Annihilation of Positrons in Liquid Helium

The annihilation scheme of positrons in liquid helium at 4.2^∘{}K was studied. Three components in the decay were observed with mean lives: τ₁<5×10^-10 sec, τ₂=(2.6±0.2)×10^-9 sec, and τ₃=(1.2±0.2)×10^-7 sec. The abundance of the τ₃ component is (16±{}2)% of all positrons which annihilate in the liquid. Three-quantum annihilation occurs in about 15% of all positron annihilations. This leads to the conclusion that the three components are due to parapositronium, free positron annihilation, and orthopositronium.