Observation of anomalously low momentum transfer in the low energy scattering of large 4He droplets from 4He and 3He atoms

The momentum transferred to large superfluid He-4 droplets ((N) over bar 1.5.10(9)) in low, energy (approximate to 4-10 K) scattering from He-4 and He-3 atoms was determined from time of Fight measurements after the droplets have passed through a low temperature (T = 1.7-4.2 K) scattering box filled with the gas of either He isotope. The results are compared with He-3 droplets scattered from either He-4 or He-3 gas for which all of the incident momentum is transferred, as expected for classical capture of the scattering gas atoms. In the case of the He-4 droplets a smaller momentum transfer is found amounting to 65% for He-4 atoms and 45% for He-3 atoms but only at the lowest collision energies. These results are consistent with transmission of some of the atoms through the droplets.