DYNAMIC FORM-FACTOR OF LIQUID-HE-4 AT INTERMEDIATE MOMENTUM-TRANSFER

Measurements of the inelastic neutron scattering from liquid 4He at T = 1.2 K in the range of wavevector transfer 3 &le Q &le 12 angstroms1 have been performed to verify the oscillations in the peak position and the width of the scattering function with Q observed by R.A. Cowley and A.D.B. Woods and by P. Martel et al. We confirm these oscillations and extend the measurements to show that the oscillations in the width continue up to Q 12 angstroms1 while oscillations in the peak position could be observed with confidence up to Q 8 angstroms1 only. We also present a straightforward RPA calculation of the dynamic form factor S(Q,ω), beginning from the pair interatomic potential, to see how well the observed S(Q,ω) can be described and to investigate the origin of the oscillations. The observed S(Q,ω) is quite well reproduced and the oscillations in the width and peak position of S(Q,ω) are seen to originate from oscillations in the interaction in the RPA which we have approximated as the 4He-4He scattering amplitude calculated from v(r).