Fast digitizing techniques applied to scintillation detectors

A 200 MHz 12-bit fast transient recorder card has been used for the digitization of pulses from photomultipliers coupled to organic scintillation detectors. Two modes of operation have been developed at ENEA-Frascati: a) continuous acquisition up to a maximum duration of similar to 1.3 s corresponding to the full on-board memory (256 MSamples) of the card: in this mode, all scintillation events are recorded; b) non-continuous acquisition in which digitization is triggered by those scintillaton events whose amplitude is above a threshold value: the digitizing interval after each trigger can be set according to the typical decay time of the scintillation events; longer acquisition durations (> 1.3 s) can be reached, although with dead time (needed for data storage) which depends on the incoming event rate. Several important features are provided by this novel digital approach: high count rate operation, pulse shape analysis, post-experiment data re-processing, pile-up identification and treatment. In particular, NE213 scintillators have been successfully used with this system for measurements in mixed neutron (n) and gamma (gamma) radiation fields from fusion plasmas: separation between gamma and neutron events is made by means of a dedicated software comparing the pulse charge integrated in two different time intervals and simultaneous neutron and gamma pulse height spectra can be recorded at total count rates in the MHz range. It has been demonstrated that, for scintillation detection applications, 12-bit fast transient recorder cards offer improved performance with respect to analogue hardware; other radiation detectors where pulse identification or high count rate is required might also benefit from such digitizing techniques.