[Solid_baffle] baffle material

[Zhiwen Zhao]

Dear All

The question about baffle material was raised by the review committee 
and need to be addressed

I had the study last year showing Copper seems not a bad choice among 
other materials.
http://hallaweb.jlab.org/12GeV/SoLID/download/baffle/talk/solid_baffle_zwzhao_20150316.pdf
I think Rich also had some study favoring Copper, but I can't find the 
record any more.

If we decide on Copper, we can
1. use that to do background and trigger study from now on.
2. Rich could further tune baffle because Copper can be machined with 
good precision.
    (for example, maybe the inner surface of baffle slit can have some 
angle to follow electrons within the 9cm length)

Practically, it would be some kind Copper alloy.
Roger from hallC has good suggestion and experience from Qweak Collimator
Seamus, maybe you can even get some cost estimation to update the very 
rough number in pCDR.

Thanks

Zhiwen