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.

Figure 1

Technical implementation of dual spectrum measurements based on a polycrystalline beryllium filter. By motorizing the filter, consecutive images with (continuous line) and without (dashed line) filter were acquired.

Figure 2

Relative attenuation (${\sigma }_{\text{rel}}$) as a function of the optical density without filter. Averaging over one hour is used to reduce the noise.

Figure 3

Temporal evolution of the liquid (LQ) fraction parameter $f_{\text{LQ}}$ during a temperature ramp (spatial averaging over a surface of approximately $1\times 5\mathrm{mm}$).

Figure 4

Spatial distribution of the calculated liquid fraction parameter for images of the water column at (a) $-1°\mathrm{C}$ and (b) $1°\mathrm{C}$ of the water column and the cathode flow channels (c) $-1°\mathrm{C}$ and (d) $1°\mathrm{C}$. Figure (e) illustrates the spatial distribution of the water content inside the fuel cell. All images are averaged over one hour to reduce the noise.

Figure 5

Temporal evolution of the calculated liquid fraction parameter $f_{\text{LQ}}$ inside the flow channels of a small-scale fuel cell (average over four channels corresponding to a total area of $2{\mathrm{mm}}^2$).