[Halld-cal] uniformity test of BCAL epoxy joints

[Zisis Papandreou]

Hi Elton et al:

I am providing information from our experience in testing the light uniformity of modules.  We did this for the Construction Prototype and Module 1 only, since the latter modules could only be tested at Ross Machine shop, in order to avoid the cost, time and risk of moving the modules back to the university for the tests and then shipping them to JLab.  We decided, in the end, to do visual inspections and take hi res photos for all remaining modules.

First, here are a couple of photos showing how we did our the light uniformity measurements.  

The first shows an aluminum grid that was attachable with bolts to the standard features on the base plate.  You can see two pairs of bolts both top and bottom.  The grid was designed to be mounted in positions offset by 1/2 readout cell size (both left-right and top-bottom), so as to to be able to perform 4 different measurements.  The size of the grid is 2x2cm^2, matching the Regina Winston Cone light guides.  The module shown in this photo is the Construction (a.k.a. "Drunken Monkey") Module.  The data below are from Module 1; it was well built.



The second photo shows two WInston Cones.  One (source) had an LED mounted on its round (smaller) side, which was controlled by a standard lab bench voltage supply.  The other (detector) had an SS281 photodiode mounted on its round end and read out by a Keithley 6485 picoammeter (Carl Zorn has the same photodiode and same or similar Keithley).  Obviously, the two LGs were mounted on opposite cells at either end of the module.



Attached you will also find an Excel file from our light uniformity measurements on Module 1.  They will give you a sense of what is achievable.
- first sheet gives the data and charts showing the row, column and averaged measurements with the appropriate standard deviations shown as errors.
- the second and third sheet show bar graphs of the data, just different orientations of the 3D plot.  Note that the outer columns of cells have different areas, due to the wedge shape of the module.  Only the central three columns (see top photo) are directly comparable (the others would need area corrections).
- the fourth and fifth sheet show the corresponding surface plots with Column B (partial) and with only central three columns (C, D, E columns on first sheet), respectively.  The last chart shows indeed the achievable uniformity.

The grid allows accurate positioning of the LGs.  I don't have the data handy to check the stability of the current readout  although I could hunt for the logbooks if you would like to know.  My feeling is that once held in place with a steady hand the fluctuation is <1%.

For the module testing you would have to replace our aluminum grid with some sort of jig to hold the LED and photodiode onto the Chile LGs.  You would obviously test opposing cells but you could also offset the detector onto neighbour cells to look at cross talk.  Making such jigs should not be a challenge considering the rectangular ends of the Chile LGs.  

I hope this stuff gives you folks some ideas on how to proceed.  The tests are fast, literally seconds for measuring one LG cell pair, and they can be quickly repeated to see if the epoxy or the joints are changing over hours or days.  Measurements at a few percent should be achievable without great pain.  

I'll check our logbooks to verify this.  Shaun Kruger was one of the folks on these tests (George and I as well) so we'll pull the collective memory to see if any other info needs to be added to what I have in this email.

Cheers, Zisis...


Dr. Zisis Papandreou
Professor of Physics, Ph.D, P.Phys.
---------------------------------------------------------
Department of Physics   
University of Regina	    
3737 Wascana Parkway
Regina, SK  S4S 0A2	 CANADA

Phone: (306) 585-5379
Fax: (306) 585-5659
Email:  zisis at uregina.ca
Website: http://kronos.phys.uregina.ca/~zisis/professional/



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