Characterizing Dead Time of Neutron Multiplicity Counters Using Rossi-Alpha Distributions

As neutrons released in a single fission event are temporally correlated, the dead time effects in neutron multiplicity counting are non-trivial resulting in rather complicated corrections of the measured data. While recent theoretical advances (e.g. Correlated Neutron Dead Time Model - CNDTM) take into account this neutron correlation, the implementation of these corrections relies on a single effective dead time. Due to the complexity of the instrument, there may be different sources of dead time, leading to a complex, detector-specific structure to the dead time, not easily characterized by a single paralyzable dead time. In practice, these dead time characteristics may not be readily available. In this paper, we present results of systematic simulation studies of various dead time effects that are artificially introduced into neutron detection time sequences (pulse trains) generated from high-fidelity MCNP simulations. Rossi-alpha distributions created from these pulse trains clearly show the differences between various dead time structures and in turn can be used as a tool to deduce the characteristics of the detector-specific dead time in the experimental data and possibly evaluate an effective value usable in analytical approaches such as CNDTM. Results from comparison with CNDTM are also discussed.