Spectral structure of 245-445 keV electrons and positrons in positron-thorium scattering

Anomalies in low-energy positron scattering from thorium are searched for. The incident positrons were emitted from a ⁶⁸Ga source with an end-point energy of 1.90 MeV. Coincidences between electrons and positrons in the energy range 245-445 keV were registered in a double beta spectrometer. The electron and positron energy spectra are best described in terms of Bhabha scattering with a small additional structure at an energy of about 320 keV for both the electrons and the positrons. The significance of this peak is estimated to be three standard deviations. The spectrometer was also used in a study of scattering of electrons emitted in the decay of ⁹⁰Y, with an end-point energy of 2.28 MeV. The electron-electron coincidence spectrum in the energy range 245-445 keV is well described as resulting from Möller scattering alone.