The four isomers α-, β-, γ- and δ-benzene hexachloride (1, 2, 3, 4, 5, 6-hexachlorocyclohexane) were used as target materials to study the effect of structure isomerism in the Szilard-Chalmers reaction. They were irradiated in solid state and also in benzene solution and frozen states, after which the recoil ³⁸Cl retained therein was stripped into 0.1M K₂SO₃ solution.
In the case of solid state, the radioactivity yields of the isomers decreased rapidly in the early state of irradiation and a corresponding diminution was also seen in the difference in yield between the four isomers. The greatest yield was registered by the δ-isomer, followed by α-, γ- and β-forms in that order, but the difference dwindled down with progress of irradiation until it became insignificant.
But, when the four isomers were irradiated in benzene solution almost the same yield was obtained for all four isomers, and which value of yield did not appear to vary very significantly with progress of irradiation. This value, moreover, was higher than in the cases of either solid or frozen states.
In the case of frozen state, the yield characteristics were intermediate between solid and liquid targets, both in absolute value and in the difference according to structure.

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This paper describes a detailed error analysis of the prompt-jump approximation (PJA) and improvements brought upon the accuracy of the PJA by application of the singular perturbation method. The correction factor derived for interpreting the validity of the PJA is effective in estimating numerical errors based on the PJA and calculating its first-order correction. Taking three representative cases (step, ramp and periodic inputs), we examine the numerical errors qualitatively and quantitatively through comparisons with exact solutions obtained by the Runge-Kutta-Merson method. As a result, it is confirmed that solutions based on the PJA would generally result in overestimation in the case of step and ramp inputs, and that the accuracy of the PJA is considerably improved by correction, as evidenced by the numerical examples given in this work.

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Experiments have been performed with the view to applying the ³He proportional counter to differential measurements of the continuous neutron spectrum in a fast reactor. The results of measurements in the JAERI Fast Critical Facility (FCA) proved the apparatus to be useful for the measurements of the main part of the fast reactor spectrum.
A specially devised electronic circuit was employed to eliminate the effect of differences in the rise-times of pulses; the energy resolution of the spectrometer was 25keV (F. W. H. M.).
The modified inverse matrix method was used to obtain the neutron spectrum from the measured pulse-height distribution, resulting in an improvement of the effective energy resolution in the resultant spectrum to about 10keV. The applicable energy range of the spectrometer is for the time being from 20keV to 2MeV.

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An experimental technique to evaluate models for lattice calculation by measuring what is known as microparameters is presented. It is shown that a generalized four-factor formula expressed in terms of the microparameters could successfully evaluate sources of discrepancy between measurement and calculation in regard to gross quantities, such as critical mass and multiplication factor. Lattices studied are of UO₂, enriched in ²³⁵U to 2.5 and 1.5W/0, with water-to-fuel volume ratio ranging from 0.43 to 3.5. Detailed description is given on the measurement of the microparameters in epithermal and fast energy regions. Analysis of the results has been made mostly with the JUPITER (modified MUFT)-THERMOS-SUNRISE (diffusion) code system.
A new experimental approach for directly measuring the non-leakage probability is also presented. The multiplication factor obtained by this method is compared with that derived from the microparameters. The resulting good consistency supports the validity of both of these experimental techniques.

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A method of determining k∞ for subcritical heterogeneous assemblies with reflector is described. This method is based on the measurement of the equivalent infinite flux volume integral in an assembly containing no poison. To take account of the experimental fact that radial flux distributions in the core are influenced by the heterogeneous core structure, the formulation for the integral was improved. On the basis of the equation thus formulated, the flux-distribution data were analyzed to yield a reliable value of the integral.
The value of k∞ is determined by combining this integral with the measured thermal utilization. The result obtained by this method is compared with that given by the buckling measurement using the critical equation. The two results are in good agreement. The present method is considered particularly useful for the determination of k∞ in unpoisoned heterogeneous assemblies.

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The effect of additives on the radiolytic yield of nitrogen from the radiolysis of 2, 2'-azobis-isobutyronitrile (AIBN) in benzene solution is examined. These additives have been known to quench excited benzene very effectively. Diacetyl, naphthalene, 2, 5-diphenyloxazole (PPO) and anthracene with excitation energies lower than that of AIBN in the lowest triplet state decrease the nitrogen yield. The rate constants of these compounds in quenching the excited benzene are seen to be equal for both singlet and triplet states. An anomalously high quenching rate constant is seen for anthracene, which could be attributed to the quenching of highlying excited state benzene.
Certain ketones with excitation energies higher than that of AIBN in the lowest triplet state also decrease the nitrogen yield with the exception of benzophenone, these compounds are all seen to provide about the same degree of reduction in nitrogen yield, in the range of concentration studied. The rate constants of self-quenching and the average life of acetone and acetophenone have been determined from kinetic studies, and the results agree with that by other investigators.
The effect of several charge scavengers on the nitrogen yield are also reported.

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