Electron tree. This pattern of branching fractures is produced by liberating a beam of high-energy electrons that has been trapped inside a perspex block. The electron beam is set so that the energy is sufficient for the electrons to imbed in the perspex. Then a small crack is made in the base of the perspex block, and the electrons flow out, leaving behind the fractures seen here. This electron tree has been produced using the electron linear accelerator at the National Physical Laboratory, Teddington, UK.