[d2n-analysis-talk] BB cer ped shifts

[Brad Sawatzky]

On Fri, 05 Feb 2010, posik at jlab.org wrote:

> So I looked at the Big bite cerenkov ped location for run 1849, which
> is a He3 production run at ~8-9 uA.  I did this by plotting the ADC
> for the first 100,000 events and making a T8 trigger cut (pulser)
> which is "(DBB.evtypebits&1<<8)==(1<<8)". Using this cut, I then fit a
> Gaussian to the ped and extract the mean. Then I compared these "beam
> on" ped positions with those of the LED runs. Here is a list of the
> mean ped values:
[ . . . ]

Interesting.  Good work!  Please generate a couple of slides showing the
LED-run pedestals overlaid on top of the beam pedestals for the 20 PMTs.
I'd like to see how the characteristics of the pedestal change.

Also, we need to determine if the pedestals change between the LED runs
and what you show here is due to beam loading or something else.  Look
for a cosmics/no-beam run in this period and redo your pedestal checks
(and generate the above plots) for a beam-off run and an adjacent
beam-on run.

If you can find appropriate runs, here's what would be ideal:
  A) peds with beam-off (cosmics or rest run)
  B) peds at low current (1 uA or so)
  C) peds at full current
Having A-C for each hardware configuration (v792, 1881, with pre-amp on
beamline side, without pre-amp on beamline side).  would help
disentangle configuration from beam-loading effects. 

> You can see that there is a significant ped drift with the beam line
> side pmts (1-10) from the LED peds, and the RHRS side (pmts 11-20) are
> fairly close to the LED peds. I also calculated the rates for this
> run, using TDC hits and cutting away beam trips, here are the results:
[ . . . ]
Rate for cell 05:    0.9315 MHz
------------------------
Rate for cell 06:    0.4108 MHz
------------------------
Rate for cell 07:    1.0756 MHz
------------------------
Rate for cell 08:    0.7693 MHz
------------------------
Rate for cell 09:    0.9036 MHz
------------------------
Rate for cell 10:    0.7387 MHz
------------------------
Rate for cell 11:    0.1087 MHz
------------------------
Rate for cell 12:    0.0224 MHz
------------------------
Rate for cell 13:    0.0802 MHz
------------------------
Rate for cell 14:    0.0816 MHz
[ . . . ]

> pmt 6 and 12 seem to be much lower than the rest of the pmts on that
> side.  This was also seen in run 1848.

There was some Pb shielding installed at the beamline height on the
beamline side.  The shadow should have been cast over more than PMTs 6,
12 though...  I'm not sure what's going on there.

I don't think I beleive the 20kHz number associated with cell 12.
There's either a bug in the calculation, or the hardware is doing
something really strange.

> Using these ped values as well as mirror and TDC cuts I plotted the
> ADC for mirror 3 (1849_mir3_pe.png) and 12 (1849_mir12_pe.png) for run
> 1849.  Please note that the x-axis is in #p.e. The red histo is the
> signal and blue histo is an off mirror background. Mirror 3 shows
> about 4-6 p.e and mirror 12 7-8 p.e. I am thinking about including
> these two images in my APS talk.

Using PMT 12 as a 'representative' histogram probably isn't a good idea.
What do 11 and 13 look like?

> So since the ped were stable with beam on/off with the v792 ADC, then
> that means the 1881 ADC is responsible for the ped shift. I assume
> then is has to do with the electronic differences between the 792 and
> 1881. I was wondering what exactly this difference is that results in
> the ped location shifting with beam on/off?

I did the with/without beam pedestal stability test before Transversity.
All I can say is that it was stable then.  That was with the v792s and
with no extra amplification on the beamline side PMTs...

I suspect/hope that the difference you see is due to some other change
that happened at the same time.  The tests mentioned at the top of the
email should clarify things.

-- Brad

-- 
Brad Sawatzky, PhD <brads at jlab.org>  -<>-  Jefferson Lab / Hall C / C111
  Ph: 757-269-5947 -<>- Pager: 757-584-5947 -<>- Fax: 757-269-7848
The most exciting phrase to hear in science, the one that heralds new
  discoveries, is not "Eureka!" but "That's funny..."   -- Isaac Asimov