This paper deals with the analysis of experimental positron lifetime spectra in polymer materials by using various algorithms of neural networks. A method based on the use of artificial neural networks for unfolding the mean lifetime and intensity of the spectral components of simulated positron lifetime spectra was previously suggested and tested on simulated data [Pžzsitetal, Applied Surface Science, 149 (1998), 97]. In this work, the applicability of the method to the analysis of experimental positron spectra has been verified in the case of spectra from polymer materials with three components. It has been demonstrated that the backpropagation neural network can determine the spectral parameters with a high accuracy and perform the decomposi-tion of lifetimes which differ by 10% or more. The backpropagation network has not been suitable for the identification of both the parameters and the number of spectral components. Therefore, a separate artificial neural network module has been designed to solve the classification problem. Module types based on self-organizing map and learning vector quantization algorithms have been tested. The learning vector quantization algorithm was found to have better performance and reliability. A complete artificial neural network analysis tool of positron lifetime spectra has been constructed to include a spectra classification module and parameter evaluation modules for spectra with a different number of components. In this way, both flexibility and high resolution can be achieved.

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 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.

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.

In recent work we have developed a general one-speed transport theory model for the calculation of the multiplicity moments, used in nuclear safeguards. Quantitative results were obtained for spheres, cylinders of various shapes, and shells. In all these works, similarly to the point model, the only neutron reaction was assumed to be fission. Since the quantitative results for highly multiplicative systems were not in agreement with recent experiments, we extended the model to include isotropic elastic scattering without energy loss of the neutrons. In the present work, we generalise the model further, to include inelastic scattering, which requires the introduction of energy dependence and anisotropic scattering. In the paper, the significance of the elastic scattering, as well as the need for including inelastic scattering is demonstrated, and the extension to energy dependent transport theory is described and illustrated with one example.