Design and development of a compact thermal ionization mass spectrometer for isotope ratio measurement of uranium.

Abstract

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 and a compact footprint in comparison with the conventional one. The main feature of the TI mass spectrometer is the magnetic sector analyser with a 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 were validated by computer simulations using SIMION 7.0, leading to the finalization of the mechanical design. Experimental evaluation was carried out using standard TI mass spectrometric methodology to determine the parameters, namely peak flatness, mass range, sensitivity, abundance sensitivity, resolution and precision. The multi-collector resolution was 496, and the maximum mass was m/z 382. The sensitivity for uranium was found to be better than 1 ion for 500 atoms, and an abundance sensitivity of 50 ppm at m/z 237 was obtained. The peak flatness for the uranium peak was 3200 ppm of mass. The precision for the isotopic ratio 235 U/238 U was found to be 0.05% for the U200 standard. A new compact TI mass spectrometer with a 20 cm sector radius has been designed and developed. The advantage of larger dispersion produced by the VDZO (developed in-house) was exploited when deciding the 20 cm sector radius. The comparison study with the conventional TI mass spectrometer shows an enhanced (1.5×) sensitivity and an improved precision (0.05%) for the 235 U/238 U ratio of U200.