On the traceably accurate voltage calibration of electrostatic accelerators

We describe in detail a calibration method for the terminal voltage of small accelerators used for ion beam analysis, with the elastic resonance of ¹⁶O(α,α)¹⁶O at 3038 keV as the intrinsic measurement standard. The beam energy relative to this resonance is determined with a precision around 300 eV and an evaluated reproducibility of 1.0 keV. We show that this method is both robust and convenient, and demonstrate consistency with calibration relative to three other independent methods: using radioactive sources and using the resonant ²⁷Al(p,γ)²⁸Si and non-resonant ¹⁶O(p,γ)¹⁷F direct capture reactions. We re-evaluate the literature and show that the peak in the cross-section function is at 3038.1 ± 2.3 keV. By comparing the results obtained with ¹⁶O(α,α)¹⁶O to the other calibration methods we show that this uncertainty can be reduced to 1.3 keV.