Barbara Gabańska

A pulsed measurement of the effective thermal neutron absorption cross-section of a heterogeneous medium

Abstract The material heterogeneity for the thermal neutron absorption has been experimentally investigated. Silver grains of different sizes in a Plexiglas matrix have been used as models of the heterogeneous material. Three silver-in-Plexiglas models have been designed and constructed in order to create a controlled lattice of absorbing centres. The grains are small cylinders of the height equal to the diameter (4, 6 and 10 mm) and their total volume content φ=0.05 in all the models is constant. These models allow the investigation of an isolated effect of the grain size only on the neutron absorption. The effective thermal neutron absorption cross-section has been measured. The results strongly depend on the silver grain size and significantly differ from the cross-section of a corresponding homogeneous material. The ratio of the absorption cross-section of the heterogeneous to homogeneous material has reached 0.34 for the largest silver grains and 0.66 for the smallest ones.

Influence of the grain size on the effective absorption cross-section of thermal neutrons in a medium containing highly absorbing centres

Abstract The thermal neutron absorption is considered in a medium which contains highly absorbing grains. The theoretical approach is based on a definition of the effective macroscopic absorption cross-section for such a medium. The parameter of the grain effect is defined as the ratio of the effective cross-section to the cross-section of a corresponding homogeneous material. It is generally obtained as a function of the ratio of the absorption cross-sections of components, of the grain size, and of the contributions of the components. A further analysis is performed for the grain size effect when the two other parameters are fixed. The presented approach is applied to an interpretation of experiments on Plexiglas models containing silver grains. A good agreement of the experimental and theoretical results is obtained when relevant effective energy-dependent cross-sections are weighted by the thermal neutron energy distribution.

A 14?MeV neutron generator as a source of various charged particles produced in fusion reactions

Measuring the energy of ions from the thermonuclear reaction in future energetic tokamaks (like ITER) is important in order to obtain information on the energetic balance in a plasma toroidal column. Detectors made of synthetic diamond can be used for the spectrometry of ions which accompany burning plasma. A fast neutron (14?MeV) generator, which is a linear accelerator of deuterons, is based on the nuclear reaction T(d,n)? in a tritium target. The energy of alpha particles produced in the D?T reaction in the neutron generator is the same (maximum 3.5?MeV) as the energy of alpha particles present in the hot D?T plasma in tokamaks. Other reactions in the target also occur and the energy spectra of various created ions can be also measured. The experiments have been performed with an ion spectrometry made possible with the use of scCVD diamond detectors at the fast neutron generator (IGN-14) at the Institute of Nuclear Physics in Krak?w, Poland.

Are geological media homogeneous or heterogeneous for neutron investigations

The thermal neutron absorption cross section of a heterogeneous material is lower than that of the corresponding homogeneous one which contains the same components. When rock materials are investigated the sample usually contains grains which create heterogeneity. The heterogeneity effect depends on the mass contribution of highly and low-absorbing centers, on the ratio of their absorption cross sections, and on their sizes. An influence of the granulation of silicon and diabase samples on the absorption cross section measured with Czubeks method has been experimentally investigated. A 20% underestimation of the absorption cross section has been observed for diabase grains of sizes from 6.3 to 12.8 mm.

Thermal neutron diffusion cooling in two-region small systems

The thermal neutron diffusion cooling effect is a result of the perturbation of the thermal neutron energy distribution owing to the leakage of neutrons outside a finite volume of the medium. In multi-zone systems an additional perturbation appears because of diffusion of neutrons between regions which have different diffusion cooling properties. A study of the thermal neutron diffusion cooling in two-region small cylindrical systems is presented in this paper. A Plexiglas shell (hydrogenous medium) completely surrounds the inner cylinder filled with an absorbing aqueous solution (hydrogenous medium) or with a mixture of hydrogenous and non-hydrogenous substances. The pulsed neutron technique has been used for the experimental investigation of the problem. The time decay constant of the thermal neutron flux in the two-zone systems has been measured. It has also been calculated as a function of the system geometry and neutron dynamic parameters, one of these being the diffusion cooling coefficient of the outer Plexiglas shell. From a combination of results of the calculation and the experiments, it has been found that the obtained function for the particular diffusion cooling coefficient of the Plexiglas shell is controlled by the scattering properties of the inner zone of the system.

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.

Pulsed thermal neutron source at the fast neutron generator.

A small pulsed thermal neutron source has been designed based on results of the MCNP simulations of the thermalization of 14 MeV neutrons in a cluster-moderator which consists of small moderating cells decoupled by an absorber. Optimum dimensions of the single cell and of the whole cluster have been selected, considering the thermal neutron intensity and the short decay time of the thermal neutron flux. The source has been built and the test experiments have been performed. To ensure the response is not due to the choice of target for the experiments, calculations have been done to demonstrate the response is valid regardless of the thermalization properties of the target.

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