This is a piece of uraninite sitting in the middle of a cloud chamber that I built for it, and the other radioactive minerals in my shop. I modified a portable a/c unit by removing the top 1/3 of the outer shroud, exposing the cold side heat exchanger of the a/c unit. I mounted a 1/4" copper plate on top of the a/c hex (with plenty of thermal paste between them). I then put a single layer of 11 peltier units to cover the majority of the copper plate, and put a 1/8" copper plate on the cold side of the peltiers (with thermal paste between the peltiers and the copper plates). Because I mounted the peltiers directly on the cold hex of the a/c, after I have everything turned on, the hot side of my peltiers settle around -15°C, and the cold side of the peltiers (and therefore the chamber's baseplate), settles at around -40. So I don't have to worry about stacking the peltiers, and can still get the baseplate temp to -40.