L. S. Kothari

Calculations on the Low‐Temperature Specific Heat of Selenium and Tellurium

A model of lattice vibrations for linear lattices is proposed in which longitudinal and transverse components of vibrations are considered separately, unlike the model proposed by Tarassov. This model has been applied to interpret the specific heat data for selenium and tellurium obtained by DeSorbo. The agreement with the experimental results is found to be good.

An eigenvalue and eigenfunction study of space-dependent fast neutron spectra in /sup 232/Th

The multigroup space eigenvalues and eigenfunctions of a one-dimensional steady-state diffusion theory operator have been used to study the spatial behavior of a fast neutron field in certain thorium systems. The nuclear data used are from the 26-group ABBN data set. It has been shown that for a fast thorium system, unlike a fast uranium system, all the space eigenvalues lie in the continuum and no discrete space eigenvalue exists. A fast thorium system behaves more like a fast nonmultiplying system. The spectra shifts continuously to lower energies as one moves away from the source; however, pseudoasymptotic conditions are established in certain distance ranges. In order to test the validity of the diffusion theory and eigenfunction expansion method, results have also been obtained using transport theory. In all cases the two sets of results are in reasonably good agreement. To see the effect of geometry, the spectra at certain distances inside a 1-m-thick thorium slab are compared with the corresponding spectra inside a thorium sphere of 1-m radius. At all distances the normalized slab and sphere spectra are nearly the same.

Decay of fast neutron pulses in uranium assemblies

The multigroup diffusion equation is solved for the pulsed problem and the timedependent energy spectra are obtained using the eigenfunction expansion method. It is shown that complete spectral equilibrium can be obtained beyond 2000 ns in a 40-cm cube of natural uranium. This time is found to increase with increasing assembly size. The earlier exponential decays observed in a pulsed uranium system are traced to the establishment of the pseudo-equilibrium condition due to the trapping of neutrons in certain energy groups. It is shown that such a pseudo-decay corresponds to the establishment of the first higher mode of decay and the time range in which it is established is a function of B/sup 2/.

Anisotropy in the scattering of thermal neutrons in crystalline moderators-beryllium oxide

Anisotropy in the scattering of thermal neutrons from beryllium oxide has been investigated. Elastic differential scattering cross sections have been calculated by replacing the omicron function occurring in the expression by a Gaussian. The effect of changing the width of the Gaussian on the differential cross section has also been studied. Anisotropy in inelastic scattering has been calculated for coherent one-phonon and incoherent one- and two-phonon processes. Using these differential cross sections, the authors have calculated the energy distribution of neutrons scattered along different directions by a beryllium oxide slab and these results are compared with corresponding measured results. The agreement between the two sets of results is found to be good.

Effect of anisotropy in scattering on neutron angular flux inside slabs

For one-speed neutrons, an investigation has been made of the effect of anisotropy in the scattering kernel on their angular distribution at various space points inside slabs of different thicknesses. This has been done by solving the one-speed space-angle-dependent neutron transport equation for a pulsed source, by numerically iterating over the space and angle variables. It is found that the fundamental decay constant decreases with increasing anisotropy, the effect being more in thicker slabs. The effect of scattering anisotropy on the angular flux inside slabs, as well as on leakage flux, has been reported. For a specific scattering kernel, the variation of angular flux with thickness of the slab is also discussed. Some of these results have been compared with those based on P

Space- and angle-dependent steady-state thermal-neutron spectra inside small beryllium assemblies. II

sub 1

A New Variational Method for Pulsed-Neutron and Diffusion-Length Problems in Nonmultiplying Assemblies

and P

A modified source term for use in solar cell theory

sub 3

Study of pulsed fast neutrons in an infinite iron assembly

approximations. 2 tables, 4 figures. (auth)

Neutron wave propagation across a temperature discontinuity-multigroup approach

Earlier calculations of steady-state space- and angle-dependent thermal-neutron spectra in small beryllium assemblies have been extended to assemblies of much greater transverse dimensions and neutron diffusion has been studied up to much greater distances from the source plane, with a view toward looking for a discrete mode of decay. It is found that in the forward direction, the neutron distribution fails to attain equilibrium inside 140-cm-thick assemblies with transverse dimensions of 150 x 150 cm/sup 2/, whereas in the backward direction, equilibrium is reached even inside an assembly of transverse dimensions of 80 x 80 cm/sup 2/. It is shown that in the forward direction, equilibrium is delayed by the presence of a penetrating beam of uncollided sub-Bragg neutrons of the source. Thus, an experimentalist can hardly hope to observe equilibrium in the forward direction. The calculated value of diffusion length is in excellent agreement with the observed as well as the theoretical values obtained by earlier workers.