Advanced neutron coincidence counting techniques applied for the assay of special nuclear materials additionally also determine the factorial moments of the detected neutron multiplicity distribution. From these factorial moments the actual neutron detection efficiency or neutron multiplication can be inferred via a so-called three-parameter-analysis in which the spontaneous fission rate together with two other unknowns can be determined. The factorial moments are generally obtained with data from multiplicity counters, but can also be derived by the Time Interval Analysis (TIA) technique based on Rossi-alpha distributions. In the past, the importance of accurate corrections for counting losses of the counting data from the multiplicity counters has already been shown and deadtime correction models for the three-parameter-analysis using multiplicity counters were presented. This article presents deadtime correction formulas for a three-parameter-analysis based on the recording of Rossi-alpha count-rate distributions. Whereas dead time correction formulas in neutron assay are generally based on a model with a single deadtime parameter, also a more general multi-deadtime parameter description is derived which can account for different counting losses in the different detection chains of a neutron counter. The multi-deadtime parameter correction formalism was tested experimentally using a fast Time Interval Analyser board which also allows to directly monitor the effect of counting losses and measure deadtime parameters.
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