POSITRON DIFFUSION AND ANNIHILATION IN HELIUM

The diffusion and annihilation of slow positrons have been studied by the method of delayed coincidences for helium at 77°k and densities of 20-70 amagat. The non-exponential shoulder in the free-positron component is confirmed and has a total breadth of more than 1300 ns amagat, corresponding to a Ramsauer-Townsend-type minimum in the diffusion cross section Q of about 0·031 πa02 at an energy of 1·7 ev. Application of electric fields of up to 33 v cm-1 amagat-1 failed to enhance positronium formation, contrary to the findings of Marder et al., we find Q > 0·115 πa02 around 17·7 ev. Zeff is found to be 3·677 ± 0·025 at thermal energy, dropping to about 3·3 near 1 ev, in agreement with recent experiments of Lee and Jones. Drachman's theoretical s-wave phase shifts (quasi-lower bounds) appear to be very reliable and we have set an upper limit on these phase shifts by constructing intermediate values and comparing thermalization times with experiment. Drachman's values of Z eff are a few per cent too low below v = 0·8 ryd 1/2, but probably not anywhere by more than 8%. The variation of the positronium annihilation rate with pressure fits an approximately parabolic form above 40 amagat, resembling similar results of Celitans et al. for oxygen-free argon, but at lower densities in helium this annihilation rate becomes anomalous.