Rapid separation of isobar nuclides of alkali and alkaline earth elements by use of a surface ion source

Abstract

Using a magnetic mass spectrometer we studied the applicability of (positive) surface ionization to rapid separation of isobar nuclides, especially of alkali and alkaline earth elements. For this purpose the nuclides to be separated were evaporated onto a tungsten ribbon, which was subsequently flashed by resistive heating to about 2500 K in a typical time of 100 ms. During the flash the adsorbed species desorb partly ionized and pass the mass spectrometer, which is tuned to the desired mass number. The recorded signal shows two peaks arising at different times after having started the flash according to the mean residence times of the respective elements. The peaks are completely separated in time scale (and thus in the corresponding temperature scale, too) if the difference of the desorption energies is sufficient. This is the case with Rb (2.09 eV) and Sr (3.7 eV), which reach their maxima of desorption at about 1100 K and 1800 K, respectively depending to some extent on the heating rate. The different peak heights in the desorption spectrum are primarily determined by the ionization energies of the elements and represent ionic yields of 95% for Rb (4.17 eV) and 10 −3 for Sr (5.69 eV). The value for Sr could be increased to about 20% when the tungsten ribbon was covered with oxygen up to nearly one monolayer during the separation cycle, a procedure which additionally improved the separation efficiency. We also did some computer calculations based on measurements of the desorption parameters of the alkali and alkaline earth elements on a tungsten surface, which reproduced the experimental results of Rb/Sr quite well. Therefore we extended these calculations to K/Ca and Cs/Ba with the following results: These elements are completely separable as well, and the yields for K and Cs of the same order of magnitude as that for Rb (70% for K, 99% for Cs), while those for Ca and Ba are 7 × 10 −4 and 3%, respectively. The latter values might be raised to about one to some ten percent by an adequate oxygen coverage of the ionizing surface.