Measurement of a probability of electron-positron pair radiation at the capture of thermal neutrons by protons

Measurements were performed by registration of γγ-coincidence spectrum at the capture of thermal neutrons (E=50 MeV) with polyethylene target protons. Gamma-spectra were measured by means of two scintillation detectors with NaI(Tl) crystals. The pair conversion probability value obtained is equal to (5.8 ± 2.4)x10-4. It agrees with up-to-date data on the probability of internal pair conversion at γ-transitions in nuclei and contradicts the recent calculations of this value within the relativistic momentum approximation. The obtained experimental value of probability excludes also the production in this reaction of a short-lived axion with τa < 5x10-11 s and ma=1.8 MeV