The C 13 nuclear magnetic resonance has been observed in the unenriched carbides of thorium and uranium ThC, TI1C2, UC, U2C3, and UC2. In contrast to nearly all nonmagnetic metals and semimetallic compounds, the Knight shift in the semimetallic thorium carbides is negative. On the other hand, the Knight shifts in all of the uranium carbides are positive, which is consistent with the P 31 shift in UP and PrP. The C 13 Knight shift in all of the carbides except U2C3 was temperature-independent. On the basis of magnetic susceptibility evidence, it is found that the uranium carbides are similar to other semimetallic uranium compounds in that their magnetic properties are best explained at higher temperatures with a localized /-electron model with large crystal-field splitting. The C 13 Knight shift in the uranium carbides is interpreted in terms of an effective hyperfme field resulting from the presence of two 5/ electrons in UC and UC2 and three in U 2 C 3 . Only the lowest crystal-field level of the 3 # 4 configuration (the nonmagnetic level Ti in the case of UC) is appreciably populated in UC and UC2. A comparison of hyperfme fields in a variety of lanthanide and actinide compounds suggests that the fields in semimetallic compounds and insulators have similar origins and that the differences are associated largely with structure, covalency, and overlap, and not with the presence or absence of conduction electrons.
The excitation spectrum of a free exciton in GaSe has been measured in magnetic fields up to nearly 200 T. The measured spectrum compares favorably with a theoretical calculation done within the effective-mass approximatio'n, consistent with a recent interpretation that the mass anisotropy is smail, p~~/p "=' 1.
Uranium carbide is found to have the property of high thermionic emission in the observed temperature range from 1400 to 1800°K. The parameters of the Richardson equation which can be used to fit the data are A=7.3×105 amp/cm2 (°K)2 and φ=4.57 v. While these parameters accurately predict the observed saturated currents, it is doubtful that such anomalous values are of theoretical significance within the framework of the Richardson-Dushman derivation. A solid solution form of uranium carbide and zirconium carbide gives substantially the same thermionic emission as uranium carbide, and for other reasons appears to be the more useful emitter. The carbides require no surface preparation or activation schedule for use as emitters.
SUMMARYMeasurements of the detonation wave structure at the axis of long cylindrical charges of nitromethane and TNT are described. The measurements show that the structure consists of three zones: (1) a reaction zone, (2) zones, is not predicted by any extrapolation of one-dimensional theory.
*In the liquid explosive nitromethane, the pressure at the head of the decay zone is essentially independent of the charge size, but the extent of the zone is very strongly dependent an the charge diameter and length.The decay zone increases in length as the detonation runs. In a 37 mm diameter charge, after a run of 16 diameters, the decay zone is about 0.6 mm long and the pressure falls about 27 kbar in the zone.
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