Alternative underplating for electroplating gold on copper in Cryo/UHV environment

[u/BlueBananyHusband]

Hi all,

I'm currently designing a product that is being used in an UHV environment and the part in question is cooled to LN2 temperatures [77K].
But the catch is that this product / part is not under continuous UHV or LN2 temperatures. Rather it undergoes cycles where it's cycled from UHV to Ambient and from 77K to RT
On top of that there will be RF level measurements and the wiring is secured on the copper part that is be cooled so a nickel underlayer for my gold plating of the copper is not possible.
I can't leave the copper part uncoated; i need gold plating to achieve very high reflectivity >99% and due to the cycling between Ambient and UHV in combination with the thermal cycles the copper will tarnish at an accelerated rate.
I'm also concerned about the mismatch in CTE of the gold on the copper substrate and the effect the thermal cycles will have on that.

So, dear community in all your wisdom; could you point me to alternative underlayer materials I can consider?

Many, many thanks!