Digital pulse processing: a new paradigm of nuclear instrumentation

Full text: High speed (100-200 MHz) and precise (12-14 bits) analog to digital converters are now available at relative low cost, the same occurring with devices known as FPGA - Field Programmable Gate Arrays. These facts made possible a large development of a new kind of data acquisition and pulse processing for nuclear physics experiments, where all after the pre-amp is digital. These new systems are already in production by the related industry, mainly for simple and specific applications, but are the very complex applications that gave a large impulse in the development of such systems. In low energy nuclear physics, for instance, new detecting systems now being planed, like the gamma-ray spectrometers GRETA in the US and AGATA in Europe, will have all pulse analysis and data acquisition based in this new technology. In these systems, the signal generated by the pre-amplifier coupled to the detector is the only analogically processed part of the acquisition system. A flash ADC samples the pre-amp pulse at a a rate of 100 millions per second, and with 12-14 bits of precision. Every pulse will then produce about 1-2K samples. All the pulse processing usually performed by analog devices, like spectroscopic amplifiers, fast amplifiers, constant fraction discriminators, etc. are now realized digitally, after the sampling of the pre-amp pulse. A large part of this processing is done in the FPGA devices, that can do it in a continuous flux at about 20-40 Mbytes per second. Several FPGA can work in parallel to process the large number of channels, normally necessary in these systems. We are now learning, testing and evaluating a simple system using this new technology, in order to develop a similar system to be used in our Laboratory. (author)