Search for correlated two-photon emission from e+(82Sr)+Th interactions

A previously observed electron line at INS at 330.3±0.4 keV in e++Th interactions has been implied by these authors to originate from the decay of an unknown neutral object X0. QED predicts a two-photon decay mode of X0 leading to a correlated two-photon coincidence peak at 841.3±0.4 keV. To search for such a peak γγ-coincidence experiments were performed. From an observed peak-like structure in the spectrum at 841.7±0.3 keV and an associated cross section of 10.6±3.1 (stat) ± 6.9 (syst) μb an upper limit for the cross section of 15.2±1.5 (syst) μb for the production and 2γ decay of the X0 was deduced with a statistical confidence level of 95%.