Limitations of the beam-foil method in lifetime measurements

We have measured the decay time of the 1s(2)2p P-2(3/2) level of Li-like Ar15+ using beam-foil spectroscopy. The experiment was done at a beam energy of 2.6 MeV/u and with an Al foil instead of a C foil which is usually used in beam-foil experiments. The signal was recorded by means of a CCD detector system installed on a 2.2 meter grazing incidence spectrometer. This system allowed recording of the spectral line-profile decay. The result shows different decay times for different parts of the measured line profile, thereby corroborating our previous studies of the same transition in Ar15+ by using a C foil and the same energy. In the latter work some difficulties were revealed in determing reliable lifetimes for excited levels in highly charged ions. We discuss the causes of the uncertainties in decay time (hence lifetime) measurements and suggest a scheme for increasing the reliability of beam-foil lifetime experiments.