asymmetric_implosion wrote: Tin activates as it is used in activation counters. Argon might be an option as it is pretty immune to neutrons

I would be very interested to see a link to an example of an activation counter based on tin. You might be thinking of indium – that has one of the highest cross-sections to thermal neutrons there is. Indium and silver are the usual materials used in activation counters. I can’t see anyone would have any reason at all to use tin instead, when indium and silver are conveniently available.

There is no element with as many stable isotopes as tin – nine stable isotopes in all, ten if you also count the ‘essentially-stable’ Sn-124.

Argon 40 (the commonest of the two naturally occurring isotopes) activates to the beta emitting Argon 41 with a half-life of 100 minutes or so, which makes its specific radioactivity non-trivial. It has a cross-section of around 1 barn to thermal neutrons, which is not as big as some cross-sections (like indium) but that makes it far from being immune to activation.

If you really want a gas to do your cooling that is as immune to neutron activation as possible, then the choice should be either oxygen (smallest neutron cross section of all the elements = 0.2 mbarns) or helium (which is a bit higher, 7 mbarns, but won’t practically activate in any case, though I guess you might end up with a bit of Li 6 from double-captures), the latter having good thermal conduction properties. I guess helium is your gas of choice, which brings the tread back, full circle!!!…