Krzysztof Drozdowicz

A method to calculate thermal neutron diffusion parameters for hydrogenous mixtures

Abstract A comprehensive method is proposed to calculate the thermal neutron diffusion parameters for homogeneous mixtures of any non-multiplying non-crystalline compounds, in particular including hydrogenous substances. The macroscopic absorption, scattering, and transport cross sections of the mixture are defined generally as functions of the incident neutron energy. The energy-averaged neutron parameters can be calculated when the energy dependence and the thermal neutron energy distribution are known. For most substances the scattering cross section can be assumed as being almost constant in the thermal neutron region (which results from the free gas model). However, serious deviations are observed for hydrogen atoms bound in molecular systems. In the paper certain effective scattering cross sections are found in this case on the basis of individual accurate data for a few hydrogenous media. The macroscopic parameters calculated are averaged over the Maxwellian energy distribution for the thermal neutron flux. Results agree well with available data.

Monte Carlo simulation of pulsed neutron experiments on samples of variable mass density

Abstract A method is presented to facilitate the interpretation of a pulsed neutron experiment (the variable geometric buckling experiment) when the mass densities of individual samples differ. A generalisation of the classic expression, which connects the fundamental mode decay constant to the thermal neutron diffusion parameters and to the geometrical buckling, is presented. The method has been tested (on polyethylene) by means of a computer simulation of the experiments. The simulation has been based on a Monte Carlo method, using the MCNP code. The described generalised buckling method is especially recommended for experiments with bulk materials.

A generalized interpretation of buckling experiments for thermal neutrons

Abstract The buckling experiment is a well-known pulsed measurement method used to determine the thermal neutron diffusion parameters of a medium. In the classic form of the method, a dependence between the geometric buckling B 2 (in cm −2 ) and the thermal neutron time decay constant λ (in s −1 ) are involved. In the present paper, dimensionless values for the decay constant and for the buckling are introduced. This unification offers a method to generalize the description of the buckling experiment and makes it possible to compare experiments made for different media. The application of this procedure is exemplified on results for polyethylene of two different densities (from a Monte-Carlo-simulated experiment) and for Plexiglas (from a real laboratory experiment). The conclusion is that the buckling experiment is relatively easy for interpretation in the dimensionless buckling range which does not exceed 0.2. It corresponds, for example, to the geometric buckling B 2 =1.1 cm −2 for polyethylene of density 0.57 g cm −3 or to B 2 =3.0 cm −2 for polyethylene of density 0.95 g cm −3 .

The effective absorption cross-section of thermal neutrons in a medium containing strongly or weakly absorbing centres

The structure of a heterogeneous system influences diffusion of thermal neutrons. The thermal-neutron absorption in grained media is considered in the paper. A simple theory is presented for a two-component medium treated as grains embedded in the matrix or as a system built of two types of grains (of strongly differing absorption cross-sections). A grain parameter is defined as the ratio of the effective macroscopic absorption cross-section of the heterogeneous medium to the absorption cross-section of the corresponding homogeneous medium (consisting of the same components in the same proportions). The grain parameter depends on the ratio of the absorption cross-sections and contributions of the components and on the size of grains. The theoretical approach has been verified in experiments on prepared dedicated models which have kept required geometrical and physical conditions (silver grains distributed regularly in Plexiglas). The effective absorption cross-sections have been measured and compared with the results of calculations. A very good agreement has been observed. In certain cases the differences between the absorption in the heterogeneous and homogeneous media are very significant. A validity of an extension of the theoretical model on natural, two-component, heterogeneous mixtures has been tested experimentally. Aqueous solutions of boric acid have been used as the strongly absorbing component. Fine- and coarse-grained pure silicon has been used as the second component with well-defined thermal-neutron parameters. Small and large grains of diabase have been used as the second natural component. The theoretical predictions have been confirmed in these experiments.

Energy calibration of neutron detectors for the neutron spectrometer TANSY

Abstract We present a method to calibrate an array of scintillation neutron detectors using a gamma source. The count rate is measured as a function of high voltage at a given discrimination level. The obtained distribution is differentiated and a maximum value is determined which corresponds to the voltage at which the gamma peak passes through the discrimination level. By repeating the measurement at different discrimination levels the experimental dependence between the discrimination level and the high voltage is found as a straight line in a log-log diagram. Two calibration parameters for each detector are determined from a fit of these straight lines. The obtained calibration parameters of the detectors give an absolute relation between the output fast pulse amplitude and the energy of radiation for any detector high voltage. Tests of the calibration method have been performed. A relation between the neutron detector calibration parameters and producers data for the photomultipliers has been investigated and the two kinds of data correlate well. The calibration method has been used to get the detector settings for the TANSY neutron spectrometer. Test procedures for the spectrometer during its regular operation are proposed and a method is given to calibrate a future spare detector to be included in the array.

Influence of granulation of the sample on the thermal neutron Σa measurement

Abstract The theoretical approach to the influence of granulation of samples on the measured thermal neutron absorption cross-section has been verified in an experiment on samples containing irregular grains. The neutron absorption heterogeneity effect is defined as the ratio G of the absorption cross-section of the grained sample to the cross-section of the corresponding homogenised medium. The parameter G=1 when the heterogeneity of a material does not influence the thermal neutron transport, otherwise G