A Search for Back-to-Back e+e- Pairs in the Spontaneous-Fission Disintegration of 252Cf

A back-to-back electron-positron pair is searched for in spontaneous-fission disintegration of 252Cf. The emission of such a pair, if observed, might be a manifestation of production and prompt decay of a heretofore unknown neutral particle. The emission rate of such a pair is found to be less than (2.5-5.5)10-10 per fission at the 95% confidence level depending on the mass of the hypothetical particle which is between 40 and 200 MeV/c2.