GRIDSA: femtosecond lifetime measurements with germanium detector arrays

We demonstrate the possibility to extract nuclear state femtosecond lifetimes from two-step γ ray cascades measured with a Ge-detector array. The technique is based on measuring the Doppler shift of a γ ray, caused by the recoil of a preceding γ ray emission. Since the two γ rays are populating/de-populating the same state they form a start/stop signal, the delay of which is compared to the slowing down motion of the nucleus within the target material. A multi-detector array combined with digital acquisition electronics in list mode, allows to measure several angular combinations and two-step cascades efficiently and simultaneously within one single experiment. The concept was demonstrated with the FIPPS array for the 35Cl(n, γ) 36Cl reaction, where we obtained good agreement with literature values showing the validity of the method.