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 ∼ 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 (γ) radiation fields from fusion plasmas: separation between γ 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 γ 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.