[en] A search has been conducted for light neutral particles like the axions which decay rapidly to electron positron pairs produced in the reaction n + p → D + a. Instead the e⁺e^- signal expected from direct internal pair conversion has been detected. A preliminary analysis gives (Γ/sub p//Γ/sub γ/)/sub expt//(Γ/sub p//Γ/sub γ/)/sub theo/ = 1.06 +- 0.12. The results exclude existing axion models over the mass range 2 m/sub e/ < m/sub a/ < 2.22 MeV/c². The axion mass would fall in this range if they were responsible for the narrow positron lines observed in recent heavy-ion experiments at the GSI