The magnetotactic multicellular prokaryote (MMP), a motile aggregate of bacterial cells, is known to exhibit an unusual "ping-pong" motility in magnetic fields greater than the earth's field. This motility is characterized by rapid excursions, opposite the direction of an applied magnetic field, and slower returns along the direction of the magnetic field. We have carried out detailed observations of the time and spatial dependence of the ping-pong motility and find 1), the outward and return excursions exhibit a uniform deceleration and acceleration, respectively; 2), the probability per unit time of an MMP undergoing a ping-pong excursion increases monotonically with the field strength; and 3), the outward excursions exhibit a very unusual distance distribution which is dependent on the magnetic field strength. At any given field strength, a characteristic distance is observed, below which very few excursions occur. Beyond this distance, there is a rapid increase in the number of excursions with an exponentially decaying distribution. These observations cannot be explained by conventional magnetotaxis, i.e., a physical directing torque on the organism, and suggest a magnetoreceptive capability of the MMP.

The lifetime spectra of positrons annihilating in helium gas have been obtained over the temperature range 4.6-300 K and the density range 0.016-0.030 gjcm3. The spectra are decomposed into slow-positron and orthopositronium annihilation components which are presented and discussed separately. At low temperatures, the density and temperature dependences of the data are found to depart markedly from the usual behavior observed at room temperature. Simple quantitative treatments are given of the cluster model for the slowpositron data and of the cavity model for the ortho-positronium results.

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The first observation of Lyman-z radiation from positronium is described. Positronium is formed in vacuum by a slow positron beam incident on a solid target. Single photons transmitted through an = 20-A-wide interference filter peaked near the 2480-A Lyman-n (2P 18) line of positronium are observed in coincidence with the annihilation y's of ground state positronium. The observed intensity corresponds to a production of one positronium "atom" in the excited state for every 104 incident positrons.