A Tandem Double Focusing Mass Spectrometer for Neutron Cross Section Measurements

A mass spectrometer has been designed and constructed which consists of two double-focusing systems arranged in tandem. The instrument includes two 20-inch radius of curvature - 90° electromagnets, and two 20-inch - 90° cylindrical electrostatic lenses. This combination of velocity and momentum focusing permits the measurement of isotopic abundance ratios that are very large. Such ratios must be measured when nuclear reactions lead to daughter nuclides which are stable isotopes, and therefore cannot be detected by radiation counters. Isotopic abundance ratios of 108 to 1, in principle, can be measured with this spectrometer, as demonstrated by initial tests. It has already been utilized to (1) measure neutron cross sections, (2) determine neutron temperatures, and (3) provide 1 part per million isotopic purity of several rare earth elements in activation experiments. Special instrumental features of the spectrometer will be described, and a brief summary will be presented of nuclear and reactor measurements which are anticipated using this new facility.