Subboundaries in hcp 4He crystals studied by SR X-ray topography

The internal structure of hcp 4He crystals was studied by SR (synchrotron radiation) X-ray topography. Subboundaries in the crystals appeared as black or white bands in the X-ray topographs. In one of the hcp4He crystals, the subboundaries turned out to be flat planes perpendicular to the basal plane. They were small-angle tilt boundaries which consisted of basal edge dislocations. The dislocation spacing in one of the subboundaries was determined to be 800 nm and the total density of the boundary dislocations to be 2.6 x 105 cm-2. The subboundaries in another hcp 4He crystal were curved and/or branching, indicating that the crystal was strained