We report neutron scattering measurements of the kinetic energy (kappa) of
He-4 atoms, close to the superfluid transition temperature T-lambda = 2.17
K. We observe a sharp reduction in kappa from 16.3 to 14.8 K over the tempe
rature range 2.17 K > T > 2.0 K, giving direct experimental evidence that s
uperfluidity is associated with the development of a Bose-Einstein condensa
te. (C) 2000 Elsevier Science B.V. All rights reserved.