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RIMSKI -Resonance Ionization Mass Spectrometer for Krypton Isotopes -is a newly developed ultra sensitive instrument with a cryogenic sample concentrator and a laser ion source designed for isotopic analysis of extraterrestrial material. Laser heating is used for extraction of krypton from samples into the mass spectrometer volume. Atoms continuously condense on a cold spot in the ion source, and are repeatedly released by laser heating. They are ionized in the evaporation plume using three tunable dye lasers and one frequency tripled Q-switched Nd:YAG laser. Two resonant transitions are employed (at 116.5 nm, produced by four-wave frequency mixing in Xe, and 558.1 nm) and are followed by single photon ionization at 1064 nm. Multiple analyses of air aliquots and <5 mg samples of meteorites, both containing $10 6 total krypton atoms, show precision $1% for major isotope ratios. They are reproducible to this level through a day's analyses, allowing calibration for linear and non-linear mass discrimination by sample-standard bracketing. Cosmogenic 81 Kr can be reliably detected at the few thousand atom level, corresponding to determination of cosmic ray exposure (CRE) ages from meteorite samples of a few milligrams. Measured CRE ages of noncumulate eucrites are:

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