Search for Short-Lived Axions in a Nuclear Isoscalar Transition

Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We have searched for such a signal in the 3.59 MeV transition in 10B with a fourfold Mini-Orange spectrometer. No axion events were found within one percent of the prediction for the standard axion- to γ-ray branching ratio.