sentences:  Helium-3 refrigerator
  
  A helium-3 refrigerator is a simple device used in experimental physics for obtaining temperatures down to about 0.2 kelvins.By evaporative cooling of helium-4 (the more common isotope of helium), a 1-K pot liquefies a small amount of helium-3 in a small vessel called a helium-3 pot.