Isotopic ratio measurement at abundance sensitivities greater than 1:10¹⁵: A comparison between mass spectrometry at keV and MeV energies

The mass abundance sensitivities of the best conventional mass spectrometers are about 10^-10 in the uranium region and 10^-10 for low masses. This is about five orders of magnitude below several measurements that have been made using the new accelerator-centered mass spectrometry technique. The reasons for this dramatic increase in sensitivity are discussed. This improvement is not the result of any single effect but rather the compounding of several, each contributing several orders of magnitude. As an example, a tandem accelerator-centered double mass spectrometer is analyzed. For this geometry, significant background elimination is accomplished by: (1) The removal of all molecular interferences by fragmentation; (2) reduction in scattering cross sections; (3) simultaneous measurement of E, wdE/dx and range for each event; (4) the use of negative ions. It is pointed out that with suitable ion sources the accelerator-centered technique has the potential of unambiguously detecting a single atom of specific mass and atomic number for many elements.