Fresh UN powder of a medium particle size (0.3 m²/g) prepared by nitridation of U powder was exposed to O₂ at room temperature in a constant volume apparatus. Oxidation was followed by pressure measurement. Under O₂ pressures higher than 3 mmHg the powder ignited instan-taneously, probably due to local heating in the surface layers of the powder particles. When the O₂ pressure was lower, oxidation proceeded in two clearly distinct stages, without N₂ release. The first stage terminated instantaneously. Irrespective of differences in powder conditions and pressure, the amount of O₂ absorbed in the first stage was found almost invariably to be about 1.7 cm³ STP/m², which corresponds to the formation of an oxidized layer somewhat thicker than 18Å. The powder particles covered with this layer did not ignite even under 1 atm O₂. This oxide coating could be of practical utility for protection against ignition. The second stage proceeded very slow and could be completely suppressed by cooling.

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Dose buildup factors and number spectra of γ-rays transmitted through stratified slabs have been estimated by the multiple scattering method, taking scattering into consideration up to the fourth order and including back scattering.
Further, the method proposed in the previous paper for estimating the contribution to the total dose buildup factor by γ-rays of the fifth and higher orders of scattering was applied to the present case of stratified slabs made up of different materials.
The calculations covered the range of source energy from 1 to 10 MeV, and various arrange-ments of strata made up of combinations of two of the three materials water, iron and lead, making up, slabs of total thicknesses up to 6 mfp. Calculations were also performed for ⁶⁰Co γ-rays normally incident on stratified slabs of water-lead-water.
The results of the above calculations were found to be in good agreement with those of other calculations, such as by Monte Carlo method and by numerical integration of the photon transport equation; The present results were also found to agree well with the dose buildup factors calculated by our own empirical formula. Comparisons with experimental results too have proved good.

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An airborne radioiodine sampler composed of cellulose asbestos filter paper (HV-70 or HE-40), charcoal-impregnated filter paper (CP-20B) and two charcoal cartridges (CHC-50) was devised with the aim of ensuring higher and more reproducible collection efficiencies under high humidity conditions encountered in routine air monitoring. This paper describes the results of tests conducted on this sampler to determine its performance in collecting airborne 131I released from an iodine processing cell in the author's institute, and discusses the factors affecting the collection efficiency.
The results are summarized as follows:
(1) The sampler is notably superior in performance to current forms consisting of single filter medium in that the collection efficiency is far less dependent on relative humidity. Its col-lection efficiency is maintained at more than 75% on the average during a long sampling period of 24 hr even under very high humidity conditions.
(2) During a short sampling period of 30 min the collection efficiency exceeds 0% for the airborne iodine sampled at the exit of an air cleaning system containing charcoal filters, . while the efficiency reaches almost 100% for the unfiltered iodine under conditions of rela-tive humidity ranging from 40 to 60%.
(3) The collection efficiency is not affected significantly by face velocity.
(4) The desorption of iodine collected in the sampler depends sensitively on relative humidity.

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A study has been made on how simple analysis of pulsed neutron experiments could be applied to the measurement of reactivity of fast systems with a blanket of metallic natural uranium. Pulsed neutron experiments have been performed on three fast assemblies of the Fast Critical Assembly at the Japan Atomic Energy Research Institute. The decay constants have been successfully observed even at delayed critical. The energy and spatial higher modes have been also observed. Simple models of Gozani and Simmons-King have been applied to derive the reactivity in dollar units.
It has been made clear that the reactivity could be measured by the pulsed neutron method down to -3, and in a certain case, down to -5, for fast systems with a core of medium size up to at least about 200l enclosed in a metallic natural uranium blanket. The establishment of the asymptotic decay has been experimentally investigated. Linearity has been ascertained to hold between the prompt neutron decay constant and the reactivity by the pulsed neutron method and between the reactivities by the pulsed method and by the positive period method down to a few dollars in negative reactivity.

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Formulas are derived for calculating the energy loss distribution of recoil protons in a cylindrical organic scintillator with a broad beam of fast neutrons in the energy range 1 to 15 MeV perpendicularly incident on its base. The assumptions are: (1) the scintillator is composed solely of hydrogen and carbon; (2) only elastic scattering from hydrogen and carbon is considered in relation to the interaction of fast neutrons with the scintillator; (3) single and double scat-tering alone are considered; (4) wall effect is neglected for recoil protons produced in the second collisions; (5) the radius and the thickness of the scintillator are greater than the range of recoil protons of the highest energy.
As an example, the calculated results are shown for Stilbene crystals of 1" dia. and 1/8" to 1" thick. In scintillators of thicknesses below 1/4", and for incident fast neutrons in the energy range from 3 to 15 MeV, the contribution of double scattering to the energy loss distribution of recoil protons is below about 10% of that of single scattering, while wall effect is quite signifi-cant, in particular, for incident neutrons of high energy.

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