Single stage closed cycle Joule-Thomson cryocooler and mixed gas refrigerant therefor. The cryocooler (1) comprises a single stage compressor (2), an after cooler 4), a recuperative counter low heat exchanger (8), an expansion device (11) and a cold end heat exchanger (10). The compressor operates at suction and discharge pressures of 0.163 MPa and1.5683 MPa, respectively and at a pressure ratio of 9.71 between the high pressure stream and low pressure stream of the refrigerant. The cryocooler employs a mixed gas refrigerant comprising 11:1 molar percent neon, 19.59 molar percent nitrogen,22.11 molar percent methane,9.14 molar percent ethane,15.99 molar percent propane and 22.11 molar percent isobutene to produce cryogenic temperature below 70 k(fig 1