[PRad] the list of current tasks and a suggestion to set the veto counters

[gasparan]

   Hi All,

  I list below and in the attachment the current tasks together with my 
suggestion how to
  make an on-line positioning of the veto counters.
  Please take a look and prepare your comments.
  Thanks
  Ashot
.......................................................................
1)	 Veto scintillators thickness: we are testing them now but, it will 
take another week
      if not more. I will contact Youri to see if this is important for 
the engineering design
      part.
2)	Veto scintillator attachment tolerance: we are working on this item 
but, it will need
  again a week or so. I will contact Youri and Dipangkar to see if a 0.5 
mm tolerance
  is realistic (for both edges of a single scintillator). In all cases we 
need survey points
  on this device.
  Weizhi, please make this item a priority before the weekend. If we need 
one more meeting, I am ready for that.
3)	Changing the vacuum windows from X17 to PRad-II experiments (as it 
was shown by Bob). This is highly undesirable since it will involve not 
only the vacuum part (that comes from the target change also) but it 
will require a new survey of the HyCal. If we decide to run PRad-II with 
the new (smaller) window, then the
Pb-glass part of the HyCal no longer in use (if even it is ready).
Weizhi and Chao, please take one more look on this part to before the 
weekend.
4)	The GEM efficiency high accuracy measurement using the so-called 
third GEM. As we discussed on Monday, this item is not fully clear to 
me. My questionwas, and still is, what is the accuracy of these 
measurements using the GEM1, GEM2 and HyCal (plus the target vertex, we 
even have 12C foil in the target cell)?
Nilanga, Weizhi, Chao, and who wants to contribute in this part, please 
come up with your arguments on this part.
My approach is that if we even will end up needing the third GEM then 
there are
several open item on this way: (a) the size of the third GEM; (b) we 
still will need to attach and survey it better than 1 mm (better than 
HyCal is providing it); (c) the electronics ...
5)	Running the X17 experiment with E = 4.4 GeV. We need to recalculate 
all resolutions and most importantly the parameter space plot for this 
new energy before the end of January (well before our ERR meeting in 
mid-February).
……………………………………………………………………………..
My suggestions for the Veto counters survey and geometrical calibration
1)	First, we require +/- 0.5 mm engineering survey accuracies on all 4 
edges of all 4 scintillators. We need to come up with a way to make 
survey points on these scintillators with this accuracy.
2)	Then, with electron beam on, target in, all devises (HyCal, …) 
calibrated we make a so-called on-line veto counters geometrical 
calibration.
a)	Position the vito counter(s) well below (close to the beam) to fully 
cover the
area of the second crystal(s) (counted from the beam hole). That can be 
done with on-line measurements of the Moller events.
b)	With the stepper motor(s) slowly moving the scintillators away from 
the beam, continues measurement of the Moller coverage area(s) by the 
reconstructed (on-line) Moller events.
c)	Fix the veto positions when the count(s) of the Moller pairs with the 
scintillator(s) and the edge of second scintillator(s) are just 
vanishing.
3)	This method needs a step size of 0.1 mm for our stepper motors. This 
is a question to Youri and Christopher (Bob).
............................
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