More on FDC

[Elliott Wolin <wolin@jlab.org>]

Hi,

I talked with Fernando about some of the issues raised in my last note.  Fernando's response:


On the possible need for gold coating:

The oxidation on copper is OK as copper oxide is conductive. Gold may be
a good choice but the overall thickness will increase a bit. Before gold is
deposited, a layer of nickel is required over the copper.


On copper thickness and skin depth:

I don't think you have to worry about skin depth in this application.
The skin depth is a few microns deep for microwave frequencies. As the rise
times in DWCs is generally in the 5 nS range, there won't be any problems.
However, you may want to consider the strip impedance and terminations for
long strips, maybe over 3 ft.

(Mehmet...please calculate all this yourself)


On gaps:

The trace and space definitions are dependent on manufacturing variables and the cost 
increases for narrower traces and spaces. The best you can get now and reliably is about 4
mil but 8 mil is the norm.

(Fernando and I know of no fundamental lower limit to the inter-cathode gap size...anyone 
know of any?).




Some additional thoughts/questions:


Wire tension:

We need to know what wire tension is required.  Standard formulas can be found in 
textbooks or articles on wire chambers.


Picture frame pc board rigidity:

The anode wire pc board will likely need to be attached (glued, pinned, screwed, whatever) 
to the spacer underneath to support the wire tension once the wires are glued/soldered 
onto it.  I don't believe the pc board alone can handle the tension.


Gain sensitivity and tolerances:

Since we are using analog readouts we may be sensitive to gain variations.  The gain 
sensitivity to the wire/cathode distance should be calculated (standard formulas, I 
believe).


				Sincerely,
					Elliott


P.S. Mehmet...in the past I found it useful to keep a list of every reasonable question 
ever asked about the device I was working one, as well as every question I came up with 
myself.  I never answered all of them satisfactorily, but by keeping the list at least I 
didn't forget to look into them.




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     "Science plays a major role in satisfying the minor needs of mankind"

                "Secular man, by playing God, becomes the devil"


Elliott Wolin
Stalag 602-11
Gulag ARC-ipelago

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