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 $n\mathrm{n̄}$ occurrences to have $>~4$ prongs (tracks and showers) and to have kinematics compatible with $\mathrm{n̄}N$ annihilation within a nucleus. We observe five candidate events, with an estimated background from atmospheric neutrino and cosmic ray induced events of $4.5\pm 1.2$ 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 $n\mathrm{n̄}$ oscillation time in iron: $T_A\left(\mathrm{Fe}\right)>\mathrm{}7.2\mathrm{}\times {10}^{31}\hspace{0.5em}\mathrm{yr}.$ The corresponding lower limit for oscillation of free neutrons is ${\tau }_{n\mathrm{n̄}}>1.3\mathrm{}\times {10}^8\hspace{0.5em}\sec .$

• Received 29 May 2002

DOI:https://doi.org/10.1103/PhysRevD.66.032004