Under a NSF Major Research Instrumentation (MRI) award, NOSAMS has built a new AMS system designed specifically for continuously monitoring 14C in a flowing gas stream. The instrument is capable of continuously analyzing chromatographic effluents and determining the abundance of 14C in individual chromatographic peaks. This system will enable a dramatic expansion of significant and well-established lines of inquiry including: (i) surveys of the distribution of radiocarbon among natural products and thus of the sources of those materials, (ii) quantification of 14C tracers at extraordinary levels of dilution, and (iii) sensitive recognition of fossil-fuel-derived pollutants in natural systems by exploiting their zero content of 14C as a ‘negative label’.
Image Caption: Mark Roberts Staff Physicist assembles the gas-ion source at the 0º port.
Development of a Gas-Ion Source
NOSAMS has been exploring the capabilities of a gas-accepting microwave ion source originally built at the Atomic Energy of Canada, Chalk River Laboratories. The source uses 2.45 GHz microwaves and a continuously flowing stream of argon gas to sustain a plasma. Carbon containing gases mixed into the argon yield C+ ions that can be extracted as an ion beam. Negative ions are obtained by passing the beam through a magnesium charge-exchange canal. Initial success with this ion source has led to the design of a new gas-ion source.
Image caption: Albert Benthien and Baoxi Han Post-docs 2004 with the gas-ion source.
Collaborative Efforts to Improve NEC Ion Source
A collaborative research effort involving National Electrostatics Corp. (NEC) and three AMS laboratories (UC Irvine, University of Arizona, WHOI/NOSAMS) is underway to improve the design of Cs-sputter ion sources manufactured by NEC and currently in use at each of these labs. The NSF-sponsored collaboration is supported for the two-year period beginning in August, 2003.
Image caption: Karl von Reden, staff physicist, works on the NEC ion source.
Development of Carbon Stripper Foils
Over the last decade, research on single-walled carbon nanotubes (cylindrical closed structures of graphitic carbon, ~1.2 nm diameter) has revealed remarkable properties: high electric and thermal conductivity and tensile strength far exceeding that of steel. That has led Karl von Reden and Enid Sichel to the idea of developing a durable carbon nanotube foil for electron stripping in accelerator mass spectrometry (AMS). The idea is to create thin mats of nanofibers into a mesh configuration that would perform like a “frozen gas”, keeping the stripping atoms largely stationary under ion beam bombardment and minimizing the structural damage known to occur in amorphous or graphitic films.
Image caption: Enid Sichel Adjunct Scientist & Karl von Reden examine a nanotube foil with an atomic force microscope.