HIGH RESOLUTION NEUTRON SPECTROSCOPY OF THE CROSSOVER IN THE VIBRATIONAL DENSITY OF STATES OF SILICA AEROGELS H. CONRAD, J. FRICKE et G. REICHENAUER J. Phys. Colloques, 50 C4 (1989) C4-157-C4-162 DOI: 10.1051/jphyscol:1989425