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In this paper we present results of a long-term observation of the decay of22Na in the presence of a nuclear fission reactor. The measurements were madeoutside the containment wall of and underneath the Koeberg nuclear power plantnear Cape Town, South Africa. Antineutrino fluxes ranged from ~5*10^11 to1.6*10^13 cm^-2 s^-1 during this period. We show that the coincidence summingtechnique provides a sensitive tool to measure a change in the total decayconstant as well as the branching ratio between EC and beta+ decay of 22Na tothe first excited state in 22Ne. We observe a relative change in count ratebetween reactor-ON and reactor-OFF equal to (-0.51+/-0.11)*10^-4. Afterevaluating possible systematic uncertainties we conclude that the effect iseither due to a hidden instrumental cause or due to an interaction betweenantineutrinos and the 22Na nucleus. An upper limit of ~0.03 barn has beendeduced for observing any change in the decay rate of 22Na due to antineutrinointeractions.

Upper limit on the cross section for reactor antineutrinos changing 22Na decay rates