I was just looking at a vapor pressure chart for xenon, and I realized it's critical temperature is about 20 degrees celsius, yet it's boiling point at STP is -100 celsius. Ignoring the expense, would xenon be useful as a second stage refrigerant? Assuming you could get -30C from the first stage, xenon would condense at ~300 psi. Maybe an alternative to those who can't get restricted refrigerants (r1150, etc.) in small quantities?

Xenon Vapor Pressure Chart

10 Liters Xenon Cylinder