VG5400 noble gas mass spectrometer
The laboratory employs a custom built furnace, CO2 and UV lasers, and ultrahigh vacuum extraction line connected to a Micromass 5400 noble gas mass spectrometer. The 5400 has both faraday cup and ion-counting electron multiplier capabilities. With the axial multiplier, the spectrometer has a mass resolution of >600. We typically run at 100µA trap current for argon analyses with a corresponding sensitity of 1.63 E-4 amps/torr. The ion-counting system has a dead-time of approximately 20 ns and dark count rates are less than 1 cps.
Furnace and UHV extraction line
The extraction line for the 5400 consists of an all-metal system. Samples are outgassed using a double vacuum resistance-heated tantalum furnace, or CO2 laser or UV laser. The furnace is controlled via a Eurotherm controller and thermocouple in contact with the bottom of the crucible and mounted on the outer (low) vacuum side of the furnace. Two SAES ST-707 getters, one hot and one cold, are used for purification of the gas prior to inlet. A Janis high-temperature, closed cycle cryogenic cold trap system will be used for applications involving separation of noble gases. The extraction line is computer controlled using a LabVIEW program and gas handling routines are automated.
Quadrupole mass spectrometer and extraction line
Helium determinations for (U-Th)/He analyses are made using a dedicated quadrupole mass spectrometer and UHV gas handling system. A double vacuum resistance-heated furnace or CO2 laser is used to outgas samples. Our 4He blanks are routinely on the order of <0.001 ncc. Helium outgassed from samples is purified in the all-metal extraction line. Gettering of active gasses is handled by small, standard SAES getters. Reservoirs containing a 3He spike and a 4He standard are attached to the line behind all metal pipettes. Our 4He pipette delivers 9.84 +0.08 ncc STP 4He per aliquot. Measurement of 4He is done by isotope dilution, using a 3He spike. Measurement of U and Th concentrations are presently performed by solution ICPMS at UA. A Janis cryogenic cold trap system is interchangeable between the He extraction line and the 5400 extraction line when required.
The CO2 laser system shown above can be used on either the MM5400 MS system or the Helium MS system. It features a CO2 laser with output at 10600 nm wavelength in the infrared band. It is used to heat multiple small grains or a single large mineral grain separated and purified from the whole rock sample. The temperature of the heating step can be controlled via the laser power output. The temperature of the step is monitored via an infrared temperature probe. The system features a variable zoom camera and an X-Y-Z stage.
Synrad CO2 laser heating system
The Excimer laser system is based on an ArF laser system operating at 193 nm in the ultraviolet band. This laser exhibits a very small laser spot size (10-50 microns) which allows for the spatially resolved ablation of discrete areas on single mineral grains. The sample cell allows for the use of "thick sections" (3mm thick) of highly polished sections of rock samples. Sections can be 100 mm x 100 mm in area.
American Beam ArF Excimer laser ablation system
The fission track laboratory comprises two Nikon Optiphot2 microscopes. One scope is fitted with a computer controlled Kinetek automated stage and Calcomp digitizing tablet. The other scope is a polarizing scope used for sample characterization and thin section analyses. Interchangeable between these two scopes is a video system with color monitor and a 35 mm camera set-up. The video system is particularly useful for teaching petrography and demonstrating fission track counting procedures. We use the external detector method and zeta calibration on CN5 glass for fission track thermochronology. Samples are irradiated in the Oregon State University Nuclear Reactor.
Crushing facilities consist of a jaw crusher and Bico disk mill. Samples are washed and initially separated on a Rodgers Gemini Table. Mineral separation facilities consist of a heavy liquid set-up within a series of fume hoods, Frantz LB-1 Barrier magnetic separator, shaker table, sieves etc. Grinding and polishing facilities include a Buehler dual wheel variable speed polisher/grinder.