Measured line peak of the SPI background 511 keV line. Each data point (vertical bar, with uncertainty) represents the line peak, determined via Eq. (11), of all detectors in one orbit (revolution) of the INTEGRAL mission. The data set used ranges up to revolution 1279. Outliers, such as around revolution 400 (E_peak= 511.075 ± 0.015 keV) have been excluded from our data set beforehand. The horizontal dashed line marks the laboratory energy.