CHARGE COLLECTION THERMAL ION MASS-SPECTROMETRY OF THORIUM

The determination of thorium isotope ratios using a new technique suit able for low intensity ion beams has resulted in significant improveme nt in analytical precision. A capacitor was substituted in place of th e usual high-value resistor at the feedback loop of an electrometer. C harge collected in a Faraday cup accumulates on the capacitor, resulti ng in a time-varying voltage proportional to the ion current. Electrom eter noise levels are reduced by a factor of up to 500, and immunity t o temperature fluctuations is improved by a factor of more than 10. Ch arge collection thermal ion mass spectrometry bridges the gap between the measurement range for resistive feedback electrometers and electro n multiplier devices. Standard design Faraday cup arrays and electrome ters equipped with capacitive feedback elements can be used for simult aneous data collection in multiple channels. Maximum working range, fo r 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 o ver 1 h. The ionization efficiency is 4% and analytical errors for a s ingle run are better than +/-0.6 parts per thousand; these results are superior to those reported for other techniques. Charge collection ma ss spectrometry will be most useful in cases where large ion beams can not easily be obtained or where the concentration of the element of in terest in geological materials is very low.