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An electrohydrodynamic (EHD) technique is used to generate ions from liquid metals. Liquid metal is fed to the tip of a capillary needle emitter with a voltage difference applied between the emitter and an extractor electrode to produce an intense electric field at the liquid surface. Electrostatic forces overcome surface tension forces to produce ions by field emission. When using liquid cesium, time-of-flight mass analysis showed the ion current to be primarily Cs+ with a small percentage of Cs2+ and Cs3+. Electron currents of over 1 mA have been produced by operating the emitter at 2 kV negative. Besides cesium, alkali ion beams have been generated using NaK/cesium alloy and sodium. Calculations show that liquid metals of low work functions appear more suitable for production of atomic ions while higher work functions metals may produce multiatomic ions and charged droplets in addition to atomic ions.

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A value has been obtained for the absolute pair production cross section of lead for the 2.76 Mev. gamma rays of Na24. A collimated beam of these gamma rays was made to fall on a specially constructed target and the positrons produced in the target were detected by counting two-quanta annihilation events by means of two scintillation spectrometers in coincidence. The strength of the Na24 source was measured by a coincidence counting technique and the counter detection efficiency for annihilation radiation was measured with a calibrated Na22 source. The effect of absorption of the 0.511 Mev. annihilation radiation in the target was measured in a separate experiment. The value obtained for the cross section was 2.38 ± 0.62 barns.

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