Data acquisition with a fast digitizer for large volume HPGe detectors

A 12-bits and 420 MSample/s waveform digitizer was used with large volume High Purity Germanium (HPGe) detectors for measurements of gamma-ray production cross-section from inelastic neutron scattering. For these time-of-flight measurements at the GELINA white neutron source, the use of a fast digitizer significantly increases the efficiency compared with a conventional data acquisition system. First, the pulse processing time required to obtain the amplitude with high resolution is significantly reduced so that pulse pile-up with the prompt gamma-burst is eliminated for neutron-induced events. Second, an improved time response is obtained for which the amplitude and rise time dependence is strongly reduced compared to that of a conventional constant fraction discriminator. Excellent energy and time resolution is obtained with algorithms suitable for on-line signal processing, so that data storage is under control. Bench tests are presented that compare methods of signal processing. For the best method, the data acquisition system based on the fast digitizer was tested during measurements of gamma production cross-sections for Pb206 and Pb208. A direct comparison was made with results obtained with conventional electronics operated in parallel.