Cryogen-free cryostat for large-scale arrays of superconducting tunnel junction ion detectors in time-of-flight mass spectrometry

Nb-based superconducting tunnel junction (STJ) detectors have a fast time resolution of a few 100 ns and high operating temperature of 0.3 K . These advantages expand their applicable fields to time-of-flight mass spectrometry (TOF-MS). In order to enlarge effective detection area, we have built arrays based on hundreds of large STJ elements. To realize the fast readout and no-cross talk, coaxial cables made of low-thermal conductivity materials were investigated. From results of thermal conduction measurements, we chose thin coaxial cables with a diameter of 0.33 mm , consisting of CuNi center/outer conductors and Teflon insulator for the wiring between 0.3 K -3He pot of the sorption pump and 3 K -2nd stage of GM cooler. Even after the installation of coaxial cables and a cold snout to the cryogen-free cryostat, we could keep arrays at 0.3 K for about a week, and reduction of the holding time at 0.3 K and temperature rise at 3He pot due to the installation were small, ∼ 0.5 day and 10 mK , respectively.