Accelerator mass spectrometry and radioisotope detection at the Argonne FN tandem facility

The Argonne FN tandem accelerator and standard components of its experimental heavy-ion research facility have been used as a highly-sensitive mass spectrometer to detect several long-lived radioisotopes and measure their concentrations by counting accelerated ions. Background beams from isobaric nuclei have been eliminated by combining the dispersion from the energy loss in a uniform Al foil stack with the momentum resolution of an Enge split-pole magnetic spectrograph. Radioisotope concentrations in the following ranges have been measured: ¹⁴C/¹²C = 10⁻¹² to 10⁻¹³, ²⁶Al/²⁷Al = 10⁻¹⁰ to 10⁻¹², ³²Si/Si = 10⁻⁸ to 10⁻¹⁴, ³⁶Cl/Cl = 10⁻¹⁰ to 10⁻¹¹. Particular emphasis was placed on exploring to what extent the technique of identifying and counting individual ions in an accelerator beam can be conveniently used to determine nuclear quantities of interest when the measurement involves very low radioisotope concentrations. We are able to demonstrate the usefulness of this method by measuring the ²⁶Mg(p, n)²⁶ Al(7.2 × 10⁵ yr) cross section at proton energies in the astrophysically interesting range just above threshold, and by accurately determining the previously poorly known half-life of ³²Si.