Time evolution of background rates for lines from radioactive species. Top: ⁶⁰Co activation, leading to gradual build-up due to the radioactive lifetime of 7.6 years. Bottom: ⁴⁸V activation, with a decay time of 16 days. Exponential decay can be seen after an activation event (insert) such as a particle storm following deformations of the Earth’s magnetosphere from solar activity; but the shorter decay timeavoids long-term build-up such as seen in ⁶⁰Co.