Dilutra

Dilutra, a frost-haired keeper of the cold, holds a tiered golden chandelier glowing in the dark.

The Physics

Below about 0.87 K a mixture of helium-3 and helium-4 separates into a ³He-rich phase floating on a dilute phase. Even at absolute zero the dilute phase keeps a few percent of ³He dissolved. Ifft, Edwards and Sarwinski measured this limit at 6.37 ± 0.05% (Phys. Rev. Lett. 19, 831, 1967); later work gives about 6.4–6.6% depending on pressure. Pumping ³He out of the dilute phase pulls more across the boundary, and that ‘evaporation’ into the dilute phase absorbs heat. It works continuously, down to a few millikelvin.

x₃(T → 0) ≈ 6.4–6.6%

• x₃: fraction of ³He atoms in the dilute (⁴He-rich) phase

This is not a compression cycle: nothing is squeezed. The cooling comes from ³He crossing a phase boundary inside the mixing chamber.

Honest caveat: The finite solubility at T = 0 is what keeps the cooling power from vanishing; real fridges are limited by heat leaks and exchanger performance.

On the sticker

Dilutra cups a golden chandelier of stacked plates, the stages of a dilution refrigerator. Printed text: DILUTRA with the line 6.4-6.6% ³He. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.