CHARGE COLLECTION THERMAL ION MASS-SPECTROMETRY OF THORIUM

The determination of thorium isotope ratios using a new technique suitable for low intensity ion beams has resulted in significant improvement in analytical precision. A capacitor was substituted in place of the usual high-value resistor at the feedback loop of an electrometer. Charge collected in a Faraday cup accumulates on the capacitor, resulting in a time-varying voltage proportional to the ion current. Electrometer noise levels are reduced by a factor of up to 500, and immunity to temperature fluctuations is improved by a factor of more than 10. Charge collection thermal ion mass spectrometry bridges the gap between the measurement range for resistive feedback electrometers and electron multiplier devices. Standard design Faraday cup arrays and electrometers equipped with capacitive feedback elements can be used for simultaneous data collection in multiple channels. Maximum working range, for unit mass separation in adjacent Faraday cups, is uncompromised. The charge collection method was tested with thorium ion beams. Filaments loaded with 15 pg of(230)Th produce intensities up to 6 x 10(-14) A over 1 h. The ionization efficiency is 4% and analytical errors for a single run are better than +/-0.6 parts per thousand; these results are superior to those reported for other techniques. Charge collection mass spectrometry will be most useful in cases where large ion beams cannot easily be obtained or where the concentration of the element of interest in geological materials is very low.