in the last channel
(making filter times equal again) will not decrease the
Window width
.
Keep at required E filter width
At all times keep the Window width at the required minimum. If a larger or smaller number
is entered, it is ignored and changed back to the required minimum.
Use this mode if there are no coincidence requirements from the experiment and
differences in the energy filter should be accommodated, but unnecessary large values
should not linger as in the first mode.