FPGA Implementation of a Digital Constant Fraction for Fast Timing Studies in the Picosecond Range

Thermal or cold neutron capture on different fission systems is an excellent method to produce a variety of very neutron-rich nuclei. Since neutrons at these energies bring in the reaction just enough energy to produce fission, the fragments remain neutron-rich due to the negligible neutron evaporation thus allowing detailed nuclear structure studies. In 2012 and 2013 a combination of clover and standard coaxial Ge detectors plus very fast LaBr3 scintillators has been installed at the PF1B cold neutron beam of the Institut Laue-Langevin (ILL). The present paper describes the digital acquisition system used to collect information on all gamma rays emitted by the decaying nuclei. Data have been acquired in a trigger-less mode to preserve a maximum of information for further off-line treatment with a total throughput of about 10 MByte/sec. Special emphasis is devoted to the FPGA implementation of an on-line digital constant fraction algorithm allowing fast timing studies in the pico second range.