Lichtenberg Figures and the Tree of Fire |
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Paul Frame, Oak Ridge Associated Universities |
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When
a dielectric material (e.g. cerium-containing glass or Lucite) serves as
the target for a beam of electrons in an accelerator, the electrons become
trapped in an excited state at various locations in the material. A
potential of several million volts can be created by this accumulated
electrical charge. The depth in the material at which the electrons are
trapped depends on their energy. As a rule, the energy of the electron
beam is chosen so that the electrons end up in the middle of the target. If
the irradiation continues long enough, the strength of the electric field
will exceed the dielectric strength of the material and a spontaneous
discharge will occur wherein the electrons jump out of their traps and
flow to some point(s) at the material’s edge or, less commonly, to some
point on the surface. The discharge is accompanied by an explosion
and light. The appearance of this light, sometimes called the
“tree of fire”, is that of numerous beams of light rapidly converging
on one point. If
the irradiation is stopped prior to such a discharge, the latter can be
initiated by striking the material with a pointed metal object (e.g.,
center punch) that is grounded. The electrons then flow to the point at
which the material was struck. This provides a way to control the
discharge and to demonstrate the phenomenon to an audience. To enhance the
effect, the discharge might be initiated in the dark so that the tree of
fire can be more easily observed. After the discharge, the path followed
by the electrons can be seen etched into the material. The etched pattern,
which resembles an inverted tree or piece of coral, is called a
Lichtenberg Figure in honor of George Lichtenberg, a physics professor at
the Lichtenberg
Figures are often described by the colorful phrase “frozen
lightening.” |
Last updated: 07/25/07
Copyright 1999, Oak Ridge Associated Universities