K. Drozdowicz

The diffusion cooling coefficient for thermal neutrons in Plexiglas

The thermal neutron diffusion cooling coefficient is a macroscopic material parameter which is needed for a description of the decay of the thermal neutron pulse in a medium and gives information on the diffusion cooling of the thermal neutron spectrum in a bounded volume. Experimental results from various measurements for Plexiglas (a hydrogenous substance) are overviewed in the paper. A method for a theoretical calculation of the parameter is presented. The formula used utilizes other thermal neutron parameters and a cooling function which describes the deviation of the neutron spectrum in a bounded system from the distribution in an infinite one. The energy dependence of the function is obtained numerically from relations which result from the eigenvalue problem of the scattering operator when both the decay constant and the spectrum of the thermal neutron flux are developed in powers of the geometrical buckling. The calculation utilizes Granadas synthetic scattering function. The obtained value of the diffusion cooling coefficient for Plexiglas is at the temperature of C. The uncertainty is estimated to be within the physical model of the scattering kernel used. Results obtained using the same type of kernel in other theoretical methods are also included.

Diffusion cooling of thermal neutrons in basic rock minerals by Monte Carlo simulation of the pulsed neutron experiments

The pulsed neutron experiment (the variable geometric buckling experiment) in spherical geometry has been simulated using the MCNP code. The time decay of the thermal neutron flux has been observed as a function of the sample size. The thermal neutron diffusion cooling coefficient C with the correction F has been determined for three basic rock minerals (quartz, calcite, dolomite) at the given specific densities. The corresponding density-removed parameters have also been obtained.

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.

Thermal neutron absorption cross-section for small samples: experiments in spherical geometry

A new method of determination of the thermal neutron macroscopic absorption cross-section Sigma a for small samples has been checked experimentally. The measurements have been performed with spherical geometry by the pulsed method. Two samples (aqueous solutions of boric acid) with known values of the absorption cross-section have been used. Good agreement of the measured values of Sigma a with the known ones has been observed.

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 parameters for plexiglass

Abstract Thermal neutron diffusion and absorption parameters for plexiglass have been measured in the spherical geometry using the pulsed method. The results obtained are: νΣ a = (4120 ± 30) s −1 , D 0 = (36 882 ± 314) cm 2 s −1 , C = (7149 ± 615) cm 4 s −1 (at 19.5°C).

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.

Detection of explosives and other illicit materials by a single nanosecond neutron pulses — Monte Carlo simulation of the detection process

Recent progress in the development of a Nanosecond Impulse Neutron Investigation System (NINIS) intended for interrogation of hidden objects (explosives and other illicit materials) by means of measuring elastically and non-elastically scattered neutrons is presented. The method uses very bright neutron pulses having durations of the order of few nanoseconds, generated by a dense plasma focus (DPF) devices filled with pure deuterium or a deuterium-tritium mixture as a working gas. A very short duration of the neutron pulse, as well as its high brightness and mono-chromaticity allows using time-of-flight methods with bases of about few meters to distinguish signals from neutrons scattered by different elements.Results of the Monte Carlo simulations of the scattered neutron field from several compounds (explosives and everyday use materials) are presented. The MCNP5 code has been used to get information on the angular and energy distributions of neutrons scattered by the above mentioned compounds assuming the initial neutron energies to be equal to 2.45 MeV (DD) and 14 MeV (DT). A new input has been elaborated that allows modeling not only a spectrum of the neutrons scattered at different angles but also their time history from the moment of generation up to the detection. Such an approach allows getting approximate signals registered by hypothetic scintillator + photomultipler probes placed at various distances from the scattering object, demonstrating principal capability of the method to identify an elemental content of the inspected objects. The extensive computations reveled also several limitations of the proposed method, namely: low number of neutrons reaching detector system, distortions and interferences of scattered neutron signals etc.Further more, preliminary results of the MCNP modeling of the hidden fissile materials detection process are presented.