Emission and evaporation characteristics of a porous tungsten cathode impregnated with the composition 5BaO·2Al2O3 are presented and are interpreted in terms of the cathode mechanism. Barium necessary for activation is generated by the reaction, 23 Ba3Al2O6+13 W=13 BaWO4+23 BaAl2O4+Ba,and is transported through partially clogged pores, the length of which increases with time, predominantly via Knudsen flow. During transport, oxygen is acquired from the tungsten, leading to a substantial content of BaO in the evaporant. The BaAl2O4 component of the impregnant is inert. Emission is substantially lower than that of an L cathode, presumably because of release of a poisoning agent accompanying the activator.

An experimental study of the L cathode is presented, including the chemistry of carbonate decomposition and barium generation, the origin of BaO in the evaporant, the barium transport mechanism through the porous plug, and the factors determining cathode life. Carbonate decomposition is accompanied in low porosity cathodes by the deleterious reaction BaCO3+13W=13Ba3WO6+CO. Barium is generated in the cavity at an equilibrium pressure by the reaction 2BaO+13W=13Ba3WO6+Baand is transported continuously through the porous tungsten predominantly by Knudsen flow. During transport the barium acquires oxygen from the tungsten, producing in the evaporant a large BaO content which helps to activate the surface. Termination of life coincides with completion of the BaO–W reaction and onset of the subsequent reaction 23Ba3WO6+13W=BaWO4+Ba,which is accompanied by release of a poisoning agent. Material decrease in evaporation rate and a corresponding increase in cathode life may be gained by addition of SrCO3.

By means of photographic magnetic spectrometers located close to the heavy water moderated reactor, the following radioactive nuclei have been examined: Se 77m (17.5 15 min), and Th 233 (23.6 min). The 27.5-hour activity assigned to Pm 151 was discovered in this work. A new vacuum gate for the camera of the spectrometer permits the study of activities with half-lives as short as ten seconds. Energy level schemes for the product nuclei are proposed on the basis of a spectrographic analysis of the gamma-rays and also of coincidence and absorption measurements. Changes in spin and parity between many of the energy levels are determined from measurements of the K/L ratios of the corresponding gamma-rays.

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