Thermally induced dislocation loops in polycrystalline ice

White-beam synchrotron X-ray topography has been used to study the circular prismatic [0001] dislocation loops which are commonly observed on the (0001) plane in polycrystalline freshwater ice. A new method, involving detailed analyses of the effects of beam divergence on the loop images, has been developed to determine whether a loop is of vacancy or interstitial type. In an 0002 image, one half of a loop (projected as an ellipse) appears as a single image and the other half as a double image. Experimentally, it was found that the 0002 vector drawn from the centre of a loop passes through the single image if the loop is of vacancy type and through the double image if a loop is of interstitial type. This method of loop characterization was confirmed by performing theoretical analysis of both the dislocation image widths and their strain fields.