L S Kothari

Steady-state fast neutron spectra in slabs on natural uranium-a comparison of diffusion and transport theories

Space-dependent fast neutron spectra have been calculated in slabs of natural uranium by solving the multigroup diffusion equation. It is shown that the eigenfunction expansion method can be used even in this case when the fission term is present, by using the biorthogonality property between the eigenfunctions of the actual matrix and those of the transpose matrix. These results have been compared with the corresponding results based on transport theory. Except close to the source and the free boundary, the authors find good agreement between the two sets of results.

A model for the lattice vibrations of solid methane

Abstract A model for the lattice vibrations of solid methane is proposed. It takes account of both translational and rotational modes of vibration. The calculated specific heat agress very well with the experimental values in the temperature range (5°–90°K). This study supports the view that CH4 molecules in solid methane are capable of free rotation.

Frequency distribution for graphite based on the unfolding technique

Abstract We have extended the “unfolding technique”, developed in Reactor Physics, to calculate the frequency distribution function for graphite, starting from the measured values of its specific heat at different temperatures. The advantages and limitations of this approach are discussed.

Effective debye temperature for surface vibrations of a crystal

Abstract Recent studies show that the mean square amplitude of vibrations of atoms on the surface of a crystal is almost twice that in the bulk. We show that this result follows from simple physical arguments.

The existence of equilibrium spectra in fast natural uranium assemblies. The effect of anisotropy in scattering

The calculated results of steady state, space- and angle-dependent fast neutron spectra inside various one dimensional slabs of natural uranium are reported. The effect of anisotropy in scattering on fast neutron spectra has been studied for three different orders of anisotropy, zero, first and third. It is shown that with a high energy, plane source of monoenergetic neutrons, spectral equilibrium is established in natural uranium slabs beyond 90 cm from the source plane, with an associated diffusion length of 19.9 cm. This is consistent with the lower bound for diffusion length in natural uranium. Further, the neutron flux is larger when anisotropy in scattering is included in the calculations.

A model for the phonon frequency distribution function for crystalline polyethylene

Abstract A model for the phonon frequency distribution function of crystalline polyethylene has been developed considering the mass of the vibrating unit to be two methylene groups and distinguishing between the ‘hard’ longitudinal and transverse dispersive modes. It explains the temperature variation of specific heat of polyethylene and can be used for other studies.

The dynamics of liquid H2O and the thermalization of neutrons

A study has been made of the asymptotic as well as time-dependent decay of neutron pulses in water assemblies at various temperatures in the range 05-60 °C. A new scattering kernel for neutrons has been proposed which takes account of low-energy collective oscillations of H2O molecules in the Debye approximation and which also includes contributions from rotational and intramolecular vibrational modes. The calculated values of the total neutron scattering cross section for water at 20 °C are found to agree rather well with the experimental results. The values of the asymptotic decay constant calculated for different B2 at various temperatures also agree well with the corresponding experimental results. The temperature variation of the diffusion coefficient D0 and the diffusion cooling coefficient C is also in agreement with most of the experimental results. Transient spectra in assemblies with different B2 and at various temperatures are successfully compared with those reported by Ishmaev et al (1965) and Menzel et al (1970). The values of thermalization times tth obtained by the authors are consistent with the waiting times quoted by Ishmaev et al (1965) The importance of introducing rotational and intramolecular vibrational modes, apart from the collective elastic modes, is demonstrated by neglecting the contribution of these other modes. Two cases have been considered: one corresponding to θD=200 K, and these other to θD=250 K. It is shown that under certain conditions the exclusion of rotational and intramolecular vibrational modes markedly affects various neutron thermalization parameters.

Atomic scattering factor for an anisotropic charge distribution

Abstract Recent X-ray diffraction experiments show that when an external electric field is applied to crystal, it produces marked changes in the intensity of the diffracted X-ray beam along certain scattering directions. We show that this can, in principle, be explained in terms of the atomic scattering factor for an anisotropic electronic charge distribution in atoms.

Effect of steady electric field on the lifetime of minority carriers in p-n junction solar cells

The Shockley-Read-Hall (SRH) theory of electron-hole recombination at traps has been modified to include the effect of steady electric field on the minority carriers in p-n junction solar cells. The modified expression predicts enhanced lifetime of minority carriers in the base region.

Pulsed neutron studies in water poisoned with non-1/ν absorbers

Thermalization parameters for water poisoned with non-1/ nu absorbers, Cd, Gd and Sm have been calculated using the neutron scattering kernel recently proposed by the authors (see abstr. A43583 of 1974). Calculated values of the parameters are compared with the available experimental data and with values based on other scattering kernels. For the first time a reasonable agreement between the theoretical and recent experimental results is obtained. Time-dependent spectra in infinite assemblies have also been calculated and reaction rates as functions of time have been computed. These are in close agreement with the measurements of Fujino et al. (1971).