threshold on an Aria markG at
Mon Jan 7 15:25:07 EST 2008

I’m having a problem with event triggering on my Aria. When I choose a threshold
parameter and value, a dead spot appears in that parameter at approximately 10X
the threshold value.

E.g. I set the threshold as side scatter with a value of 1000 in log mode. I
load some beads and see a tight population on a side scatter histogram. If I
adjust the side scatter PMT voltage, the population moves around
correspondingly, and of course disappears when it drops below the threshold at
1000. In a narrow range near 10,000, however, it also disappears, only to
reappear once the voltage is moved either up or down. Oddly, when the beads
fall in this dead spot, the event rate doesn’t change, although no other PMTs
detect anything.

I’ve talked to a couple different people at BD and they keep telling me that
this is normal and has to do with flooding the PMT at a high voltage. I’m
confused by this because the PMT works when the voltage is increased to move
the population even higher, just not in that one spot. Also, the dead spot
moves if the threshold value is changed.  Can someone please explain this
phenomenon to me better than the BD technical support? Or if this shouldn’t be
happening, what is likely failing?

Thanks for any help you can give this first-time poster,

Mark Gebhard
Graduate Research Assistant
University of Texas at Austin

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