Slow Neutron Inelastic Scattering Study of Light Water and Ice

Dual spectrum neutron radiography: identification of phase transitions between frozen and liquid water

In this Letter, a new approach to distinguish liquid water and ice based on dual spectrum neutron radiography is presented. The distinction is based on arising differences between the cross section of water and ice in the cold energy range. As a significant portion of the energy spectrum of the ICON beam line at Paul Scherrer Institut is in the thermal energy range, no differences can be observed with the entire beam. Introducing a polycrystalline neutron filter (beryllium) inside the beam, neutrons above its cutoff energy are filtered out and the cold energy region is emphasized. Finally, a contrast of about 1.6% is obtained with our imaging setup between liquid water and ice. Based on this measurement concept, the temporal evolution of the aggregate state of water can be investigated without any prior knowledge of its thickness. Using this technique, we could unambiguously prove the production of supercooled water inside fuel cells with a direct measurement method.

High-resolution neutron scattering measurement of the dynamic structure factor of heavy water

The low momentum collective dynamics of heavy water has been investigated by means of neutron three-axis spectroscopy. Working at the resolution limit of this instrument, an energy resolution of 2.6 meV was achieved for a constant analyzer energy of 120 meV. This good resolution allowed us to establish definitely that the collective dynamics of water is dominated by the presence and interaction of opticlike and acousticlike branches, coupling at wave-vector transfers of 0.3 to 0.35 A(-1). The transition from slow to fast sound has been attributed to the interaction between opticlike modes and a solidlike fast acoustic mode.