Design and development of a compact Thermal Ionization Mass Spectrometer for isotope ratio measurement of actinides.

RATIONALE A new compact Thermal ionization (TI) mass spectrometer, for the isotope ratio analysis of the elements relevant to nuclear applications, has been designed and developed. The new development offers superior performance in terms of sensitivity, precision along with a compact footprint in comparison with the conventional one. The main feature of the new development is the magnetic sector analyser with sector radius of 20 cm (instead of 30/27 cm in conventional/commercial geometry). An apparent reduction in dispersion has been adequately compensated by employing variable dispersion zoom optics (VDZO). The theoretical calculations of the design were validated by computer simulations using SIMION 7.0 and verified experimentally. The system was characterized in terms of the performance parameters viz. sensitivity, resolution, mass range, precision and abundance sensitivity using the Uranium standard U200. METHODS The theoretical calculations were validated by computer simulations using SIMION 7.0 leading to the finalization of mechanical design. The experimental evaluation was carried out using standard TI mass spectrometric methodology to determine the parameters viz. peak flatness, mass range, sensitivity, abundance sensitivity, resolution and precision. RESULTS The multi-collector resolution was measured to be 496 and the maximum mass was determined to be m/z 382. The sensitivity for Uranium was found to be better than 1 ion for 500 atoms and the abundance sensitivity of 50 ppm at m/z 237 was obtained. The peak flatness for Uranium peak was measured to be 3200 ppm of mass. The precision for the isotopic ratio 235 U/238 U was found to be 0.05% for the U200 standard. CONCLUSION A new compact TI mass spectrometer with a 20 cm sector radius has been designed and developed. The advantage of larger dispersion offered by the variable dispersion zoom optics (VDZO) (developed in-house) was exploited when deciding the 20cm sector radius. The comparison study with the conventional TI mass spectrometer shows an enhanced (1.5x) sensitivity and an improved precision (0.05%) for the 235 U/238 U ratio of U200.