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

A compact dry He-3- He-4 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 mu W 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.