Neutron activation analysis of eWaste using a medical cyclotron

Abstract

Neutron activation analysis (NAA) is an analytical technique that harnesses a decay product of certain radioactive isotopes. This works in a two-step process: neutron activation followed by gamma spectroscopy analysis. While a research reactor is typically used for NAA, a medical cyclotron can also be used as neutrons are produced as the byproduct of its radionuclide production. A potential source of these neutrons is from the reaction 18O(p,n)18F, where a H218O target is irradiated by a proton beam to generate 18F for radiotracer synthesis. This fact paired with its easier access compared to a research reactor demonstrates a potential benefit of using medical cyclotron for NAA. This work focused on using a medical cyclotron to analyse electronic waste (eWaste) samples using NAA. A variety of elements were assessed, including aluminum, silicon, copper, iron, magnesium, tin, chromium, arsenic, cadmium, neodymium, gold, palladium, and nickel. Analysis of single-element calibration standards gave a high degree of linearity. A variety of multi-element samples were then prepared and analyzed. These included small circuit board fragments from a variety of eWaste, custom-made standards containing a selection of the previously mentioned elements, and heterogenous samples containing similar selections. Each of the samples were irradiated for 20 minutes or less and had their gamma rays collected via gamma spectroscopy for 10 minutes or less. Spectral data was analysed with InterSpec, with peak areas being corrected for radionuclide decay, neutron beam exposure, and gamma count time. Across all the sixty-one gamma peaks analysed, approximately two thirds (62%) of the calculated masses were above the true or expected values for the element. Except for arsenic and palladium (35% higher and 40% lower, respectively), all NAA-determined elemental masses were within 26% of their expected value in the heterogeneous samples. These samples provided the most accurate assessment of this technique, as they were the only samples in which the elemental composition was known prior to NAA. Thus, the comparison of initial element masses to those determined through NAA had a high level of validity and accuracy.