Lightning grounding conductors

I hope it is 24 times #14AWG
Aluminum is OK.
In some countries galvanized steel tape is also used, as long as it conducts electricity, has decent crosssection, it is OK. Steel will not last as long as copper.
Make sure all bends in the wire are no less than 12" radious.

How often will the copper run across these caps. The Contractor should be able to take measures to avoid the contact by using the correct mounting hardware. If aluminum is used you need to get a credit.

Most specs that I've seen include something like this: "All lightning protection materials shall be copper or bronze. Aluminum components shall be used in locations where system components are mounted to aluminum surfaces to avoid galvanic corrosion of dissimilar metals. All bolts, nails and screws are to be stainless steel."

That leads me to think you'd want copper.

However, Aluminum and Zinc are fairly close on the galvanic series tables, which would indicate that you'd get less corrosion with Aluminum.

That leads me to think you'd want aluminum.

Hope that clears things up for you.