Fast digital spectrometer for mixed radiation fields

We present a digital system for measurement of neutron spectra in mixed gamma-neutron fields. The signal from a standard scintillation detector is sampled at rate up to 1 GHz, using 12-bit analog-to-digital converters. First stage of the signal processing is performed on a Virtex 6 FPGA and partial results are streamed through a fast Ethernet link to a computer, where the final processing takes place. The combination of high performance ADCs and flexible FPGA fabric allows the system to reliably process up to 100,000 detection events per second. This greatly reduces the time needed for data acquisition, compared to its analog predecessor that reached count rates approximately 30 times lower. The second main advantage over old analog spectrometers and earlier digital systems with similar parameters is a great reduction of storage space requirements, achieved by performing particle discrimination on-the-fly using the FPGA. A typical experiment that previously required several gigabytes of storage space to store all the raw data samples now requires only a few megabytes to store the measured energy and discrimination parameter for each detected particle.