In measurements of fast neutron spectra by a 3He semiconductor detector, the unfolding method is not usually required. The unfolding method based on principle of maximum likelihood that incorporates the Gaussian approximation of counting statistics is developed and implemented in the MLMHE3 numerical code for application in fast neutron spectrometry by 3He semiconductor detectors. The derived likelihood equations have been solved using method of the singular value decomposition of the response matrix. For this inverse problem, the detector responses were generated by the Monte Carlo technique.

Methods applied in the calculation and interpretation of the measurements of the fast neutron spectrum in the NERBE coupled fast-thermal system are validated in this paper. When advantages and disadvantages of a He-filled semi-conductor-sandwich detector are compared to other neutron detectors, the former is found more appropriate. The neutron detection is based on the reaction {sup 3}He(n,p)T + 0.764 MeV and simultaneous detection of the reaction products in the silicon diodes. The pulses from the diodes are amplified and shaped in separate {open_quotes}energy{close_quotes} channels and summed to produce a single pulse with height proportional to the energy of the incident neutron plus the Q value of the reaction. A well-known measuring system of the He neutron spectrometer is used for the HERBE fast neutron spectrum measurement and calibration in a thermal neutron field.

The delayed neutron parameters and methods used for calculation in reactor safety studies are verified by measurement of the effective delayed-neutron fraction {beta}{sub eff} in the coupled fast-thermal system HERBE. The HERBE system is strongly heterogeneous. It consists of the fast core loaded with the natural uranium fuel elements. The thermal core is composed of the 80% enriched UO{sub 2} moderated and reflected by heavy water. Methods applied in the calculation and interpretation of {beta}{sub eff} measurement are described in this paper.

The results of measurements {beta}{sub eff} and {beta}{sub eff}/{Lambda} and calculation results based on various sets of evaluated six-group delayed neutron parameters for the coupled fast-thermal system HERBE are shown in this paper.