Search for neutron-antineutron oscillations using multiprong events in Soudan 2

We have searched for neutron-antineutron oscillations using the 5.56 fiducial kiloton-year exposure of the Soudan 2 iron tracking calorimeter. We require candidate nn ̄ \= occurrences to have $>~4$ prongs (tracks and showers) and to have kinematics compatible with n ̄ \=N annihilation within a nucleus. We observe five candidate events, with an estimated background from atmospheric neutrino and cosmic ray induced events of 4.5±1.2 events. Previous experiments with smaller exposures observed no candidates, with estimated background rates similar to this experiment. We set a lifetime lower limit at 90% C.L. for the nn ̄ \= oscillation time in iron: TA(Fe)>7.2×10³¹ yr. The corresponding lower limit for oscillation of free neutrons is τ_nn ̄ \=>1.3×10⁸ sec.