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: ${\tau }_1<5\mathrm{}\times {10}^{-10\mathrm{}}$ sec, ${\tau }_2=(2.6\mathrm{}\pm 0.2)\times {10}^{-9\mathrm{}}$ sec, and ${\tau }_3=(1.2\mathrm{}\pm 0.2)\times {10}^{-7\mathrm{}}$ sec. The abundance of the ${\tau }_3$ 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.