Abstract:
We have developed a special instrument for three-dimensional mass spectrometric analysis (TDMSA) with differential ion counting, which allows rather detailed detection of time-dependent changes that occur in the isotopic composition of elements that constitute a crystal undergoing breakdown. This breakdown is induced by heating, and is accompanied by thermionic emission in the ion source of the mass spectrometer. The instrument constitutes an improvement of devices based on the idea of multiple scanning of mass spectra with differential ion counting, but also embodies a new approach to the automatic grouping of spectral information to suit the investigator. The TDMSA method and the instruments developed for it can also be used in other problems of crystal physics, geophysics and geochemistry, as well as in analytical studies. For example, attempts have recently been made to determine the absolute ages of minerals (chiefly zircon) from the isotopic composition of radiogenic lead isolated from them by thermionic emission. Our experiments demonstrate that the TDMSA instrument can be used to monitor this process with a high degree of differentiation of the lead fractions and will be very useful in studying this problem.