Development of Compact Dry ³He-⁴He Dilution Refrigerator for Transition Edge Sensor Microcalorimeter X-ray Detector Operation on Electron Microscopes

A compact dry ³He- ⁴He dilution refrigerator (DR) was developed for operation of a superconducting transition edge sensor (TES) microcalorimeter x-ray detector mounted on a transmission electron microscope. A Gifford-McMahon (GM)-type mechanical cryocooler was used as a DR pre-cooler to operate the TES microcalorimeter without consuming liquid helium. To reduce the noise and vibrations generated by the GM cryocooler, the DR was separated from the cryocooler in a split system that allows a remote helium cooling loop to pre-cool the DR. In the DR unit, the dilution components of the still, the heat exchangers, and the cascade pair of mixing chambers were assembled into a compact cylinder. A snout made from an oxygen-free copper rod was attached to the DR mixing chamber for placement of the TES microcalorimeter x-ray detector. The lowest temperature and the cooling power in the mixing chamber were measured to be 87 mK and 20 μW at 100 mK, respectively. The DR unit was shown to maintain the temperature on top of the snout such that it was below the temperature required for multi-pixel-array TES microcalorimeter x-ray detector operation.