Removal of particles from air is discussed in terms of three classes of filtering media--air-sampling, respirator, and air-cleaning. Plots of efficiency versus filtering velocity are presented. The pressure drop characteristics of the filters are discussed in terms of initial pressure drops of the clean filters, variations in pressure drop of the individual filters, and comparison of loading rates.

Several improvements have been made in the gamma scintillation counter (GSC). The Model V gamma scintillation counter uses a canned thallium activated sodium iodide crystal as a detector. Although the electronic components remain unchanged, a modification of the sample support has been made to improve reproducibility of geometry. To assure comparable results between various counters, they must be operated at the same energy threshold and counting yield. Methods have been developed to assure operation at a preferred energy threshold of 0.1 Mev and an arbitrarily fixed counting yield of 3.36% for Cs-Ba-137. Partial pulse height discrimination against U 237 is also accomplished.

In 1949 Barad presented two solutions to the general diffusion equation. Basic in both solutions is the assumption that in very stable atmospheres a point source may be replaced by a vertical area of uniform concentration at a short distance downwind. This vertical area is considered to exist at the distance at which the plume finally "levels-off" and assumes a flat ribbon-like appearance. In addition if the distance over which diffusion takes place is limited to one or two miles and if only the stable atmosphere with its narrow range of eddy sizes is considered, the average diffusion co-efficient (K) has a signficance which it does not have in the case of neutral equilibrium.

In obtaining the results reported here a double focusing magnetic type spectrometer was used. Momentum spread was about 1%, and in most cases 2560 counts per point were taken. An end window Geiger tube was used as a detector. The window was of mica with areal density of 1.6 mg/cm2.

Various organizations in the Biophysics Section of the Radiological Sciences Department participated in evaluating local radioactive particle fallout as influenced by the nuclear explosions at the Nevada Proving Grounds during the early part of 1953. Data collected from approximately 50 air monitoring stations operated by the Regional Survey forces were used to determine the trend of radioactive particle concentrations in the atmosphere between March and June. These measurements were supplemented with the findings of associated organizations in the Biophysics Section during and immediately after the peak influx on May 26.

Some data on the temperature effect in pulse columns are reviewed. New data obtained with the "vibrational" stirrer are reported. The effects on both the dispersion and coalescence and the net rate of extraction suggest that improved operation of the RA column and lowered waste losses would be expected at elevated temperatures.

Seven runs were made with the system Penn. Ref. Co. "Supersol" (dispersed) water (continuous) to investigate the effects of pulse frequency, amplitude plate spacing, volume flow ratio, and throughput on backmixing, using the technique of continuous injection of a tracer solution into the packed section. Backmixing was found to be surprisingly insensitive to all the above variables over the range studied with the exception of pulse amplitude and continuous phase flow rate, showing greatest dependence on the later. Backmixing increased with decreasing continuous phase flow rate and increased with pulse amplitude.

The monitoring of thermal and resonance neutron flux in a thermal reactor having high flux over periods of time from 1 to 12 months using think Co foils is considered. Special attention is paid to the many correction factors to be applied to the activation data; neutron temperature, effective cadmium cutoff energy, burnout of Co59 and Co60, and decay of Co60. Results on a homogeneity test of 10 mil, 0.08% Co-A1 alloy foils is given.

A first iteration of the diffusion theory result is executed for the thermal flux distribution in a 7.5" graphite-uranium lattice cell using the integral equation formulation of transport theory.