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The ERC cold-cathode ion source has been in routine production use on several PR-30 systems for the past three years. This source has been further developed to improve target current, lifetime, and stability. The ion-optical lens has been changed from circular to elliptical geometry in order to provide an asymmetric beam for entry into the PR-30 analyzing magnet. This measure, as well as the use of higher extraction voltages, provides higher beam currents on the PR-30 target wafers. Beam steering in the nondispersive direction has been provided to correct the effects of minor machine misalignments, further enhancing target current. The discharge chamber has been modified to increase source lifetime. A new gas-feed control system and a new method of oven temperature control have been devised to provide good source and ion beam stability. The source operates with only occasional attention by unskilled personnel, and has been used principally for boron and arsenic implants. Target currents of 1-mA boron and 4-mA arsenic can be obtained routinely. Lifetimes are of the order of 40–80 h, depending on ion species. The source has also been used to provide 5-mA phosphorus, 4-mA argon, 3-mA helium and neon, and 0.3-mA nickel and palladium ion beams.

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Marked deviations from simple exponential decay are known to characterize the two-photon annihilation of positrons in the noble gases. In this experiment, a quadruple coincidence system was used to examine the detailed shape of three-photon positron lifetime spectra in argon. No deviation from a simple exponential was observed over the density range 2.57 to 33 amagat, except when a high electric field was applied. The ability to distinguish three-photon ortho-positronium annihilation from other modes of positron annihilation permitted, for the first time, a detailed investigation of the density dependence of the annihilation of ortho-positronium in a simple monatomic gas. Over the whole density range, the ortho-positronium annihilation rate was found to depend linearly on density, with a value of 7.15 ± 0.10 μs−1 for the vacuum rate of ortho-positronium annihilation and 0.250 ± 015 μs−1 amagat−1 for the quenching rate of positronium in argon. With a field of 125 V cm−1 amagat−1 (at 2.57 amagat), a deviation from simple exponential decay was observed, a deviation whose shape is shown to be consistent with an interpretation in terms of time-delayed enhanced positronium production from the equilibrium free positron population.

No abstract