Lovely little Lichtenbergs

"A Lichtenberg Figure (sometimes called a Lichtenberg Tree, Lichtenberg Pattern, Electron Tree, or Spark Tree) can be formed by penetrating an acrylic block using high-speed electrons. Electrons are tiny, negatively charged particles that orbit the nucleus of all atoms. A special machine, called a linear accelerator (or LINAC), is used to accelerate and focus free electrons into a tight, high-energy beam...

"An acrylic block is placed in the path of the electron beam. As the block is bombarded ('irradiated') by the beam, electrons are driven deep inside... As the acrylic is irradiated, huge numbers of electrons accumulate inside, creating a cloud-like layer of negative electrical charge called a space charge. Since acrylic is an excellent dielectric (an insulating material), most of the excess electrons cannot escape, and they temporarily become trapped. As they accumulate under continued irradiation, a huge negative space charge develops within the acrylic.

"As the space charge grows, the resulting electric field also increases. Eventually, the electric field overcomes the dielectric strength of the acrylic, and some of the chemical bonds that hold the acrylic molecules together are ripped apart, which strips away free electrons in a process called ionization. The newly-freed electrons are accelerated by the electric field, causing them to collide and ionize other acrylic molecules, creating additional free electrons.

"As the number of free electrons grows exponentially, they rapidly create electrically conductive pathways within the acrylic in a process called dielectric breakdown. As the previously trapped charge suddenly rushes out (accompanied by a loud bang!), the electrically conductive pathways form brilliant branching 'lightning bolts' within the acrylic."

via Fergus Murray