Linearity tests of a digital positron lifetime spectrometer

We present linearity test results for a completely digital positron lifetime spectrometer (DPLS) based on the 1 ch, 2 GS/s Acqiris DP210 digitizer. In general the performance of the apparatus is comparable to that of an analog spectrometer. A particular feature of the linearity of the digital apparatus is that one can observe "semidifferential" nonlinearity arising from the fact that the anode pulses are sampled with finite number of samples with discrete amplitudes and thus the number of different digitized anode pulses is finite. This means that also the possible time intervals measured by the apparatus are discretized. Our measurements show that in positron lifetime measurement setup this semidifferential nonlinearity is smoothed out even at 2 GS/s.