More than four decades ago A. F. Andreev predicted theoretically the existence of quantum states of 3He atoms on free surface of liquid 4He . In other words, at low temperatures, even for extremely dilute solutions, a substantial number of 3He atoms are adsorbed at the surface, forming 2D Fermi liquid. We therefore propose use of neutron reflection to study the 2D 3He layer adsorbed on liquid 4He. According to calculations , the reflected neutrons should exhibit oscillatory behaviour caused by existence of 3He layer. One of the most interesting and important aims of the proposed experiment is to study the influence of temperature on the process of formation of the Andreev states.