The cross sections of boron and gold, which are widely used as standards for slow neutron measurements, have been determined as functions of wavelength. Total cross sections were measured in the wavelength range 1.5-10A with the Brookhaven slow chopper, the long wavelengths being used to minimize the effects of scattering. The absorption cross section of boron of "normal" isotopic constitution is 749±4 barns at 2200 m/sec (1.80A); the variation of cross section with boron source is expected to be only about 1 percent. The measurement of the gold cross section is complicated by the deviation from 1/v resulting from a nearby resonance and by grain orientation effects in the scattering. The 2200-m/sec value, resulting from measurements at long wavelength together with a small correction for the 4.9-ev resonance, is 98.7±0.6 barns.

Studies have been made of the relationship between the charge transfer ΔQ and the mass loss ΔM that accompany the disintegration of water drops of radius R, mass M, and charge Q falling through an intense electric field. Calculations show that, if the resultant drop, of radius r, is on the verge of disintegration, the ratio of the charge transfer to the fractional mass loss will be given by the equation MΔQ/ΔM = [4.6(R3T)1/2(γ2 − γ3/2) + (1 − γ2)Q]/(1 − γ3), where γ equals r/R and T is the surface tension. Studies in which ΔQ and ΔM were measured directly showed that the experimental values of MΔQ/ΔM were only slightly greater than the predicted minimum values. This finding suggests that raindrops falling through the strongly electrified lower regions of thunderclouds may undergo a series of disintegrations during which appreciable loss of mass and charge may occur. This conclusion is supported by photographic evidence.

SUMMARYCalculations have been made, on the basis of the ionic balance equations derived by Phillips (1967), of the electrical conductivity of clouds in the presence, and in the absence, of secondary ion production resulting from corona.In weakly electrified clouds -no corona -the immobilization of ions on cloud droplets resulting from diffusional capture results in a reduced ionic concentration and conductivity. The normalized conductivity 1 (the ratio of the in-cloud value to the clear-air value at the same altitudez) was calculated for three different cloud types (cumulus congestus,-strato-cumulus and fog) as a function of Z, liquid water content L, droplet charge Q and field strength E. 3, was found to be very sensitive to variations in L and E, but only slightly affected by changes in Z, Q and the manner in which the charge is distributed over the size-spectrum. For example, when E = 0 I possessed the values 0.034 and 0.17 in-a cumulus congestus cloud for L = 0.5 gm/m3 and 0.1 gm/m3 respectively, whereas when E = 90 kV/m I was 1.5 x and 7.6 x respectively for the same two values of L.The experimental measurements of corona onset fields and currents from ice hydrometeors, E, and i, respectively, made by Griffiths and Latham (1974) were used to calculate the normalized conductivity within strongly electrified clouds. Corona from ice provides a copious supply of positive and negative ions which causes I to increase from between to greater than 1 when E, is achieved. The value of 1 once corona is initiated depends on L and the concentration of corona sites, J. In this situation altitude Z plays an important role because E, decreases quite rapidly with decreasing pressure. For example, with a typical distribution of temperature and pressure within a thundercloud, corona will be initiated from hailstones at E, = 720 kV/m if Z = 2.5 km (0" C) and 540 kV/m at 5 3 k-m (-18" C); the corresponding values of E, for snowflakes are 540 and 390 kV/m respectively. At 2.5 km I is slightly higher than and lower than 10 respectively, as L is increased from 1 to 5 gm/m3 with J =

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A simple theory is developed for taking into account non-random orientations in microcrystalline material of the hexagonal system. The transmission of slow neutrons through extruded and through powdered graphite is measured experimentally and compared with the theory.