Zero deadtime spectroscopy without full charge collection

The Savannah River Technology Center has built a remote gamma monitoring instrument which employs data sampling techniques rather than full charge collection to perform energy spectroscopy without instrument dead time. The raw, unamplified anode output of a photomultiplier tube is directly coupled to the instrument to generate many digital samples during the charge collection process, so that all pulse processing is done in the digital domain. The primary components are a free-running, 32 MSPS, 10-bit A/D, a field programmable gate array, FIFO buffers, and a digital signal processor (DSP). Algorithms for pulse integration, pile-up rejection, and other shape based criteria are being developed in DSP code for migration into the gate array. Spectra taken with a two inch Na(I) detector have been obtained at rates as high as 59,000 counts per second without dead time with peak resolution at 662 KeV measuring 7.3%.