| 英文摘要: | Thermal ionization mass spectrometry remains the technique of choice for high precision determinations of the elemental isotopic abundances in such disciplines as geochronology and nuclear forensics. However, the precision of the isotopic measurements by this method depends on the number of ions measured at the collector end of the instrument, which is itself dependent on the efficiency of the techniques used in the mass spectrometer to generate ions from elements extracted from natural and manufactured material. The preferred ionization method for high ionization potential metals such as lead and silver is the Si-gel technique, which consist of doping a suspension of silica with the metal of interest and then drying and heating this mixture to temperatures over 1,300C on a metal ribbon in the source of the mass spectrometer. At these temperatures, the silica mixture emits a steady stream of metal ions which is analyzed in the instrument. This ionization method is inefficient, typically producing only about 1% ionization. The goal of this project is to improve the efficiency of this class of ion source. The approach being used builds on the fact that the Si-gel technique is actually a liquid glass ion emitter which releases ions generated in the liquid during high temperature evaporation in the mass spectrometer. As a result, the fraction of metals ions generated in the liquid glass likely can be increased using electrolysis techniques, in which the liquid glass serves as electrolyte when placed in contact with two metal electrodes of opposite electrical polarity (an 'electrochemical cell'). The electrochemical cell consists of a micro-interdigitated electrode array (IDA) produced by sputtering tungsten onto an undoped Si or sapphire wafer. The device is being developed through a series of prototype IDA assemblies that will consist of IDA, itself, and a high melting temperature ceramic holder that will also hold IDA in place for required electrical connections and will allow the placement of a metal ribbon from below that will serve as a heater to bring the IDA to operating temperatures (1,200C to 1,300C). The various prototype configurations are designed to optimize heating and electrical continuity within the IDA and to maximize the ion currents created within the liquid glass. A main goal will be to improve the ionization efficiency of lead by at least a factor of five in order to improve the precision uranium-lead age determinations for picogram size lead samples. A secondary goal is to develop and commercialize a new class of ion source for use in private sector thermal ionization mass spectrometers.
The thermal ionization mass spectrometer remains the gold standard for high precision isotopic ratio determinations and is the key to current efforts to improve the precision of U-Pb age determinations. The precision of isotope ratio determinations is controlled by the number of ions counted and a major limitation in TIMS is that current methods of producing thermalized ions by emission from the surface of a resistively heated metal ribbon have low ionization efficiencies (~1%, rarely ~10%). This proposal involves the development of a new class of ion sources that builds on the 'Si-gel' technique, a molten silicate liquid ion source which was first implemented in TIMS in the late 1950s. More recent work has suggested that most metal atoms doped into a molten glass ion emitter for isotopic analyses are present and released during evaporation as neutral atoms, not ions, and so are never delivered to the analyzer portion of the instrument. The goal of this project is to increase the proportion of metal ions in these melts by treating the melt as an electrolyte in an electrochemical cell. The major effort in the proposed work will be the design, fabrication and testing of an electrochemical cell in which a micro-interdigitated electrode array (micro-IDA) serves as the working and counter electrodes and as the substrate upon which the metal doped silicate is deposited, melted and manipulated electrically to induce ionization of the metal dopant. In this method, the ionization of the metal dopant is induced by tuning the relative potential of the two electrodes to the value required to remove an electron from the neutral metal. The metal ions are then released to the source of the mass spectrometer during evaporation of the molten silicate. The small micro-IDA electrode arrays are ideal for this purpose because they can be wholly placed in the focal plane of the mass spectrometer, their submillimeter size electrodes and electrode spacing should remain wetted by electrolyte even as the electrolyte evaporates and releases metal ions into the mass spectrometer, and because micro-IDA are readily fabricated to user specifications with conventional semiconductor fabrication techniques. The project will involve establishing the combinations of micro-IDA design, substrate and holder/heater that produce effective melting of silicate on micro-IDA surface and produce a 2-10 fold increase in Pb ionization efficiency as determined by measurements of ion beam intensity and duration in the TIMS currently operating at the University of Colorado Boulder. The latter instrument is already fitted with the custom potentiostat required for powering the micro-IDA. |