Positronium formation in liquid nitrogen as a function of electric field

The intensity of the orthopositronium component and its lifetime have been measured in liquid nitrogen as a function of electric field. While the lifetime remains constant at about 12.5 ns, the fraction of positronium formed is about 20% at zero field and decreases monotonically to an asymptotic value near 6% as the field is raised to 3.2 MV m-1. The decrease is roughly exponential with a drop off rate of 2.0 MV m-1. The results are a further indication of the importance of Mogensen's spur mechanism in the positronium formation process. (author)