The purpose of the present work is to confirm experimentally that the prompt neutron decay constant of fundamental mode, α₀₀₀, satisfies the following two requirements from the concept proposed in Part (I): To be uniquely measurable free from space dependence and detector specification, and to be calculable from the Boltzmann equation. To prove this, a series of pulsed experiments was made at various subcritical states in a reflected graphite-moderated 20% enriched fuel system (SHE).
The experimental results confirmed that α₀₀₀ was obtained as a common and unique value at any point of both core and reflector, determined by separation of spatial harmonics, using bare and Cd-covered BF₃ counters. The state of SHE was changed by attenuating the core, with or without central control rod, and by adding distributed poison to the core and/or to the reflector.
Direct comparison was made between the experimental results and the calculated values from multigroup treatment using P₁ and S₄ approximation. Calculated values of α₀₀₀ for subcritical states with attenuated core, with or without control rod, showed fairly good agreement with the measured values. In addition, the time-dependent spectral changes of the subcritical systems were also investigated in the experiments.

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To shed further light on the neutron slowing down process, slowing down times were measured in graphite systems by the resonance filter method. The distribution of the slowing down time from 14 MeV to a few energy points from thermal equilibrium was obtained in the form of differences of time response between compensation-filter covered and resonance-filter covered BF₃ counters. From the time distributions thus determined were derived the most probable slowing down time t_max and the average slowing down time <t>, which were compared with the theoretical values calculated with the use of various scattering kernels. The effects of finite medium and space dependence were also evaluated.
The values of t_max obtained were 9.5±0.5, 10.5±0.5, 20±1, 55±4, 71±4 and 101±8 μs for 5.2, 4.91, 1.46, 0.28, 0.2 and 0.11eV, respectively. Of these values, those over 1.46eV show good agreement with the theoretical values by 0°K free gas kernel, while those under 0.28eV agree well with the values by Williams' model, in which the effects of chemical binding and thermal vibration are considered.

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With the view toward measuring neutron flux in a subcritical assembly, a detector consisting of a γ-ray insensitive scintillator based on ₆LiF, ZnS(Ag) and polyethylene combined with a long light guide has been developed. A description is given of performance obtained with this detector, in comparison with BF3 counters and glass scintillator. It has proved that the newly deviced detector with 16 mm diam.×0.42 mm thick scintillator is almost comparable in detection efficiency to a BF₃ gas filled counter (40 cmHg, 96% ₁₀B) having the effective dimensions of 16 mmdiam.×70 mm. The fine spatial resolution obtained with the detector is demonstrated by actual measurements of neutron flux distribution in the assembly.

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A 'one collision operator' is introduced, which expresses the macroscopic-stochastic existence of neutrons in a multiplying system. Using this 'one collision operator', the one-time doublet Boltzmann equation considered by Osborn, et al. is extended in the form of a two-time doublet Boltzmann equation. As an application of the two-time doublet Boltzmann equation, we discuss a method of analyzing the neutron fluctuations in a simple reactor model. In the Rossi-α experiment in the one point model with an infinitely small detector, the amplitude of the main correlation term is reduced in magnitude by half as compared to the corresponding term derived by Orndoff, There has further been found an additional correlation term that decays faster than the main term. We will also discuss the minimum width for a detector to disturb the neutron fluctuations.

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Studies of vibratory compacted fuel have become very active in view of the cost reduction possibilities in fuel fabrication. Recent advances in UO₂-PuO₂ fuels, symbolized in the realization of ATR and FBR, have also given impetus to these studies. This report describes a fabrication technique applicable to various vibratory compaction fuel rods, and also specifies some requirements governing the compaction oflong size fuels to high density.
The principal results are:
(1) Compaction to 85-89% T.D. is possible even using firm fused UO₂ powder of homogeneous particle size.
(2) Addition of 35% of highly fired UO₂ (-20-+325 mesh) to coarse fused UO₂ (-3-+10 mesh) shortens the compaction time, and improves the reproducibility of the final compaction densities.
(3) The addition of a loading weight on top is effective in raising the density when starting with powders of single particle size and in small quantity, but the effect becomes minor for mixed powders and in large quantity compaction.
(4) The quantity charged is one of the major factors determining the vibration time required to obtain high density of compact.
(5) Increase of impact force results in higher densities with the effect of crushing and compression of UO₂ particles.
(6) Vibswaging improves density distribution.

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