NEUTRON THERMALIZATION MEASUREMENTS USING AN ELECTRON LINEAR ACCELERATOR

Discussions are presented concerning integral experiments primarily on water and graphite systems. A description of experimental facilities is included along with details of die-away experiments, infinite-medium spectrum measurements, spatially dependent spectrum measurements, spectrum measurements for graphite systems, and development of a subcritical assembly program. (J.R.D.

A Cell‐Type Gamma‐Counter

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INTEGRAL NEUTRON THERMALIZATION. Annual Summary Report, October 1, 1961- September 30, 1962

3 3 9 9 7 6 : < 6 9 9 4 9 7 pulse of within a factor of two of the highest now available; operation was more than satisfactory. A pressure vessel to be used with the linac to measure neutron spectra of poisoned solutions for moderator temperatures up to about 600 l F was constructed. A 150-kv positive-ion accelerator was added to the linac facility to produce 2.5 and 14-Mev neutrons from T(d,n) and d(d,n) reactions. The 14-Mev neutron yield is about 10/sup 11/ neutrons/sec. Facilities for measuring neutron spectra were rearranged to eliminate systematic errors; reorientation of the 16-m detector bank necessitated remeasurement of the sensitivity of the bank as a function of neutron energy. Sensitivity of the 50-m bank was also remeasured. The effects on spectra of the scatterer, the location of pulsed neutron sources, the precollimator location and opening, and the linac intensity were also evaluated. To obtain information on the reliability of flux values predicted by standard transport theory approaches, angular dependent and scalar spectra were measured in 4- and 2-in. slab geometry and were compared with theoretical spectra. One-dimensional slab measurements were analyzed with the DSN and THERMOS codes. Results show that an anisotropic component of the scattering kernel is by far the most important refinement to the isotropic calculation. A multiplying assembly, nearly free of the large spatial effects induced by the external source, was constructed; and spectra were measured under distributed source conditions and compared with one-dimensional transport theory calculations. Neutron die-away was also studied. The feasibility of making spectral measurements by time-of-flight techniques in a system with a relatively long neutron die-away time was considered. Fluxes and spectra were obtained and compared with DSN calculations. Influence of various types of absorption, 1/v or resonance, on infinite-medium neutron spectra for various common moderators was studied. Spectra for various Sm compounds in H/sub 2/O were measured and compared with experimental and theoretical values; the total cross section of H/ sub 2/O was obtained using Nelkin's kernel and Nelkin's kernel with Egelstaff's data; spectra for gadolinium chloride in H/sub 2/O were compared; and spectra for D/sub 2/O and polyethylene were considered. The thermal spectra in lithium hydride were also examined. The theoretical part of the integral thermalization program was concerned with the formulation of analytical descriptions of the scattering laws for low-energy neutrons. Comparisons between the predictions of these scattering laws and available differential scattering data were made. In addition, the theoretical scattering laws were used to calculate thermal-neutron spectra that were also compared with experimental data. The differential scattering cross section for neutrons on graphite was measured to determine the confidence with which the phonon frequency distribution can be derived. Spectra for anisotropic and isotropic graphite were computed. Thermalization of neutrons by zirconium hydride was investigated to determine the applicability of a model based on harmonic lattice vibrations. The model was found not to agree with experimental evidence. Evidence necessitating an alteration of Nelkin's model for neutron scattering by H/sub 2/O was developed. A scattering kernel for lithium hydride was calculated, and gave good agreement between experimental and theoretical neutron spectra. Asymptotic and transient spectra were computed for different moderators and scattering kernels by the times-moment method. The diffusion parameters of H/sub 2/O were studied, and the eigenvalue problem derived from the Boltzmann equation for neutrons approaching thermal equilibrium was considered. Modifications to the THERMOS, DSN, and ECTOPLASM codes were made, and the CERBERUS and CLAUDIUS codes were combined with the ORPHEUS code to give CORC. The new SPAN code i

Los Alamos National Laboratory Report LA-1939 (Del.)

The transmissions of neutrons through spherical shells of normal uranium, oralloy, and Pu239 are reported. One group of measurements was made using a pure fission neutron source produced by thermal neutrons. The other group of measurements was made using monoenergetic neutrons. From both groups of measurements, the inelastic scattering cross sections have been determined according to methods developed in Los Alamos Scientific Laboratory Report LA-1429 (Jaunary 1955)

INTEGRAL NEUTRON THERMALIZATION. Quarterly Progress Report for the Period Ending June 30, 1963

Decay constant and modal analysis measurements were performed in a U/sup 235/-water critical assembly, and infinitemedium neutron spectra in water poisoned with a 1/V absorber were measured at various pressures and temperatures. The results were compared with theoretical predictions. Neutron penetration of water shielding was also measured. Theoretical work involved models and codes for neutron thermalization in ZrH, Be, and water. (D.C.W.