ANNIHILATION OF POSITRONS IN LIQUID HELIUM AND THE POSITRON–HELIUM BOUND STATE

Abstract

It is shown that it is possible for a positron to form a bound state with a helium atom, with a binding energy of about 0.55 electron volt. The lifetime of positrons in liquid helium is substantially longer than the predicted lifetime for annihilation from this state, indicating that capture into such a state takes a time considerably longer than the annihilation lifetime. On the other hand, the observed lifetime implies an electron density at the positron larger than the mean electron density in the material, consistent with the expected attraction of the positron toward the helium atom.