The high-precision isotopic analysis of uranium hexafluoride

Several factors limit the precision and accuracy which can be achieved in the isotopic analysis of uranium hexafluoride by a mass spectrometer. These include inadequate resolution, memory errors, statistical fluctuations in ion currents, amplifier noise, current noise, inefficient collection of ions, voltage coefficients in resistors and chemical contamination of samples. A description is given of a 90° sector field mass spectrometer of 30 cm radius, which has been built and used to investigate the accuracy which can be achieved in practice. The precision (1 σ) of this instrument when measuring the fractional difference in isotope ratio between two samples is 5 × 10 -5 at the 50% level of enrichment and 1.4 × 10 -4 with natural uranium, for comparisons lasting 12 minutes. By suitable design of ion source and inlet system, the memory error in the measurement of the concentration difference between two samples has been reduced to ±0.3% of the difference.