Image courtesy of Los Alamos National Laboratory
Left photo shows researchers collecting sea-water samples containing diatoms from an oceanographic sampling device. Photo at right shows microscopic image of a small colony of diatoms in the sea water.
In a collaboration initiated by the U. S. Department of Energy Isotope Program, scientists from TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, delivered two proton irradiated potassium chloride (KCl) targets to radiochemists at Los Alamos National Laboratory (LANL). The highly radioactive targets, containing spallation-produced Si-32 and other isotopes were processed remotely using complex radiochemical extraction procedures to successfully produce a solution of silicic acid labeled with purified silicon-32 (Si-32). The solution is well suited for oceanographic research applications.
The DOE Isotope Program has made available silicon-32 for marine research which is valuable to understanding and modeling the global climate. Marine biologists have successfully used silicic acid labeled with various silicon isotopes as a tracer to measure silica production rates in coastal seawater. The radiotracer Si-32, a pure beta-emitting isotope, has advantages over stable isotope tracers because it is significantly easier to use due to the easily detected low energy radioactive emissions. This enables real-time quantitative radioanalytical measurements of silicic acid uptake rates in samples at sea.
The availability of Si-32 in the early to mid-1990s stimulated a flurry of research activity to assess biogenic silica production by microscopic single-cell phytoplankton known as diatoms. Silicon in the form of silicic acid is an essential nutrient for diatom blooms in the oceans. The diatoms transform the silicic acid to a solid silica shell surrounding each cell. The rate of bloom of these tiny photosynthesizing sea-dwellers provides researches with insight into carbon dioxide consumption at various times and locations in the oceans. This is because atmospheric carbon dioxide (CO2) is dissolved in the surface seawater and is metabolically transformed to sugars by the diatoms consuming large amounts of the green-house gas. The rate of consumption of the vital nutrient silicic acid can be followed using the Si-32 radiotracer giving a measure of the total mass of silica produced and the amount of carbon-dioxide consumed by the diatoms in the oceans. This information is important in assessing the role of the oceans in climate models.
Efficient production of the Si-32 tracer requires the use of high energy proton beams accelerated onto potassium chloride (KCl) targets. During the late 1990s a reconfiguration of U.S. high-energy accelerator operations limited access to irradiation facilities of sufficient proton beam energy and current, resulting in a lull in the supply of Si-32 to researchers. This stimulated a joint agreement between TRIUMF and the U.S. Department of Energy (U.S. DOE) to use the 500 MeV irradiation facilities in Vancouver, BC for Si-32 production. The irradiated KCl targets were shipped from Vancouver to Los Alamos, NM and processed at LANL in shielded hot cells. Final purification chemistry for the Si-32, a pure beta-emitter, is extremely labor intensive to recover the isotope from the activated KCl and obtain a high-purity research-grade product. Due to this new production effort, oceanographic researchers now have a new supply of the Si-32 to support their on-going research efforts.
Dr. Kevin John
Los Alamos National Laboratory
U.S. Department of Energy, Office of Science, Office of Nuclear Physics, Isotope Development and Production for Research and Applications.
Mark A. Brzezinski and Dennis R. Phillips, “Evaluation of 32Si as at tracer for measuring silica production rates in marine waters,” Limnol. Oceanogr., 42(5), 1997, 856-865.