Fission-Fragment Tracks and Directional Effects in the Surface of LiF Crystals

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Abstract
Elongated damage structures resulting from fission fragments traveling nearly parallel to the surface have been observed in (100) cleavage faces of bulk LiF crystals. These tracks have lengths up to 2.3 μ and are about 100 Å wide. The direction in which the fission fragment has entered the LiF crystal can be determined from the characteristics of tracks which cross typical fracture steps in the crystal surface. When the fragment enters the surface it registers a surface disturbance in the form of a trough defined by two ridges. Deeper in the crystal the fragment produces a single ridge and finally ceases to register a surface disturbance when it has penetrated more than about 30 Å into the crystal. Track length distributions have been determined from a large number of tracks for two specimens. These show good agreement with a theoretical distribution based on straightforward geometrical considerations. Details of the theoretical development are outlined in an appendix. There are, however, a disproportionately large number of tracks visible in crystallographic directions. This would be explained if fragments registered surface disturbances from a greater depth in these particular directions.